Mega Deck Flashcards
State two conditions for any object to be in equilibrium
Resultant force zero
Resultant moment about any point zero
State three vector quantities
Any 3 of the following:
Velocity
Acceleration
Force
Displacement
Weight
Momentum
State three scalar quantities
Any 3 of the following:
Speed
Distance
Mass
Energy
Power
Temperature
How can force vectors be arranged to show that an object has constant velocity?
- Vectors make a closed shape when rearranged (by scale drawing)
- Or resolve into components and show
- Total up forces = Total Down forces
- Total left forces = Total right forces
What is the difference between a vector quantity and a scalar quantity?
Vector has a direction
Scalar does not
What is meant by centre of mass?
The point in a body where the weight of the object appears to act
Also the resultant moment about this point = 0
Define the moment of a force
Product of the force and the perpendicular distance from the line of action of the force to the point
![](https://s3.amazonaws.com/brainscape-prod/system/cm/357/137/980/a_image_thumb.png?1632406493)
Resolve F into its vertical and horizontal components…
![](https://s3.amazonaws.com/brainscape-prod/system/cm/357/137/981/q_image_thumb.png?1632406493)
FH = FcosØ
Fv = FsinØ
![](https://s3.amazonaws.com/brainscape-prod/system/cm/357/137/981/a_image_thumb.png?1632406493)
What mistake has been made in rearranging the vectors for a scale drawing?
![](https://s3.amazonaws.com/brainscape-prod/system/cm/357/137/982/q_image_thumb.png?1632406494)
6N vector has been translated (moved) but also rotated
Should be:
![](https://s3.amazonaws.com/brainscape-prod/system/cm/357/137/982/a_image_thumb.png?1632406494)
What are the steps in working out the resultant force using a tip-tail scale drawing?
![](https://s3.amazonaws.com/brainscape-prod/system/cm/357/137/983/q_image_thumb.png?1632406494)
- Set a scale
- Draw the horizontal or vertical vector first (if there is one)
- Move each vector in turn to the end of the previous one (DO NOT ROTATE THE VECTORS)
- Resultant vector goes from the very start to the very end
![](https://s3.amazonaws.com/brainscape-prod/system/cm/357/137/983/a_image_thumb.png?1632406494)
How is the balancing force different from the resultant force?
The balancing force brings the object into equilibrium so makes the resultant force = 0
For a scale drawing, it is the vector that closes the shape
![](https://s3.amazonaws.com/brainscape-prod/system/cm/357/137/984/a_image_thumb.png?1632406495)
If vectors are parallel they can be resolved by…
![](https://s3.amazonaws.com/brainscape-prod/system/cm/357/137/985/q_image_thumb.png?1632406495)
Adding or subtracting the values
![](https://s3.amazonaws.com/brainscape-prod/system/cm/357/137/985/a_image_thumb.png?1632406496)
If vectors are perpendicular they can be resolved by…
![](https://s3.amazonaws.com/brainscape-prod/system/cm/357/137/986/q_image_thumb.png?1632406496)
Making a right angled triangle and using trigonometry and pythagoras
![](https://s3.amazonaws.com/brainscape-prod/system/cm/357/137/986/a_image_thumb.png?1632406496)
What is wrong with this?
![](https://s3.amazonaws.com/brainscape-prod/system/cm/357/137/987/q_image_thumb.png?1632406496)
Vectors of different types can’t be combined
(Here, force and velocity cannot be combined)
![](https://s3.amazonaws.com/brainscape-prod/system/cm/357/137/987/a_image_thumb.png?1632406496)
How do you solve this if the object is in equilibrium?
(3 vectors with 2 unknown sizes)
![](https://s3.amazonaws.com/brainscape-prod/system/cm/357/137/988/q_image_thumb.png?1632406497)
- The vectors must form a closed shape
- Start as you would with a scale drawing
- But draw the third vector meeting for where it connects to the start of the first
- Draws vectors as dotted lines
(x=2.54N, y=3.89N)
![](https://s3.amazonaws.com/brainscape-prod/system/cm/357/137/988/a_image_thumb.png?1632406497)
The box is in equilibrium with no external forces applied
Label the forces acting on the box
![](https://s3.amazonaws.com/brainscape-prod/system/cm/357/137/989/q_image_thumb.png?1632406497)
Notice the angle between weight and perpendicular is also Ø
![](https://s3.amazonaws.com/brainscape-prod/system/cm/357/137/989/a_image_thumb.png?1632406497)
How do you calculate the resultant moment? (2 ways)
![](https://s3.amazonaws.com/brainscape-prod/system/cm/357/137/990/q_image_thumb.png?1632406498)
- Multiply perpendicular component of force by distance
- Multiply perpendicular component of distance by force
(First method is shown)
![](https://s3.amazonaws.com/brainscape-prod/system/cm/357/137/990/a_image_thumb.png?1632406498)
What’s wrong with this?
![](https://s3.amazonaws.com/brainscape-prod/system/cm/357/137/991/q_image_thumb.png?1632406498)
Weight must form the hypotenuse of the triangle
![](https://s3.amazonaws.com/brainscape-prod/system/cm/357/137/991/a_image_thumb.png?1632406498)
What is a couple?
A pair of equal and opposite coplanar forces which do not act along the same line of action
![](https://s3.amazonaws.com/brainscape-prod/system/cm/357/137/992/a_image_thumb.png?1632406498)
What does it mean if an object is uniform?
