Work done and energy transfer

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Notes de leçon

What is Work Done?

Work is done when a force causes an object to move over a distance.
The force must act in the direction of the object's displacement for work to be done.
If a force is applied but the object does not move, no work is done.
Work is done on a ball when it is lifted to a height above the ground.
Air resistance (drag) does work against a bird as it flies through the air.

Work is done when a force is used to move an object over a distance

Calculating Work Done

The equation for work done is: W = F × s
W = work done in joules (J) or newton-metres (N m)
F = force in newtons (N)
s = distance moved in the direction of the force, in metres (m)
Example: A force of 500 N moves an object 23 m, so work done = 500 × 23 = 11 500 J.

Units for Work and Energy

Work is measured in joules (J) or newton-metres (N m).
1 J = 1 N m – the number of joules equals the number of newton-metres.
One joule is the work done by a force of one newton acting through one metre.
Conversions are straightforward: 1000 J = 1000 N m.
Always include the correct units in your answer – you may use either J or N m.

Work Done and Energy Transfer

Whenever work is done, energy is transferred from one store to another.
Mechanical working (or electrical working) is an energy transfer pathway.
The amount of energy transferred (in joules) is equal to the work done (also in joules): energy transferred = work done.
If a force acts in the direction of motion, the object gains energy (transferred to its kinetic store).
If a force acts opposite to the motion, the object loses energy (transferred to the thermal store of the object and surroundings).

Work Done and Gravitational Potential Energy

When an object is lifted, work is done against gravity and energy is transferred to its gravitational potential store.
The work done in lifting an object equals m × g × h (mass × gravitational field strength × height).
This is the same as the equation for gravitational potential energy: E = m g h.
Example: Lifting a 10 kg bucket 15 m with g = 9.8 N/kg gives work done = 10 × 9.8 × 15 = 1470 J.
The bucket gains 1470 J of gravitational potential energy.

Gravitational potential energy of a lifted mass

Work Done and Friction

Friction is a force that opposes the motion of an object, slowing it down.
When friction is present, energy is transferred by heating – raising the temperature of the object and its surroundings.
The work done against frictional forces causes this temperature rise.
Imperfections at the interface between surfaces bump and rub against each other, transferring energy to thermal stores.
Air resistance is a type of friction that slows objects moving through air; particles bump into the object, transferring energy by heating.

Work Done and Energy Transfer in Context

A car braking: the brakes apply a force opposite to motion, doing work and transferring kinetic energy to thermal energy in the brakes.
A sledge sliding down a slope: friction does work against the sledge, transferring energy to thermal stores.
When a force moves an object, the energy transferred equals the work done – this links work, force, distance and energy.
Changes in speed relate to kinetic energy; changes in height relate to gravitational potential energy; changes in shape relate to elastic potential energy.

Key Points to Remember

Work done = force × distance moved in the direction of the force.
No movement means no work done, even if a force is applied.
Use the distance along the line of action of the force, not total distance.
Work done and energy transferred are equal and both measured in joules.
Work done against friction is not destroyed – it is transferred to thermal stores.

Diapos

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1.What are the correct units for work done?
Easy
- AN
- Bm
- CN/m
- DN m
2.Which equation correctly links work done (W), force (F), and distance moved (s)?
Easy
- AW = F s
- BW = F / s
- CW = F − s
- DW = s / F
3.Which of the following expressions is correct for the relationship between work done and energy?
Easy
- AWork done = energy transferred
- BWork done = energy transferred × distance
- CWork done = energy transferred + distance
- DWork done = energy transferred ÷ time
4.Which of the following statements about friction are correct? (select all that apply)
Medium
- AFriction always acts against motion
- BEnergy is transferred by heating
- CFriction is not a force
- DThe units for friction are watts
- EAir resistance is a type of friction
5.Work is done when a force is applied to an object but the object does not move.
Easy
True or false?
6.Match each quantity to its correct unit.
Medium
- Work done
- Force
- Distance
- Joule
- Newton
- Metre
7.Which of the following is the correct conversion between joules and newton-metres?
Easy
- A1 J = 1 N m
- B1 J = 1 N/m
- C1 J = 1 N/m²
- D1 J = 1 N m²
8.Which of the following are examples of work being done? (select all that apply)
Medium
- AA ball is lifted to a height above the ground.
- BA bird flies through the air, experiencing air resistance.
- CA person holds a heavy box stationary.
- DA car applies brakes and comes to a stop.
- EA force is applied to a wall but it does not move.

