P5 - Forces Flashcards
What is a vector? Give some examples
A measurement with quantity AND direction.
FORCES, velocity, displacement, acceleration, momentum are vector quantities
For an arrow showing the vector quantity of something, what do the length and direction show?
The length shows the magnitude (larger arrow, more force)
The direction shows the direction of the quantity
Which direction is -v?
Backwards/ to the left/
Give the definition of a force
A push or a pull on an object caused by it interacting with something. All forces are either contact or non-contact
What type of force is it when 2 objects have to be touching for a force to act?
Contact force
What type of force is when the objects do not need to be touching for the force to act?
Non-contact force
Name some contact and non-contact forces
Contact - friction, air resistance, tension in ropes, normal contact force
Non-contact - magnetic force, gravitational, electrostatic
An equal but opposite force of attraction is felt by both the sun and the Earth/ What is this force?
Gravitational force - non-contact
An equal but opposite forces are felt by both a chair and the ground. But what is this force?
Normal contact force- a chair exerts a force on the ground, whilst the ground pushes back at the chair with the same force
What is gravitational force?
What are the 2 important effects?
The force of attraction between masses
On the surface of a planet, it makes all things fall toward the ground, and it gives everything a weight
What are the differences between weight and mass?
Mass has the same value anywhere in the universe (kg)
Weight is the force acting upon an object due to gravity - the pull of the gravitational force on the object. (N)
Name some factors that make the gravitational field strength stronger
It’s stronger the closer you are to the mass causing he field (earth’s core here). And, stronger for larger masses
The weight of an object will never change True or false?
False - it changes with its location, as it depends on the strength of the gravitational field at the location of the object
Give the formula for weight, and how it is measured
Weight = Mass x Gravitational field strength
N = kg x N/kg
Measured using a calibrated spring, whereas mass is measured with a mass balance
What is a uniform object?
One that has he same density throughout its regular shape - its centre of mass is at the centre
Weight and mass are:
a) directly proportional
b) indirectly proportional
a) directly proportional
Talk about the forces acting upon a skydiver
Weight pulls them towards the ground
Drag (air resistance) acts in the opposite direction to their motion. It is weaker, so the skydiver will be slower with a drag but still fall
What is a free body diagram?
Where it has a picture of an object, and force arrows coming off it - the sizes of the arrows show relative magnitudes of the forces, and the directions show the directions of the forces acting on the object
What is resultant force?
The overall force on a point or an object.
If you have a number of forces acting at a single point, you can replace them with a single, Resultant force (as long as that has the same effect as all the original forces together)
1500 ^ N
|
1200N 1000N
|
1500 N
Calculate the resultant force from the free body diagramVertical = 1500 - 1500 = 0N (no resultant force) Horizontal = 1200 - 1000 = 200N
Resultant force = 200N to the left - the object is going backwards
If a resultant force moves an object, what happens?
Work is done.
When a force moves an object through a distance, energy is transferred and work is done on the object
What does the force do in order move an object?
It does work. It needs a source of energy to do this, like fuel or food. Energy gets transferred from one store to another
What is the calculation for work done?
Work done = Force x distance
J = N x m
What is the difference between energy transferred and work done?
There is none
Talk about what happens when you push something along a rough surface, like a carpet
You are doing work against frictional forces. Energy is being transferred to the kinetic energy store of the object because it starts moving, but some is also transferred to the thermal energy stores due to the friction, causing the overall temperature to increase
When is 1 joule of work done?
When a force of 1N causes an object to move 1metre. 1 J = 1 Nm (Newton metre)
What can help you to resolve forces or work out the resultant forces?
A scale drawing - 1cm = 1N, you draw all the forces on an object to scale, the connect the first force to the last force, to find the resultant force
A man is on a bike with a 4N driving force north. The wind has a 3N force east. Find the magnitude and direction of the resultant force
3cm
^------>
|
4cm |
|
Measure hypotenuse with ruler - 5cm
The resultant force is 5N on a bearing of 037 degreesWhat makes the object an equilibrium?
If all the forces on it are balanced - there is no resultant force so it is in equilibrium. In a scale diagram, the first force and the last force will meet
How can you make slanted forces easier?