It has an constant density so its centre of mass acts from the physical centre point of the object
(Weight vector starts from middle of object)
![](https://s3.amazonaws.com/brainscape-prod/system/cm/357/137/993/a_image_thumb.png?1632406499)
When should you use moments?
Any situation that has two unknown forces acting on an object
Take moments about one of the unknown forces to find the other
Then use total up force = total down force to find the other
![](https://s3.amazonaws.com/brainscape-prod/system/cm/357/137/994/a_image_thumb.png?1632406499)
State the principle of moments
Sum of the clockwise moments about a point is equal to the sum of the anticlockwise moments for a system in equilibrium
What is displacement and how is it different to distance?
Displacement is a measure of the line connecting the starting point to the finishing point.
Distance is a measure of the total length of the path travelled.
Also distance is a scalar and displacement is a vector.
![](https://s3.amazonaws.com/brainscape-prod/system/cm/357/138/092/a_image_thumb.png?1632406542)
What does a straight line on a distance-time graph represent?
A constant speed.
![](https://s3.amazonaws.com/brainscape-prod/system/cm/357/138/093/a_image_thumb.png?1632406542)
How is acceleration defined?
Acceleration is the rate of change of velocity.
![](https://s3.amazonaws.com/brainscape-prod/system/cm/357/138/094/a_image_thumb.png?1632406542)
How is speed different to velocity?
Speed is the rate of change of distance.
Velocity is the rate of change of displacement.
![](https://s3.amazonaws.com/brainscape-prod/system/cm/357/138/095/a_image_thumb.png?1632406542)
Describe the motion of this ball
![](https://s3.amazonaws.com/brainscape-prod/system/cm/357/138/096/q_image_thumb.png?1632406542)
Ball is moving to the right and speeding up.
![](https://s3.amazonaws.com/brainscape-prod/system/cm/357/138/096/a_image_thumb.png?1632406542)
Decribe the motion of this ball.
![](https://s3.amazonaws.com/brainscape-prod/system/cm/357/138/097/q_image_thumb.png?1632406543)
Ball is moving to the left and speeding up.
![](https://s3.amazonaws.com/brainscape-prod/system/cm/357/138/097/a_image_thumb.png?1632406543)
Describe the motion of this ball.
![](https://s3.amazonaws.com/brainscape-prod/system/cm/357/138/098/q_image_thumb.png?1632406543)
Ball is moving to the right and slowing down.
![](https://s3.amazonaws.com/brainscape-prod/system/cm/357/138/098/a_image_thumb.png?1632406543)
Describe the motion of this ball.
![](https://s3.amazonaws.com/brainscape-prod/system/cm/357/138/099/q_image_thumb.png?1632406543)
Ball is moving to the left but slowing down.
![](https://s3.amazonaws.com/brainscape-prod/system/cm/357/138/099/a_image_thumb.png?1632406543)
Is the ball moving to the right?
![](https://s3.amazonaws.com/brainscape-prod/system/cm/357/138/100/q_image_thumb.png?1632406543)
Only if the velocity vector is also acting to the right.
![](https://s3.amazonaws.com/brainscape-prod/system/cm/357/138/100/a_image_thumb.png?1632406544)
What does a straight line on a displacement-time graph represent?
![](https://s3.amazonaws.com/brainscape-prod/system/cm/357/138/101/q_image_thumb.png?1632406544)
A constant velocity.
What does a curve with an increasing gradient represent on a displacement-time graph?
![](https://s3.amazonaws.com/brainscape-prod/system/cm/357/138/102/q_image_thumb.png?1632406544)
An increasing velocity (acceleration)
What does a curve with a decreasing gradient represent on a displacement-time graph?
![](https://s3.amazonaws.com/brainscape-prod/system/cm/357/138/103/q_image_thumb.png?1632406544)
A decreasing velocity (decceleration)
What does a negative gradient on a displacement-time graph represent?
![](https://s3.amazonaws.com/brainscape-prod/system/cm/357/138/104/q_image_thumb.png?1632406544)
A negative velocity (travelling back to where it started)
What does a straight line on a velocity-time graph represent?
![](https://s3.amazonaws.com/brainscape-prod/system/cm/357/138/105/q_image_thumb.png?1632406545)
A constant acceleration.
What does a curve with an increasing gradient represent on a velocity-time graph?
![](https://s3.amazonaws.com/brainscape-prod/system/cm/357/138/106/q_image_thumb.png?1632406545)
An increasing acceleration.
What does a curve with a decreasing gradient represent on a velocity-time graph?
![](https://s3.amazonaws.com/brainscape-prod/system/cm/357/138/107/q_image_thumb.png?1632406545)
A decreasing acceleration.
What does a negative gradient on a velocity-time graph represent?
![](https://s3.amazonaws.com/brainscape-prod/system/cm/357/138/108/q_image_thumb.png?1632406545)
A negative acceleration.
What does this graph show?
![](https://s3.amazonaws.com/brainscape-prod/system/cm/357/138/109/q_image_thumb.png?1632406545)
A ball bouncing off a surface
(Dotted lines represent the bounce)
(Red lines represent the ball accelerating towards the ground)
What does the acceleration time graph of a ball in freefall look like?
Constant acceleration of 9.81ms-2
![](https://s3.amazonaws.com/brainscape-prod/system/cm/357/138/110/a_image_thumb.png?1632406546)
What’s wrong with this?