Plus de questions

9.Which of the following statements about work done and energy transfer are correct? (select all that apply)
Medium
- AWork done = energy transferred
- BEnergy transferred is measured in joules.
- CWork done is measured in newtons.
- D1 J = 1 N m
- EWork done and energy transferred are unrelated.
10.Place the steps for calculating work done in the correct order.
Medium
- List the known quantities: force and distance.
- Write down the equation W = F × s.
- Substitute the values into the equation.
- Calculate the work done and include the correct unit (J or N m).
11.A car is braking to a stop. Order the sequence of energy transfers that occur.
Hard
- The car has kinetic energy due to its motion.
- The brakes apply a force opposite to the car's motion.
- Work is done by the brakes against the car's kinetic energy.
- Energy is transferred to the thermal store of the brakes and surroundings.
12.One joule of work is done when a force of one newton causes a displacement of one metre.
Easy
True or false?
13.A crate is dragged 8 m along the floor by a rope pulling at 30° to the horizontal. Which distance should be used in W = F × s to find the work done by the rope?
Medium
- AThe vertical height the rope is raised
- BThe component of the 8 m along the line of action of the force
- CThe total 8 m of rope length
- DThe 8 m divided by the angle
14.A student holds a heavy box stationary at arm's length for two minutes. Which statement is correct?
Medium
- AWork is done because the student's arms are exerting a force
- BWork is done because the box has weight
- CNo work is done on the box because it does not move
- DWork is done because time passes while the force acts
15.A 1500 kg car travelling at 15 m/s is brought to rest by brakes applying 12 000 N. What is the braking distance?
Medium
- A14 m
- B9.4 m
- C18.8 m
- D22.5 m
16.A builder pushes a 50 kg crate up a slope, gaining 2450 J of gravitational potential energy while pushing with 200 N over 15 m. Which statements are correct? (select all that apply)
Medium
- AThe work done by the builder is 3000 J
- BThe crate gains 2450 J of gravitational potential energy
- CThe work done by the builder equals the gravitational potential energy gained
- DSome of the builder's work is transferred to thermal stores by friction
- EThe work done by the builder is less than the gravitational potential energy gained
17.When a force acts in the opposite direction to an object's motion, the object loses energy from its kinetic store.
Easy
True or false?
18.Which statement best explains why the downward-sloping exit track improves efficiency?
Hard
- AThe motor does more work to move the train away
- BThe train's gravitational potential energy is transferred to its kinetic store, reducing the motor's work
- CFriction does more work on the train
- DThe brakes transfer more energy to the thermal store
19.Match each change to the energy store that is affected.
Medium
- A change in speed
- A change in height
- A change in the shape of a material
- Kinetic store
- Gravitational potential store
- Elastic potential store
20.Place the stages of a sledge descending a slope in the correct order.
Medium
- The sledge accelerates down the slope
- Friction does work against the sledge, transferring energy to thermal stores
- The sledge reaches a constant speed
- The gravitational potential energy of the sledge decreases
21.A rocket's return module becomes very hot as it passes through the atmosphere. Which statement explains this?
Medium
- AAir resistance does work on the module, transferring energy to its thermal store
- BThe module's kinetic energy is destroyed by friction
- CFriction creates new energy as heat
- DThe module's gravitational potential energy is converted directly to light
22.Which of the following are correct units for work done? (select all that apply)
Medium
- AJoule (J)
- BNewton-metre (N m)
- CNewton (N)
- DWatt (W)
- ENewton per metre (N/m)
23.1 joule is equal to 1 newton-metre.
Easy
True or false?
24.A penny of mass 2.5 g falls 158 m and 0.967 J of work is done against air resistance. Which expression gives the work done against air resistance?
Hard
- A½mv²
- Bmgh + ½mv²
- Cmgh − ½mv²
- Dmgh
25.Which equation correctly links the change in gravitational potential energy (E) to mass (m), gravitational field strength (g) and change in height (h)?
Medium
- AE = m g h
- BE = m g / h
- CE = m h / g
- DE = g h / m