They can be split into 2 components at right angles to each other, usually horizontally and vertically (awkward force is hypotenuse, others make up the other part of the triangle)
Acting together, they have the same effect as the single, original awkward force
Complete the sentence:
When you apply a force to an object you may cause it to…
Stretch, compress or bend, if you have more than one force acting upon the object (or it would just move)
What does elastically deformed mean?
The object can go back to its original shape and length after the force has been removed. Elastic objects can do this.
An object is inelastically deformed if it doesn’t return to its original shape & length - it has gone beyond the limits of proportionality
Extension and force are:
a) indirectly proportional
b) directly proportional
a) directly proportional
What is the equation for the force on a spring?
Force = spring constant (k) x extension
N = N/m x m
What does the spring constant depend on?
The material that is being stretched
Complete he sentence:
The stiffer the spring…
Greater the spring constant
When plotting a graph for force of a spring, what are the axis? What is the gradient?
Extension and force (extension-force graphs). The gradient is the spring constant (k, N/m)
How does the appearance of the spring change when it has extended?
It is longer, length of coils increased, increased gaps between coils, spring has become thinner
When about to do the practical to investigate the link between force and extension, what can you do before to check the masses are a good size?
1) Us an identical spring to the on you will test, load it with 5 masses one at a time. Measure the extension each time you add another mass
2) Work out increase in the extension of the spring for each of the masses. If any have a bigger increase than the previous masses, you’ve passes the spring’s limit of proportionality.
If that happens, smaller masses will need to be used
Give the practical to find out the Link Between ̶W̶o̶ force and extension of a spring
1) Use a weigthed stand (so it doesn’t fall over), a clamp with a spring hanging off, a fixed in place ruler, a hanging mass with more masses ready to be added
2) Measure the natural length of the spring with no added mass with the clamped millimetre ruler. Take the reading at eye level, using a marker (e.g. tape) to make it more accurate
3) Add a mass to the spring and allow it to come to rest - stopping the oscillation (bouncing). Record the mass and measure the new length of the spring. The extension is the change in length
4) Repeat this process until you have enough measurements (no fewer than 6)
5) Plot a force-extension graph of the results. It will only start to curve if it exceeds the limit of proportionality, but it will be straight
How can you calculate the elastic potential energy/ the work done in stretching or compressing a spring? (only if the spring hasn’t gone over the limits of proportionality)
Elastic potential = 0.5 x spring constant x extension²
J = 0.5 x N/m x m²
This calculates the energy stored in a spring’s elastic potential energy store. Also, it’s the energy transferred to the spring as it’s deformed (or transferred by the spring as it returns to its original shape)
Distance is…
Displacement is…
Scalar - just how far an object has moved. There is no direction
Vector - it measures the ditance and direction in a straght line from the start to the end (even if the journey were to go through houses)
I travel 50m north, then 5m south. What is my
a) distance?
b) displacement?
a) 10m
b) 0m - you haven’t gone anywhere, it was the most pointless journey of your life unless you went to fetch some biscuits
Objects can travel at a constant speed with a changing velocity. True or false?
True. This happens when the object changes direction whilst staying at the same speed
How do you calculate the distance travelled with speed?
distance = speed x time
m = m/s x s
Because nothing ever truly stays at a constant speed easily, this is just an average
What are the typical speeds that these travel at?
a) A person walking
b) A person running
c) A person cycling
d) A car
e) A train
f) A plane
a) 1.5 m/s
b) 3 m/s
c) 6 m/s
d) 25 m/s
e) 30 m/s
f) 250 m/s
Wat factors may affect someone’s speed?
Their fitness, age, distance travelled, terrain, etc
What is the speed of sound in the air?
330 m/s, but this does vary on what the sound waves are travelling through
What can affect wind speed?
Temperature, atmospheric pressure, any large buildings/ structures
What is acceleration?
How quickly one is speeding up. It is the change in velocity in a certain amount of time
Give the formula for acceleration
Acceleration = change in velocity ÷ time
m/s² = m/s ÷ s
If something slows down, the change in velocity is negative, so there is a deceleration (a minus acceleration)
What symbol can be used to show an approximate answer?
~
What is the definition of uniform acceleration?