![](https://s3.amazonaws.com/brainscape-prod/system/cm/357/138/112/q_image_thumb.png?1632406546)
Displacement takes direction into account.
It should be…
![](https://s3.amazonaws.com/brainscape-prod/system/cm/357/138/112/a_image_thumb.png?1632406547)
Why can’t you use SUVAT’s when working with this graph?
![](https://s3.amazonaws.com/brainscape-prod/system/cm/357/138/115/q_image_thumb.png?1632406548)
Because the acceleration (gradient) is changing
What does it mean if an object is in freefall?
Only weight is acting on the object
It has a constant acceleration of 9.81ms-2 acting downawards (on Earth)
![](https://s3.amazonaws.com/brainscape-prod/system/cm/357/138/116/a_image_thumb.png?1632406548)
If one ball is dropped as another is projected horizontally which hits the ground first?
They both hit the ground at the same time…
Both in freefall so accelerate at 9.81ms-2
Vertical motion independent of horizontal motion
![](https://s3.amazonaws.com/brainscape-prod/system/cm/357/138/117/a_image_thumb.png?1632406548)
What’s wrong with this labelling?
![](https://s3.amazonaws.com/brainscape-prod/system/cm/357/138/118/q_image_thumb.png?1632406548)
Initial velocity and final velocity are not 0
![](https://s3.amazonaws.com/brainscape-prod/system/cm/357/138/118/a_image_thumb.png?1632406548)
In projectile motion when is the vertical component of the velocity 0?
At the peak of a parabola
![](https://s3.amazonaws.com/brainscape-prod/system/cm/357/138/119/a_image_thumb.png?1632406549)
Not at the start or end
How do you start a question involving angled projectile motion?
![](https://s3.amazonaws.com/brainscape-prod/system/cm/357/138/120/q_image_thumb.png?1632406549)
Resolve the velocity into vertical and horizontal components and fill out the corresponding SUVATs
![](https://s3.amazonaws.com/brainscape-prod/system/cm/357/138/120/a_image_thumb.png?1632406549)
What is wrong here?
![](https://s3.amazonaws.com/brainscape-prod/system/cm/357/138/121/q_image_thumb.png?1632406549)
The acceleration is only 9.81ms-2 if the object is in freefall
![](https://s3.amazonaws.com/brainscape-prod/system/cm/357/138/121/a_image_thumb.png?1632406550)
What does the area of a speed-time graph represent?
How about a velocity-time graph?
![](https://s3.amazonaws.com/brainscape-prod/system/cm/357/241/773/a_image_thumb.png?1632484207)
When can you use this equation?
![](https://s3.amazonaws.com/brainscape-prod/system/cm/357/241/774/q_image_thumb.png?1632484208)
When the acceleration = 0 (constant velocity)
Or to work out an average speed
When can you use SUVATs?
![](https://s3.amazonaws.com/brainscape-prod/system/cm/357/241/964/q_image_thumb.png?1632484323)
When acceleration is constant
Or if object has stages of constant acceleration
What is the term given to an object rotating at a steady rate?
Uniform circular motion
If an ball on a string is travelling in a circle in the vertical plane, where are the points of minimum and maximum tension?
Minimum tension at the top
Maximum tension at the bottom
![](https://s3.amazonaws.com/brainscape-prod/system/cm/357/922/152/a_image_thumb.png?1633018010)
Why do planes turn when at an angle?
![](https://s3.amazonaws.com/brainscape-prod/system/cm/357/922/153/q_image_thumb.png?1633018010)
The lift force is comprised of a horizontal and vertical component.
The horizontal component provides the centripetal force causing it to turn.
![](https://s3.amazonaws.com/brainscape-prod/system/cm/357/922/153/a_image_thumb.png?1633018010)
Define centripetal force
The resultant force that makes the object move in a circle
What kind of motion will a pendulum perform?
Simple harmonic motion
What is the period of oscillation?
The time for one complete cycle of oscillation.
If the graph of displacement is sin(x), what will the respective graphs of velocity and acceleration look like?
Velocity as cos(x)
Acceleration as -sin(x)
Describe a freely oscillating object
It oscillates with a constant amplitude because there is no friction acting on it.
(Its energy is constant)
What is natural frequency?
The frequency of free oscillations of an oscillating system.
What are forced vibrations?
Making an object oscillate at a frequency that is not it’s natural frequency
When does resonance occur?
When the frequency of driving force or oscillation matches the natural frequency of the system.
What is the outcome of resonance?
An increase in amplitude of the system’s oscillation.
What is damping?
The term used to describe the removal of energy from an oscillating system.
Describe heavy damping (over damping)
System not allowed to oscillate.
Slowly returns to equilibrium.
![](https://s3.amazonaws.com/brainscape-prod/system/cm/357/922/154/a_image_thumb.png?1633018011)
Describe critical damping
The oscillating system returns to the zero position of the oscillation after one quarter of a time period.
![](https://s3.amazonaws.com/brainscape-prod/system/cm/357/922/155/a_image_thumb.png?1633018011)
How do you convert degrees -> radians
![](https://s3.amazonaws.com/brainscape-prod/system/cm/357/922/156/a_image_thumb.png?1633018011)
How do you convert radians -> degrees
![](https://s3.amazonaws.com/brainscape-prod/system/cm/357/922/157/a_image_thumb.png?1633018011)
Define angular displacement
The angle through which an object in circular motion travels in a given time
![](https://s3.amazonaws.com/brainscape-prod/system/cm/357/922/158/a_image_thumb.png?1633018011)
What are the three levels of damping?
