4.1 Flashcards
Examples of scalars?
- density
- charge
- resistance
- work done
- energy
two forces act on an object, when is equilibrium achieved?
When the two forces are equal and opposite
What is the centre of gravity of an object?
The point through which the entire weight of the object may be considered to act
Word equation to calculate the moment of a force?
Force x Perpendicular distance from the line of action of the force to the pivot
What is the principle of moments?
For any object that is in equilibrium, the sum of the clockwise moments about a point is equal to the sum of the anticlockwise moments about that same point
What is a couple?
A pair of equal and opposite forces, not acting in the same straight line
What is the turning effect that couples create called?
A torque
How to calculate the torque of a couple?
torque = one of the forces x perpendicular distance between them
What are the two conditions for equilibrium?
- there is no net (resultant force)
2. there is no turning effect (moment) about any point
What is the centre of mass of a body?
The point on an object where the mass may thought to be concentrated
When an object is in equilibrium, what is true of the horizontal forces?
They are equal in magnitude and opposite in direction
What does velocity measure?
The rate of change of displacement
What does acceleration measure?
The rate of change of velocity
What is the gradient in a distance-time graph?
Speed
What is the gradient in a displacement-time graph?
Velocity
What is the gradient in a velocity-time graph?
Acceleration
What is the unit for momentum?
kgms⁻¹ or Ns
What do objects have when stationary, in terms of momentum and inertia?
They have no momentum, but still have inertia
What is inertia?
The resistance to change velocity
What is the principle of conservation of energy?
Energy can be transferred from one form to another, but it cannot be created or destroyed
What is the definition of 1J
The work done when a force of 1N moves through a distance of 1m (in the direction of the positive force)
What is power?
The rate at which work is done
What can work done also be referred to as?
Energy transferred
What is the area underneath a force-time graph?
Impulse (kgms⁻¹)
What are coplanar forces?
Forces that are all in the same plane
What is a moment?
The turning effect of a force around a poin
What is the unit for moments?
Nm
What is a couple?
- A pair of coplanar forces of equal size that act parallel to each other but in opposite directions.
- Produce a turning effect.
How can the centre of mass of an irregular object be found?
- Hang object from a point
- Draw vertical line downwards from point (using a plumb bob as guidance)
- Repeat with different point
- Centre of mass is where the lines cross
When will an object topple and why?
- When the vertical line from its centre of mass falls outside of the base area.
- Because the centre of mass causes a resultant moment around the pivot.
Describe the displacement-time graph for an accelerating object.
- Curved graph
* If acceleration is constant, the rate of change of the gradient is constant.
How is uniform acceleration shown on a velocity-time graph?
Straight line
How is a changing acceleration shown on a velocity-time graph?
Curved line
What piece of equipment can be used in motion experiments?
Ultrasound position detector
How does an ultrasound position detector work?
- Records distance of object from the sensor several times a second
- Connected to computer with graphing software to get graph
What are some advantages of data-loggers over traditional methods of recording data?
1) Data is more accurate - don’t have to allow for human reaction times.
2) Higher sampling rate than humans (for example, ultrasound position detectors can take a reading ten times every second)
3) Data displayed in real time
Describe an experiment to calculate g.
1) Set up a circuit with a switch that controls two parallel circuits: one with an electromagnet and ball, the other with a timer and trapdoor
2) Measure the height from the bottom of the bearing to the trapdoor.
3) Flick the switch to start the timer and release the bearing.
4) Bearing falls, hits trapdoor and stops the timer. Record the time.
5) Repeat 3 times at this height and average the time.
6) Repeat at various heights.
7) Plot a graph of height (m) against time taken squares (s²).
8) a = 2 x Gradient
(See diagram pg 52)
In the experiment to calculate g, how is error reduced?
- Bearing is small and heavy -> Means air resistance is negligible
- Computer releasing and timing fall -> Reduces uncertainty
In the experiment to calculate g, what is the biggest source of error?
RANDOM error: The measurement of h (using a ruler = uncertainty of + or - 1mm).
Describe two experiments to find g.
1) Card drop through a light gate.
2) Ball bearing with timer
What does a v-t graph look like for terminal velocity?
What does an a-t graph look like for terminal velocity?
Describe how and why the speed of a skydiver changes as he falls.
- Skydiver accelerates until air resistance equals weight.
- Travelling at terminal speed until parachute opens.
- Air resistance is now bigger than his weight.
- This slows him down until his speed has dropped so that the air resistance is equal to the weight again.
- This is the new terminal speed.
Describe the velocity-time graph of a falling skydiver.
- Velocity increases at a decreasing rate
- Until terminal speed (flat part of graph)
- Sharp drop in velocity when parachute opens
- New terminal speed (lower flat part of graph)
What is momentum?
The tendency of an object to keep moving in the same direction.
The total momentum before a collision is…
…equal to the total momentum after the collision.
Is momentum conserved in a system?
- Yes, assuming no external forces act.
What is an elastic collision?
- One where kinetic energy is conserved (as well as momentum).
- No energy is dissipated as heat, sound, etc.
What is an inelastic collision?
- One where kinetic energy is not conserved (but momentum is conserved).
- Some energy is dissipated as heat, sound, etc.
Unit for impulse
Ns
Give some ways in which vehicles have safety features to reduce the force of an impact.
- Crumple zones -> Crumple on impact and increase time of crash
- Seat belts -> Stretch slightly to increase stopping time
- Air bags -> Slow passengers down more slowly and prevent them from hitting hard surfaces
When work is done…
…energy is transferred.
What is work?
The amount of energy transferred from one form to another.
What is the equation that relates work done, force and distance?
Work done (J) = Force (N) x Distance (m)
W = F x d
Define a watt.
The rate of energy transfer equal to 1 joule per second.
When will the principle of conservation of energy come up?
When doing questions about changes between kinetic and potential energy.
State the principle of conservation of energy.
- Energy cannot be created or destroyed.
*Energy can be transferred from one form or another, but the total amount of energy in a closed system will not change.
What are two types of potential energy?
- Gravitational
- Elastic
What is the equation for GPE?
ΔGPE (J) = Mass (kg) x G.F.S. (N/kg) x ΔHeight (m)
ΔE = m x g x Δh
Describe the energy transfers when a ball is thrown vertically up.
- When the ball goes up, the kinetic energy is converted into GPE
- When the ball comes down, the GPE is converted back into kinetic energy
Describe the energy transfers when a person slides down a slide.
The gravitational potential energy is converted into kinetic energy.
Describe the energy transfers when a person jumps on a trampoline.
- When bouncing up, Elastic energy -> Kinetic energy -> GPE
- When coming down, GPE -> Kinetic energy -> Elastic energy
Projectile
Travels in a parabola through the air with no horizontal driving force. Vertical motion is independent of horizontal motion. Acceleration is vertically downwards.
Newton’s 1st Law
Until acted on by a resultant force, objects at rest stay at rest and objects moving at constant velocity will stay moving at that velocity. This is called inertia
Newton’s 2nd Law
Acceleration is proportional to the resultant force. The larger the resultant force the more the object accelerates.
Terminal velocity
Maximum, constant velocity. Air resistance and weight balance.
Newton’s 3rd Law
When two objects interact, the forces they exert on each other are equal and opposite. Action and reaction are equal and opposite