A constant acceleration
Give the formula for uniform acceleration
Final velocity² - initial velocity² = 2 x acceleration x distance
m/s - m/s = 2 x m/s² x m
Distance has a symbol of s for some reason, but I will use m
A van is travelling at 23m/s and starts to deccelerate uniformly at 2m/s² as it travelled 112m away. What speed will it be when it reaches its destination?
v² = u² + 2am v² = 23² + (2 x -2 x 112) v² = 81 v = 9m/s
A distance-time graph shows what? When can it be drawn?
Journeys when the object travels in a straight line. The gradient is the speed at that time.
What do these mean on a distance-time graph?
a) Flat sections
b) Straight, uphill sections
c) Gradient
d) Curves
e) A steepening curve
f) A levelling off curve
g) Tangent
a) It is stationary
b) Travelling at a steady speed
c) Speed - the steeper, the faster
d) Acceleration or deceleration
e) Speeding up
f) Slowing down
g) The gradient of it is a speed at that point on the curve
DISTANCE IS Y AXIS, TIME IS X AXIS
What do these mean on a velocity-time graph?
a) Flat sections
b) Straight, uphill sections
c) Gradient
d) Curves
e) A steepening graph
f) Downhill accelerations
g) The area under the graph
a) Travelling at a steady speed
b) Constant acceleration
c) Acceleration
d) Changing acceleration
e) A greater acceleration/ deceleration
f) Deceleration
g) The distance travelled in that time interval
VELOCITY IS Y AXIS, TIME IS X AXIS it will come back to the bottom of the velocity again, returning to the starting point
If an object has no force propelling it along, why will it always slow down?
Due to friction (unless it’s in a space with nothing to rub against.
Talk about friction
Always travels in the opposite direction to movement.
To travel at a steady speed, the driving force must balance the frictional forces
You can reduce friction between surfaces by using a lubricant
When does drag increase?
When speed increases
What is drag?
The resistance you get in a fluid (gas or a liquid).
This can be reduced by making the shape of an object more streamlined, where fluid can flow easily across it, reducing drag. Parachutes work in the opposite way
What happens with frictional forces from fluids?
They always increase with speed. A car has more friction to work against when travelling at 100mph compared to 30mph - the engine must work must harder to maintain a steady speed
What do objects travelling through fluids reach?
Terminal velocity - when an object (skydiver) first sets off, the force of gravity is much more than the frictional force slowing it down, so it accelerates. As the speed increase, the friction builds up. This gradually reduces the acceleration until the frictional force is equal to the accelerating force (0 resultant force)
It has reached a maximum speed/ terminal velocity, so will fall at a steady speed
What is terminal velocity?
Maximum speed - when the air resistance is large enough to equal the accelerating force I think
What does terminal velocity depend on?
Shape and area - the less streamlined an object is, the lower its terminal velocity
Objects with larger surface areas tend to have lower terminal velocities
If you dropped a marble and a beach ball off a high building, which would have a higher terminal velocity?
The marble, because there is more air resistance on the beach ball, so the beach ball spends less time accelerating (and doesn’t speed up as much) before the air resistance is large enough to equal the accelerating force
Give Newton’s 3 Laws of motion
1) A force is needed to change motion, or the object will remain stationary
2) Acceleration is proportional to the resultant force - force is equal to mass times acceleration
3) When 2 objects interact, the forces they exert on each other are equal and opposite - for every action, there is an equal and opposite reaction
Give Newton’s 1st law
If the resultant force on a stationary object is 0, the object will remain stationary. If the resultant force on an moving object is 0, it will have a constant velocity (same speed and direction)
What will a non-zero resultant force always produce?
Acceleration or deceleration in the direction of the force
Acceleration is directly proportion to what, and indirectly proportional to what?
Acceleration is directly proportional to force - the larger the resultant force on an object, the more the object accelerates
Acceleration is indirectly proportional to the mass of the object - an object with a larger mass will accelerate less than one with a smaller mass for a fixed resultant force
Give the formula that links resultant force, acceleration and mass, and what this is
Resultant force = acceleration x mass
N = m/s² x kg
Newton’s Second Law
What is inertia?
The tendency for motion to remain unchanged
Until acted upon by a resultant force, objects at rest will stay at rest and objects moving at a steady speed will stay that way. This tendency for Newton’s 1st Law to continue in the same motion is called inertia
What does an objects inertial mass measure?
How difficult it is to change the velocity of an object
How can you find the inertial mass?