Light
Heavy
Critical
Describe light damping of a system
The system oscillates over a long time frame before coming to rest.
The amplitude of the oscillations exponentially decay.
What is the equation for linear velocity?
![](https://s3.amazonaws.com/brainscape-prod/system/cm/357/922/159/a_image_thumb.png?1633018011)
In circular motion which direction do the acceleration and centripetal force vectors act?
Always towards the centre
![](https://s3.amazonaws.com/brainscape-prod/system/cm/357/922/160/a_image_thumb.png?1633018012)
What is the condition for circular motion to happen?
A velocity needs to be acting perpendicular to a resultant force
![](https://s3.amazonaws.com/brainscape-prod/system/cm/357/922/162/a_image_thumb.png?1633018012)
What is Fcentri for an object at the top of the vertical circle?
![](https://s3.amazonaws.com/brainscape-prod/system/cm/357/922/163/q_image_thumb.png?1633018012)
![](https://s3.amazonaws.com/brainscape-prod/system/cm/357/922/163/a_image_thumb.png?1633018012)
What is Fcentri for an object on top of a vertical circle?
![](https://s3.amazonaws.com/brainscape-prod/system/cm/357/922/164/q_image_thumb.png?1633018012)
![](https://s3.amazonaws.com/brainscape-prod/system/cm/357/922/164/a_image_thumb.png?1633018012)
What is Fcentri for an object at the bottom of a vertical circle?
![](https://s3.amazonaws.com/brainscape-prod/system/cm/357/922/165/q_image_thumb.png?1633018013)
![](https://s3.amazonaws.com/brainscape-prod/system/cm/357/922/165/a_image_thumb.png?1633018013)
How do you find out the minimum velocity for an object travelling in a vertical circle?
![](https://s3.amazonaws.com/brainscape-prod/system/cm/357/922/166/q_image_thumb.png?1633018013)
Set R=0 (or tension if ball on string)
And rearrange for v
![](https://s3.amazonaws.com/brainscape-prod/system/cm/357/922/166/a_image_thumb.png?1633018013)
How do you find out the maximum velocity for an object travelling over a vertical circle? (eg car over a hill)
![](https://s3.amazonaws.com/brainscape-prod/system/cm/357/922/167/q_image_thumb.png?1633018013)
Set reaction R=0
Then rearrange for v
![](https://s3.amazonaws.com/brainscape-prod/system/cm/357/922/167/a_image_thumb.png?1633018013)
When solving angled circular motion problems what are the 3 usual steps?
- Set vertical component of force = weight
- Work out horizontal component using trig
- Fcentri = horizontal component
![](https://s3.amazonaws.com/brainscape-prod/system/cm/357/922/168/a_image_thumb.png?1633018014)
Why can’t a ball be swung around in a circle with the string horizontal?
![](https://s3.amazonaws.com/brainscape-prod/system/cm/357/922/169/q_image_thumb.png?1633018014)
There must be a vertical component of the tension to match the weight
Otherwise ball is not in vertical equilibrium
![](https://s3.amazonaws.com/brainscape-prod/system/cm/357/922/169/a_image_thumb.png?1633018014)
What are the two conditions for SHM?
- Acceleration must be proportional to displacement
- Acceleration must be opposite to displacement
![](https://s3.amazonaws.com/brainscape-prod/system/cm/357/922/170/a_image_thumb.png?1633018014)
How does the time period differ for the two pendulums?
![](https://s3.amazonaws.com/brainscape-prod/system/cm/357/922/171/q_image_thumb.png?1633018014)
Time period is independent of amplitude
![](https://s3.amazonaws.com/brainscape-prod/system/cm/357/922/171/a_image_thumb.png?1633018015)
Label up the maximum and minimum velocities and accelerations on the simple pendulum…
![](https://s3.amazonaws.com/brainscape-prod/system/cm/357/922/172/q_image_thumb.png?1633018015)
![](https://s3.amazonaws.com/brainscape-prod/system/cm/357/922/172/a_image_thumb.png?1633018015)
Label up the maximum and minimum velocities and accelerations on the mass spring system…
![](https://s3.amazonaws.com/brainscape-prod/system/cm/357/922/173/q_image_thumb.png?1633018015)
![](https://s3.amazonaws.com/brainscape-prod/system/cm/357/922/173/a_image_thumb.png?1633018015)
Label up the maximum and minimum potential and kinetic energies on the simple pendulum…
![](https://s3.amazonaws.com/brainscape-prod/system/cm/357/922/174/q_image_thumb.png?1633018016)
![](https://s3.amazonaws.com/brainscape-prod/system/cm/357/922/174/a_image_thumb.png?1633018016)
What are the kinetic energy, potential energy and total energy lines for one cycle of SHM?
![](https://s3.amazonaws.com/brainscape-prod/system/cm/357/922/175/q_image_thumb.png?1633018016)
![](https://s3.amazonaws.com/brainscape-prod/system/cm/357/922/175/a_image_thumb.png?1633018016)
How do you calculate KEmax or PEmax or ET in SHM?
![](https://s3.amazonaws.com/brainscape-prod/system/cm/357/922/176/a_image_thumb.png?1633018017)
What two factors affect the time period of a mass spring system in SHM?
