# P2.3- Work, energy and momentum

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What happens to an object when work is done on it?
A force makes the object move.
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When a heavy object is lifted, 10 J of work is done on it. How much energy has been transferred to the object?
When a heavy object is lifted, 10 J of work is done on it. How much energy has been transferred to the object?
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In words, write down the equation used to calculate the work does by a force.
Work done = force applied × distance moved in the direction of the force
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How much work is done pushing a box 2 m across the ground with a force of 6 N?
Work done = force applied × distance moved ∴ 6 N × 2 m = 12 J
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Why do your hands become warm if you rub them together?
Muscles do work to overcome friction, which is transferred into energy that warms the hands.
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What happens to the kinetic energy of a vehicle when the brakes are applied?
It is transferred into heating the brake pads and the wheel discs as well as the surrounding air.
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What is gravitational potential energy?
The energy stored in an object due to its position in the Earth’s gravitational field
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If 500 J of work is done lifting an object, how much gravitational potential energy has the object gained?
500 J
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What happens to the gravitational energy of the object when it is put down again?
It decreases.
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What two things does the work done on an object moving up or down depend on?
1) The change of height 2) The weight of the object
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Write down the word equation for calculating the change of gravitational potential energy.
Change of gravitational potential energy = weight of the object × change of height
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Why can astronauts lift objects much more easily on the moon than they can on Earth?
Because the gravitational field strength on the surface of the moon is much weaker than on the surface of the Earth.
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Write the equation for change of gravitational potential energy
Change of gravitational potential energy = mass x gravitational field strength x change of height
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Rewrite this equation using symbols.
EP = m × g × h
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Calculate the gravitational potential energy gained by a ball of mass 0.2 kg when it is lifted 1 m into the air. The gravitational field strength on the surface of the Earth is 10 N/kg.
Gain of GPE = mass × gravitational field strength × change of height = 0.2 kg × 10 N/kg × 1 m = 2 J
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What is power?
The rate of transfer of energy
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What is the unit of power?
watt
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Write down a word equation that could be used to find power.
Power = energy ÷ time
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What two things does the kinetic energy of an object depend on?
1) The mass of the object 2) The speed of the object
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Complete the kinetic energy equation.
Kinetic energy = ½ × mass × speed²
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What unit is kinetic energy measured in?
joules, J
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What do the letters KERS stand for?
Kinetic energy recovery systems
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What are KERS used for?
Storing kinetic energy when cars brake so that it can be used later on
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What is elastic potential energy?
The energy stored when work is done on an object.
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When firing an arrow from a bow, what happens to the elastic energy stored in the bow?
It is transferred into kinetic energy.
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How is the momentum of a moving object calculated?
Momentum = mass × velocity
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What is the unit of momentum?
Kilogram metre/second (kg m/s)
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Which symbol is used to represent momentum?
p
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Calculate the momentum of a person of mass 60 kg walking at a velocity of 4 m/s.
Momentum = mass × velocity = 60 kg × 4 m/s = 240 kg m/s
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Write down the law of conservation of momentum.
In a closed system, the total momentum before an event equals the total momentum after the event.
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What happens to the momentum when a vehicle crashes into the back of a line of cars?
The momentum is transferred along the line of cars, to the car at the front.
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When a skateboarder jumps off his skateboard, the skateboard flies away from him. What can you say about the momentum of the skateboard?
It is equal and opposite to the skateboarder’s momentum.
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Why does the skateboard move away more quickly than the skateboarder?
Because the mass of a skateboard is lower than the person
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Does momentum have a direction?
Yes
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If the total momentum before an explosion is zero, what will the total momentum be after the explosion?
Zero
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Which scientific law does this follow?
Conservation of momentum
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What happens to the barrel of a gun when the gun is fired?
The barrel recoils backwards.
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How can this effect be minimised in artillery guns?
By using a spring to slow down the barrel
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What happens to the momentum of a car during a front-end impact?
It is reduced.
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How would a crumple zone at the front of the car reduce the force of an impact?
By increasing the impact time, which reduces the impact force
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Write down an equation used to calculate the impact force of a tennis ball from its mass and (negative) acceleration on impact with a racket.
Impact force = mass × acceleration
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If the deceleration of the ball is 100 m/s2 when it hits the racket and the mass of the ball is 0.1 kg, calculate the impact force on the racket.
Impact force = mass × acceleration = 0.1 kg × -100 m/s2 = -10 N
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When two vehicles collide, what happens to the total momentum?
It stays the same.
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That no external force acts
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If a heavy lorry crashes into the back of a light car, what happens to the car?
It gains momentum and accelerates forwards.
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Give three ways that seat belts help to protect people in accidents.
1) By stopping people from continuing forwards when a car suddenly stops 2) By increasing the impact time (reducing the decelerating force) 3) By spreading the force out across the chest
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What advantage do air bags have over seat belts?
They spread the force of an impact across the upper part of the body and so the force is lessened compared with a seat belt.
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Who does the child car seat law apply to?
Children up to 12 years old or up to 1.35 metres in height.
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To reduce deaths and serious injuries of children in cars
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Who is responsible for making sure that children in a car are seated safely?
The driver
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Why do car manufacturers always include seat belts in their cars?
Because they are required by law
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Why do car makers often choose not to include side impact bars in their cars?
Because they are not required by law and add to the cost of a new car
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## Other cards in this set

### Card 2

#### Front

When a heavy object is lifted, 10 J of work is done on it. How much energy has been transferred to the object?

#### Back

When a heavy object is lifted, 10 J of work is done on it. How much energy has been transferred to the object?

### Card 3

#### Front

In words, write down the equation used to calculate the work does by a force.

### Card 4

#### Front

How much work is done pushing a box 2 m across the ground with a force of 6 N?

### Card 5

#### Front

Why do your hands become warm if you rub them together?