Using Newton’s 2nd Law of F = ma, rearranging to get m = F ÷ a
SO the inertial mass is just the ratio of force over acceleration
Give Newton’s Third Law
When 2 objects interact, the forces they exert on each other are equal and opposite - every action has an equal and opposite reaction
Pushing anything causes a normal contact force to act equally in the opposite direction
Talk about how Newton’s 3rd Law works in an equilibrium situation (a man pushing against a wall as there’s nothing better to do in your spare time)
As the man pushes the wall, there is a normal contact force acting back upon him. These forces are the same size.
As the man applies a force and pushes the wall, the wall pushes back on him with an equal force
A book resting on the ground is in equilibrium as the weight of the book is equal to the normal contact force, causing no resultant force. So why is it not Newton’s Third Law?
Because the 2 forces are different types, and both are acting upon the book
Rearrange these instructions which show how mass and force affect acceleration - Newton’s 2nd Law:
1) The weight of the hook and any masses attached to it will provide the accelerating force, equal to the mass of the hook (m) x acceleration due to gravity
2) Place the trolley on the starting line, holding the hook so the string is taut, and release it
3) Set up the apparatus (light gate that the card will block twice, to register it to a computer)
You know what? this isn’t a required practical so read the book, page 213, if you REALLY want
What is an emergency stop?
WHen the maximum force is applied to the brakes in order to stop say a car in the shortest possible distance
Give the formula for stopping distance
Stopping distance = thinking distance + braking distance
Why are speed limits important?
Speed affects the stopping distance a lot - slower, easier to stop. More people - more dangerous - need to go slower
What are these affected by?
a) Thinking distance
b) Braking distance
a) Your speed - the faster you’re going, the further you’ll travel in the time you take to react
Your reaction time - the longer your reaction time, the longer your thinking distance
b) Your speed - for a given braking distance, the faster a vehicle travels, the longer it takes to stop
The weather/ terrain - if it’s wet or icy, or leaves or oil, there is less grip/ friction between the tyres and the road, which can cause skidding (so a longer distance)
The condition of the tyres - If they’re bald (no tread left), they can’t get rid of water in wet conditions. Then they skid on top of water
The brakes - If they’re worn or faulty, they won’t apply as much force as well-maintained brakes, which is dangerous
Complete the sentence:
The longer the stopping distance…
The more space you must leave in front in order to stop safely at any point
What does braking rely on?
The friction between the brakes and the wheels
How does a car brake?
When the brake pedal is pushed, it cause the brake pads to pressed onto the wheels. This contact causes friction, which causes work to be done. The work done between the brakes and the wheels transfers energy from the kinetic energy stores of the wheels to the thermal energy stores of the brakes (brakes get hotter)
A larger braking force means a larger deceleration. Why can this be dangerous?
A large deceleration can cause the brakes to overheat (becoming faulty), or cause the vehicle to skid
Give the typical braking distances for a car at these speeds:
a) 30mph
b) 60mph
c) 70mph
a) 14m
b) 55m
c) 75m
Give the practical you can use to measure reaction times
1) Sit with your arm resting at the edge of a table (stopping your arm from moving during the test). Get someone else to hold the ruler so it hangs between your thumb and forefinger, lined with the 0
2) Without giving any warning, the person holding the ruler drops it. Close your thumb and forefinger to try and catch it as quickly as possible
3) The measurement on the ruler at the point where it is caught is how far the ruler dropped in the time it took you to react
4) The longer the distance, the longer the reaction time
5) You can calculate how long the ruler falls for (the reaction time) because acceleration due to gravity is constant
6) You should do lots of repeats and calculate an average. The results are better if the ruler falls straight down
7) Fair test - same ruler for each repeat, same person dropping it
What factors affect reaction time?
Distractions, drugs, alcohol, tiredness
Give the formula for momentum
Momentum = mass x velocity (p=mv)
The greater the mass of an object, the greater its velocity, so…
The more momentum the object has
Momentum is a vector quantity (with size and direction). True or false?
True
In a closed system, the total momentum before an event (e.g. a collision) is equal to what?
The total momentum after the event.
What is conservation of momentum?
When in a closed system - the total momentum before = the total momentum after
In an explosion, the momentum before is 0. Why does the conservation of momentum still apply, even though there is more momentum afterwards - they are not equal?
The pieces fly in all directions - so they cancel out to make 0 - so it is equal