- Mass on the end of the spring
- Spring constant (stiffness) of spring
![](https://s3.amazonaws.com/brainscape-prod/system/cm/357/922/177/a_image_thumb.png?1633018017)
What two factors affect the time period of a simple pendulum in SHM?
- Length between top of string and centre of bob
- Gravitational field strength
![](https://s3.amazonaws.com/brainscape-prod/system/cm/357/922/178/a_image_thumb.png?1633018017)
What does the graph of energy against displacement look like in SHM?
![](https://s3.amazonaws.com/brainscape-prod/system/cm/357/922/179/a_image_thumb.png?1633018017)
How do you deal with rpm? (revolutions per minute)
÷60 to convert to rps (revolutions per second)
Then set rps = frequency
When an SHM system is lightly damped what happens to its amplitude and time period?
Amplitude decreases (as it loses energy)
But time period remains constant
![](https://s3.amazonaws.com/brainscape-prod/system/cm/357/922/180/a_image_thumb.png?1633018017)
How is natural frequency determined for a mass spring system?
![](https://s3.amazonaws.com/brainscape-prod/system/cm/357/922/181/a_image_thumb.png?1633018017)
How is natural frequency determined for a simple pendulum?
![](https://s3.amazonaws.com/brainscape-prod/system/cm/357/922/182/a_image_thumb.png?1633018018)
Define frequency
The number of complete oscillations per second
Why is an object in circular motion accelerating?
Its linear velocity does not change in magnitude
But is constantly changing in direction
The graph below shows driven oscillations with varying frequencies.
Add two lines if the system is:
- Undamped (free oscillations)
- Over damped
![](https://s3.amazonaws.com/brainscape-prod/system/cm/357/922/183/q_image_thumb.png?1633018018)
![](https://s3.amazonaws.com/brainscape-prod/system/cm/357/922/183/a_image_thumb.png?1633018018)
For Barton’s pendulum which two balls oscillate?
![](https://s3.amazonaws.com/brainscape-prod/system/cm/357/922/184/q_image_thumb.png?1633018018)
P and Y because they have the same length
So natural frequency of y matches frequency of driving force from P
![](https://s3.amazonaws.com/brainscape-prod/system/cm/357/922/184/a_image_thumb.png?1633018019)
What happens if…
Fcentri > Fmax
Circular motion does not happen
(Eg car skids off the road or moves to a higher radius)
What happens if…
Fcentri max ≤ Fmax
Circular motion happens
(eg friction is large enough to keep car on track)
Define amplitude.
The maximum displacement of an obejct/particle/point from equilibrium position
When do you use?
x=Acos(wt)
When do you use?
x=Asin(wt)
x=Acos(wt) -> displacement in SHM when x=A when t=0
x=Asin(wt) -> displacement in SHM when x=0 when t=0
Does circular motion count as SHM?
When projected onto a flat surface, yes it does
What happens if the frequency of driving force is less than the natural frequency of a system?
f0
Low amplitude oscillations
With 0 phase difference.
What happens if the frequency of driving force matches the natural frequency of a system?
f=f0
Resonance occurs
Large amplitude oscillations
π/2 radians out of phase
What happens if the frequency of driving force is more than the natural frequency of a system?
f>f0
Low amplitude oscillations
With phase difference of π
What is Newton’s 1st Law of Motion?
If no resultant force acts on a body, then it will either remain at rest, or continue moving with constant velocity (no acceleration)
What is Newton’s 2nd Law of Motion?
The rate of change of momentum (acceleration) of a body is directly proportional to the resultant force acting on it
Fres ∝ a
What is Newton’s 3rd Law of Motion?
When two objects interact, they exert an equal and opposite force on each other and the forces are of the same type
If the forces acting on an object are balanced what can you say about its motion?
There is no resultant force so it will continue moving at a constant velocity. It won’t accelerate.
What’s wrong with this?
In F=ma, F must be the resultant force!!!
How does an object reach terminal velocity?
As it speeds up, air resistance increases, decreasing the resultant force.
Eventually air resistance = driving force, Fres=0 so a=0.
What two things are the case for tension?
- Tension always acts away from the contact points
- Tension is constant throughout the rope/wire/ cable
Why are objects never truly in freefall?
There will always be air resistance opposing the weight
(Apart from when v=0)
What is the condition for terminal velocity?
The drag force = driving force (or weight) so Fres=0 and so a=0
What factors the drag force on an object?
- Fluid density
- Shape of object
- Cross sectional area of object
- Velocity of object
Why does air resistance increase with velocity?
The object is colliding with more air molecules per second
What does the velocity time graph of an object reaching terminal velocity look like?
What’s wrong with this?
The acceleration is not constant so you cannot use SUVATs
Instead use area under graph
How is momentum calculated?
What’s wrong with this?
Direction must be taken into account (as momentum is a vector)
What two things is impulse equal to?
- Rate of change of momentum
- Impact force x impact time
What are the units of impulse and momentum?
What does the area under a force-time graph represent?
The change of momentum or impulse
What is the conservation of momentum?
For a system of interacting objects, the total momentum remains constant…
…provided no external resultant force acts
In any interaction, what is conserved?
Total momentum is always conserved
Total energy is always conserved
Kinetic energy is only conserved if collision is elastic
What is an elastic collision?
