Physics P2.3 : Work, Energy and Momentum

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  • Created by: TillyF
  • Created on: 28-12-16 07:37
What happens if the speed of a falling ball increases?
As the speed of a falling ball increases, the kinetic energy of the ball increases.
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What is a falling balls kinetic energy directly proportional to?
A ball's kinetic energy is directly proportional to the square of its speed.
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Give the equation to calculate kinetic energy?
kinetic energy = 0.5 x mass x speed^2 Kinetic energy in Joules/ mass in kilograms/ speed in metres per second squared (don't forget to square speed when calculating kinetic energy)
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What is a kinetic energy recovery system?
This is a store of energy in a vehicle for when the vehicle brakes, this energy from braking is then used later.
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What is elastic potential energy?
Elastic potential energy is the energy stores in an elastic object when work is done on it to change its shape.
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How can we tell if an object is elastic or not?
An object is elastic if it regains its shape after being stretched or squashed eg. a rubber band
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What do we say when an object is moved by a force?
We say work is done on the object by its force, the force therefore transfer's energy to the object.
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What is the amount of energy transferred to an object equal to?
The amount of energy transferred to an object = the work done to the object
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What does 'work done' by a force depend on?
The size of the force and the distance moved.
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What is the equation for work done?
work done (J) = force applied (N) x distance moved in the direction of the force (m)
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What is work done to overcome friction mainly transferred into?
Work done to overcome friction is mainly transferred into energy by heating.
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Explain energy transfers and work done when you rub your hands together?
When you rub your hands vigorously, they become warm. Your muscles do work to overcome the friction between your hands, the work you do is transferred into energy that warms up your hands.
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Why do brake pads become hot if the brakes are applied for too long a time?
Friction between the brake pads and the wheel discs opposes the motion of the wheel. The kinetic energy of the vehicle is transferred into energy that heats the brake pads and wheel disks as well as surrounding air,some will be transferred into sound
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What is gravitational potential energy?
Gravitational potential energy is the energy stored in an object because of its position in the Earth's gravitational field.
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What happens to the gravitational potential energy of an object when it is moved up?
When an object is moved up, its gravitational potential energy increases. The increase in gravitational potential energy is equal to work done on it by the lifting force.
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What happens to the gravitational potential energy of an object, when the object moves down?
When an object moves down, its gravitational potential energy decreases. The decrease of its gravitational potential energy is equal to the work done by the force of gravity acting on it.
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What does the work done on an object depend on when it moves up and down?
1. How far it is moved vertically (change in height) 2. The objects weight
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What is the equation for calculating the change in an objects gravitational potential energy?
the change in gravitational potential energy = weight x height (gravitational potential energy in Joules, weight measured in Newtons and change in height measured in metres)
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Why can astronauts on the moon lift objects more easily than they can on the earth?
On the moon the gravitational field strength is only about a sixth of the Earth's gravitational field strength, so there is less of a force of gravity acting on the object, therefore the objects weight is decreased and it feels lighter.
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What is the equation for calculating a change in an objects gravitational potential energy?
change of gravitational potential energy = mass x gravitational field strength x change of height
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Write the symbol equation for calculating the change in an objects gravitational potential energy?
Ep = m x g x h where Ep = change in GPE (Joules) m = mass (kg) g = gravitational field strength (N/kg) h = change in height (m)
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What is power?
Power is the rate of transfer of energy?
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What is the equation for power?
P = E/T where power is measured in watts
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How can we calculate the momentum of a moving object?
The momentum of a moving object = mass x velocity where momentum is measured in kg m/s mass in kg and velocity in m/s
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What does the law of conservation of momentum state?
In a closed system, the total momentum before an event is equal to the total momentum after that event.
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Is momentum conserved?
Yes, momentum is conserved in any collision or explosion providing no external forces act on the objects.
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Describe the direction and size of two objects recoiling in different directions?
Objects recoil with equal and opposite momentum when travelling and crashing in different directions.
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What does a '-' sign tell us after the equal sign in a momentum equation?
The minus sign after the equal sign tells us that the momentum of B is in the opposite direction to the momentum of A, in the equation (mass A x Velocity A) = - (Mass B x Velocity B) = A and B move apart with equal/ opposite momentum
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Is momentum after an explosion the same as before the explosion?
Total momentum after an explosion is the same as before the explosion. (mass A x velocity A) + (mass B x velocity B) = 0 hence why (mass A x velocity A) = - (mass B x velocity B)
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When two object push one another apart, what affects their speeds and what are there momentums?
When two objects push each other apart, they move with different speeds if they have unequal masses but they move with equal and opposite momentum so their total momentum is zero, according to the law of conservation of momentum.
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What is the function of a crumple zone on a car?
The effect of a crumple zone is to increase the impact time and therefore lessen the impact force.
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What is the result of a longer impact time?
The longer the impact time, the more the impact force is reduced
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How can we reduce the impact of a crash?
By increasing the impact time, as this would allow the object to travel a further distance before it stops and the momentum of the object would be lost over a longer time and its kinetic energy would be transferred over a greater distance.
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Why is impact force decreased with distance?
work done = force x distance, the impact force is therefore reduced because distance is increased.
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What car safety features increase impact time?
Crumple zones and side bars increase impact time and so reduce the impact force.
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How can we calculate impact time?
acceleration = change in velocity / time taken and force = mass x acceleration
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Describe the impact forces and momentum when two cars collide with one another?
When two vehicles collide, they exert equal and opposite impact forces, the change of momentum of one vehicle is equal and opposite to the change of momentum of the other vehicle. The total momentum is the same before and after the impact.
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Describe what happens to the momentum of a truck and car when a truck runs into the back of a stationary car?
The impact decelerated the truck and accelerates the car, assuming the trucks mass is greater than the mass of the car, the truck loses momentum and the truck gains momentum. So the momentum is conserved and the total momentum is unchanged.
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What happens when two objects collide?
- they exert equal and opposite forces on each other - their total momentum is unchanged and conserved
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How does a seatbelt increase the safety of a person in a car?
-time taken to stop someone in a car is longer, decelerating force is reduced by wearing a seatbelt -seatbelt acts across the chest so the force spreads out, if there was no seat belt the force would act on the head when it hits the windscreen.
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How do air bags reduce the impact of a collision for a passenger?
-they spread the force of an impact across the upper part of the body -increases duration of the impact time, so the effect of the force is decreased
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A ball's kinetic energy is directly proportional to the square of its speed.

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Card 4

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Card 5

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