A collision where kinetic energy is conserved
(as well as momentum)
What is wrong here?
You have to calculate kinetic energies separately for each object
In physics terms what is an explosion?
The total momentum = 0
How do you answer flow rate questions? (momentum of a flowing liquid)
Consider the cylinder made by a liquid’s flow after 1 second
Where the length of the cylinder = velocity of the fluid
And use density equation to get volume of cylinder
How do you work out the area of a curved graph?
- Split into boxes
- Count the boxes (pairing up incomplete boxes)
- Multiply number of boxes by area of each box
What is the principle of conservation of energy?
Energy cannot be created or destroyed, only transferred from one type to another.
How does an object gain energy from a force?
When the force does work on the object (same direction as movement)
How does an object lose energy?
By doing work against a force (usually frictional)
What’s wrong here?
In W=Fs you multiply the parallel components
When using W=Fs what must be the case?
The components must be parallel
What is the work done by the weight of this block?
0 because the weight is perpendicular to the movement
So there is no parallel component to displacement
If two objects are dropped from the same height and air resistance is negligible, which hits the ground first?
Both hit at the same time because they both accelerate at 9.81ms-2
How do you calculate the velocity the object hits the ground using GPE and KE (assuming no air resistance)
Energy equivalency (only works when air resistance = 0)
What is power?
The rate of transfer of energy
or
The rate at which work is done
(Measured in Watts [W])
What is wrong here?
For P=Fv, F is not the resultant force
How do you calculate efficiency?
If the system is not 100% efficient would this be correct?
No, because some GPE is converted to thermal and kinetic energy of the snow (working against friction)
What is 1 mole?
A collection of 6.02×1023 molecules
(Avogadro’s constant)
What is the molar mass of a substance?
The mass of each mole (every 6.02×1023 molecules)
Eg for He each mole has a mass of 4g
How do you calculate the molar mass of a compound eg NO2
Add up the nucleon numbers
(14+16+16=46gmol-1)
How do you calculate the number of molecules in a substance?
N = n × NA
(Number of molecules = moles × Avogadro’s constant)
What is the molecular mass and how is it calculated?
The mass of each molecule of the substance
m = M/N
(molecular mass = total mass / number of molecules)
How is the total mass of a substance calculated?
M = n × mr
(Total mass = moles × molar mass)
How do you convert a temperature from °C to K?
T(K) = T(°C) + 273
Define absolute zero
The point at which an ideal gas exerts no pressure
(0K, -273°C, molecules have no kinetic energy)
What is Boyle’s Law?
The pressure in a gas is inversely proportional to the volume it occupies
at a fixed temperature
and a fixed mass of gas
(P ∝ 1/V)
What does the P-V graph look like for an ideal gas?
How do you prove Boyle’s law graphically?
Plot a graph of P against 1/V
Should be a straight line passing through the origin
What is Charles’ Law?
Volume a gas occupies is directly proportional to the temperature of the gas
at a fixed pressure
and a fixed mass of gas
(V ∝ T)
How do you prove Charles’ law by graph?
Plot a graph of V against T
Should be a straight line passing through the origin
For an ideal gas, what does a graph of V against T(°C) look like?
Note: x-intercept represents absolute zero
What is the Pressure law?
The pressure of a gas is directly proportional to the temperature of the gas
at a fixed volume
and a fixed mass of gas
How do you prove the pressure law graphically?
Plot a graph of P against T
Should be a straight line passing through the origin
For an ideal gas, what does a graph of P against T(°C) look like?
Note: x-intercept is absolute zero
![](https://s3.amazonaws.com/brainscape-prod/system/cm/361/822/145/a_image_thumb.png?1652546121)
What is the ideal gas relationship?
When can you use the ideal gas relationship?
If the mass of the gas is constant
How do you calculate the work done compressing or expanding a gas?
Calculate the area under the curve
What is the general equation for pressure?
P = F / A
(Pressure = Force / Area)
How does a gas exert a pressure on a container?
- The gas molecules collide with the container walls changing their momentum.
- This creates a force on the molecule and the wall
- Exerting a pressure
What are the 5 conditions for an ideal gas?
- Volume of the molecules must be much smaller than the volume of the gas itself
- The intermolecular forces are negligible
- The collision time of molecules with each other and the walls is much less than the time between them
- The collisions are elastic (no loss in KE)
- The molecules’ motion is random
How does Brownian motion explain the random motion of smoke?
- Air molecules are moving randomly
- They collide with the smoke changing momentum and exerting a force on the smoke particles
- If at one moment there are more collisions on one side than the other
- The smoke particle has a resultant force so accelerates in that direction
Explain Boyle’s Law using the molecular Kinetic Theory
- When volume of container is decreased
- More collisions per second
- So total momentum change bigger (▲p)
- So force exerted bigger
- So pressure bigger (From P = F/A)
Explain Charles’ Law using the molecular kinetic theory
- When temperature is increased
- Volume increases to increase the distance travelled between collisions
- Molecules have greater kinetic energy but travel further so frequency stays same
- Change in momentum (▲p) stays constant
- So pressure is constant (P = F/A)
Explain the Pressure law using the molecular kinetic theory
- As temperature increases
- The average kinetic energy of the molecules increases
- Increasing the number of collisions per second with container walls
- So greater change in momentum
- Greater force and pressure exerted (P = F/A)
How would you use this equation to work out the density of a gas?
How do you calculate crms from a list of speeds?
- Square the speeds and add up
- Take a mean of the squares
- Square root the value
How is cms calculated?
cms = (crms)2
What are the units of cms?
[m2s-2]
What does the maxwell-boltzmann distribution tell us about gases?
Molecules have a range of kinetic energies.
So temperature of the gas is a measure of the average kinetic energy.
For these equations how do you calculate the internal energy of the gas?
Multiply each by the number of molecules of the gas.
How do two objects brought into contact reach thermal equilibrium?
- There is a net flow of thermal energy from the hotter object to the colder object
- Until both objects are at the same temperature
- And there is now no net flow of thermal energy
Define specific heat capacity
The energy required to increase 1kg of a substance by 1K [Jkg-1K-1]
When would you use this equation?
To calculate the mass flowing per kg of a fluid
Why does the temperature of a substance changing state not increase?
The thermal energy is used to break some of the intermolecular bonds (solid → liquid) or the rest of the intermolecular bonds (liquid → gas)
Define specific latent heat of fusion
The energy required to change the state of 1kg of a solid to a liquid at its melting point.
Define specific latent heat of vaporization
The energy required to change the state of 1kg of a liquid to a gas at its boiling point.
What is wrong with this?
Haven’t considered the change of states. Need to break it into 3 equations:
How is density defined?
The mass per unit volume. [kgm-3]
How do you convert 2,3,4 etc… units to SI units?
(eg 5cm3 to m3)
Whatever you do to the unit, you do the same to the prefix
(eg 5cm3 = 5x(10-2)3m3 = 5x10-6m3)
How do you measure the density of an irregular solid?
- Read off the volume from the beaker or measuring cylinder without and with the object submerged in water
- The difference in volumes is the volume of the solid
- Measure the mass using a balance
Calculate density using ρ=M/V
How do you calculate the average density of an alloy?
(Eg 200cm3 5kg rod of 60% copper (8960kgm-3) and 40% aluminum (2700kgm-3) by volume?)
- Work out the mass of each and the volume of each
- Add together to get the total mass and volume
- Then do the density calculation
Define Hooke’s Law
When a material is stretched, its extension is proportional to the force applied, up until the limit of proportionality
F=kx
Define the limit of proportionality
The point at which the material stops obeying Hooke’s law.
The graph is no longer a straight line.
Define the elastic limit
The point at which when stretched further the material no longer returns to its original length (there is a permanent extension)
What do the gradient and area of a force extension graph tell you for a spring.
Gradient → The spring constant (must be taken before limit of proportionality)
Area under line → The strain energy stored loading the spring or energy released unloading the spring
What equation calculates energy stored when a material is stretched?
E=½Fx
What is the difference between the elastic limit and the limit of proportionality?
Limit of proportionality is the point at which a stretched spring (or wire) stops obeying Hooke’s law.
The elastic limit is the point at which it doesn’t return to its original length when unloaded.
Will this spring return to its original length if it has been stretched to 35mm?
Yes, because it has not passed the elastic limit
What is a ductile material?
A material with a large plastic region.
What is a brittle material?
A material with a small plastic region.
What is the fracture point of a material?
The point at which a material breaks
How do you know the rubber hasn’t stretched passed its elastic limit?
It still returns to its original length when unloaded.
What is the formula for Young’s Modulus that you need to remember?
What does the gradient and area under a stress-strain graph give?
Gradient → Young’s modulus (before the limit of proportionality)
Area → strain energy per unit volume
If this box is in equilibrium how would you go about calculating the frictional force and the reaction force?
![](https://s3.amazonaws.com/brainscape-prod/system/cm/363/410/240/q_image_thumb.png?1632383619)
![](https://s3.amazonaws.com/brainscape-prod/system/cm/363/410/240/a_image_thumb.png?1636991573)
Define current (I)
The rate of flow of charge
How do you work out the number of electrons carrying a charge (eg 10C)?
Divide charge by the charge of each electron (6.25x1019)
What is the difference between conventional current and electron flow?
Conventional current flows from the +ve terminal to the -ve terminal
Electron flow shows the direction the electrons flow, from -ve to +ve
How is the current in a circuit related to potential difference and resistance?
Increasing potential difference increases the current
Increasing resistance decreases the current
What is Ohm’s law?
The current flowing through a metallic conductor is proportional to the potential difference applied across it at constant temperature
When does Ohm’s law apply?
When the component has a fixed resistance (eg a fixed resistor at a constant temperature, or a filament at a low current)
Define potential difference
The work done (energy transferred) by each coulomb of charge moving between two points
(Eg a 12V battery adds 12J of energy to each coulomb of charge passing through)
How does a circuit ‘short circuit’?
If there is an available path with 0 resistance
Current → ∞
And the circuit heats up
What is the I-V graph for a fixed resistor?
What is the I-V graph for a filament bulb?
What is the graph for a semiconductor diode?
What’s wrong with this?
Resistance is not calculated using the gradient (of a tangent) of an I-V graph!!!
Instead just use the voltage and current at that point
Explain the shape of the I-V graph for a filament
As current increases, temperature of filament increases
This increases lattice ion vibrations.
Which increases the number of collisions per second with electrons.
So resistance increases.
How does the I-V graph for a fixed resistor prove it is ohmic?
The straight line passing through the origin
proves that current ∝ voltage
Explain the shape of the semiconductor diode (in positive bias)
- As the potential difference increases weakly bound electrons in the conductor gain energy
- After the threshold pd, some electrons become free to carry a current
- The lattice vibrations still increase but this is less significant
What happens if a semiconductor diode is connected in reverse bias?
No current flows until the breakdown voltage is reached (~50V)
The diode breaks and all current flows through
What is the difference between a series and a parallel circuit?
Parallel circuits have junctions (3 or more wires connect)
Why doesn’t adding voltmeters in parallel affect the circuit? (it is still series)
Voltmeters have ~ ∞ R so no current flows through
What are the p.d and current rules for a series circuit?
P.D is shared across the components (by resistance)
Current is constant throughout
What are the p.d and current rules for a series circuit?
P.D is shared across the components (by resistance)
Current is constant throughout
What are the p.d and current rules for a parallel circuit?
P.D is same for parallel branches
Current separates at junctions (according to branch resistance)
What is Kirchoff’s 1st Law?
At any junction in a circuit the sum of the current flowing into the junction is equal to the sum of the current flowing away from it.
What is Kirchoff’s 2nd Law?
In any complete “loop” of a circuit the sum of p.d’s equals the source p.d.
How do you combine series resistors in the same branch? (no junction between them)
Add up their resistances
How do you combine resistors in parallel branches? (one junction between them)
Use the following equation…
What is the advantage of placing resistors in parallel arrangements?
The total resistance is always less than the smallest resistance
Will the current split equally?
No, because the resistance of each branch is different
Will each component receive the same voltage?
No, because the resistance of the components are different
Why would you place batteries in parallel?
- The power delivered is the same
- But they take longer to run flatter
What is a potential divider circuit?
A circuit with 2 or more resistors connected in series with a power supply. (usually one is a thermistor or LDR)
How does resistance change for an NTC Thermistor?
As temperature increases, resistance decreases
How does resistance change for a Light Dependent Resistor (LDR)?
As light intensity increases, resistance decreases
What is the advantage of setting up a rheostat as a variable resistor?
- Simpler circuit
- Current constant throughout
- But cannot get 0V across bulb
What is the advantage of setting up a rheostat as a potential divider?
- Bulb can receive full range of voltage 0V → Vsource
- Current through bulb can be reduced to 0A
- But maximum current is lower
How does changing the dimensions of a piece of metal affect its resistance?
- Increased length → increased resistance
- Increases cross sectional area → decreased resistance
- Increased resistivity (using different material) → increased resistance
How do you calculate the cross sectional area of a wire?
Assume it to be a cylinder (unless told otherwise)
A=∏r2
Why do metals with a greater cross sectional area have a lower resistance?
There are more paths for the electrons to propagate
How do you calculate the potential difference across branches?
- Work out the P.D of each component
- Make a loop connecting the branches
- Subtract the PDs of one branch from the other
What is a superconductor?
A material with 0 resistance at and below the critical temperature
Why does a material become superconducting at and below its critical temperature?
- The lattice ion vibrations reduce to 0
- So electrons can pass through without collision
What is the advantage of superconductors and name a use?
- Transmit large currents with 0 resistance
- So negligible thermal energy losses
- Used to create high power magnets → MRI machines
- High processing power circuits → Supercomputers
Define emf of a power source
The potential difference across the terminals when no current is flowing through
Define terminal potential difference of a circuit
The potential difference across the terminals when a current is flowing through
What is the lost voltage in a circuit?
The potential difference used up pushing a current through the battery (vlost = emf - TPD)
How should you work with a circuit involving internal resistance?
Treat the internal resistance as another resistor in series with the components
Then solve as a regular circuit (using ohm’s law, kirchoff’s laws, P=IV etc)
What is the photoelectric effect?
Light incident on a metal surface causes electrons to be emitted from the surface
Why are electrons emitted from this surface by shining green and blue light on it? (not red)
Blue and green light are above the threshold frequency of this metal
So the photons of light have an energy > work function (φ)
Why are no electrons emitted when red light shines on this metal?
The red light photons are below the threshold frequency
So the energy of each photon < work function (φ)
Why does making the red light brighter not cause electrons to be emitted? (Photoelectric effect)
Electrons in the metal interact with photons in a 1-1 interaction
They only absorb photons which have an energy > work function (φ)
- Why do both light source cause electrons to be emitted? (from the surface)
- What is different about the electrons emitted due to the blue light?
- Both light sources have frequency above the threshold frequency (f0) of the metal
- The electrons emitted due to the blue light have a greater maximum kinetic energy (because blue photons have a greater energy from E=hf)
What does threshold frequency (f0) of a metal mean?
The minimum frequency of the incident light needed to cause electrons to be emitted from the surface
- What can you say about the green light incident on this metal?
- What difference does the brighter lamp make?
- The green light is above the threshold frequency so the photelectric effect happens
- The brighter lamp causes more photons of light to collide with electrons so more photons are emitted per second (But the electrons have the same maximum kinetic energy)
You are shining a light (above f0) on a metal. How do you:
- Increase the maximum kinetic energy of the emitted electrons?
- Increase the number of emitted electrons per second?
- Increase the frequency of the light source
- Increase the brightness of the light source
This is a graph for the photelectric effect. What information do the 3 features of the graph provide?
- Y-intercept
- X-intercept
- Gradient
- Y-intercept = - work function
- X-intercept = threshold frequency
- Plancks’ Constant