26.A student is investigating work done against friction by pulling a wooden block across a rough surface. Which of the following would be appropriate to measure directly in order to calculate the work done against friction? (select all that apply)
Hard
- AThe force needed to pull the block at constant speed
- BThe distance the block is pulled
- CThe mass of the block
- DThe temperature of the block before and after
- EThe time taken to pull the block
27.Match each experimental measurement to the quantity it is used to calculate when investigating work done.
Medium
- Force applied to move an object
- Distance moved in the direction of the force
- Mass lifted and height raised
- Work done by the force
- Work done against gravity
- Work done against friction
28.Place the following steps in the correct order to determine the work done against air resistance when a penny falls from a tower.
Medium
- Measure the mass of the penny and the height of the tower
- Calculate the gravitational potential energy lost by the penny using m × g × h
- Measure the speed of the penny just before it hits the ground
- Calculate the kinetic energy of the penny using ½mv²
- Subtract the kinetic energy from the gravitational potential energy to find work done against air resistance
29.A car of mass 1500 kg is travelling at 15 m/s. The brakes apply a force of 12 000 N and bring the car to rest. Calculate the braking distance. Give your answer to 2 significant figures.
Hard
- A14 m
- B18.75 m
- C9.4 m
- D37.5 m
30.A student is investigating the energy transfers when a sledge descends a slope. The sledge starts from rest and accelerates, but later travels at constant speed. Which statement best explains why the sledge eventually travels at constant speed?
Challenge
- AThe component of weight down the slope becomes zero
- BThe frictional force becomes equal and opposite to the component of weight down the slope
- CThe kinetic energy becomes zero
- DThe gravitational potential energy becomes zero
31.Which of the following is the correct unit for work done?
Easy
- Anewton (N)
- Bjoule (J)
- Cwatt (W)
- Dmetre per second (m/s)
32.Work is done whenever a force causes an object to move in the direction of the force.
Easy
True or false?
33.Which of the following statements about work done are correct? (select all that apply)
Medium
- AWork is done when a force moves an object in the direction of the force.
- BWork is done when a force is applied but the object does not move.
- CThe amount of work done depends on the size of the force and the distance moved.
- DWork done is measured in newtons.
- EWork done is equal to the energy transferred.
34.Match each term to its correct definition.
Medium
- Work done
- Force
- Distance
- Friction
- A push or pull that can change an object's motion
- The energy transferred when a force moves an object
- A force that opposes motion between surfaces
- How far an object moves in the direction of the force
35.A student pushes a box with a force of 50 N for a distance of 4 m. How much work is done on the box?
Medium
- A12.5 J
- B54 J
- C200 J
- D2000 J
36.One joule is equal to one newton-metre.
Easy
True or false?
37.Which of the following is the correct rearrangement of the work done equation to find distance?
Medium
- As = W × F
- Bs = W / F
- Cs = F / W
- Ds = W + F
38.Which of the following statements about work done against friction are correct? (select all that apply)
Medium
- AFriction always acts in the same direction as the motion.
- BWork done against friction causes a rise in temperature.
- CEnergy is transferred to the thermal store of the object and its surroundings.
- DFriction is measured in watts.
- EAir resistance is a type of friction.
39.Which of the following statements about the energy transfer when work is done are correct? (select all that apply)
Medium
- AMechanical working is an energy transfer pathway
- BThe energy transferred in joules equals the work done in joules
- CWork done against friction transfers energy to the thermal store of the object and surroundings
- DEnergy is destroyed when work is done against friction
- EWork done by a force acting in the direction of motion transfers energy to the kinetic store of the object
40.A student pushes a heavy box across a rough floor at a constant speed. Which of the following statements are correct? (select all that apply)
Medium
- AThe student does work on the box
- BFriction does work against the motion of the box
- CEnergy is transferred to the thermal store of the floor and box
- DThe box gains kinetic energy as it moves
- ENo work is done because the speed is constant
41.A person does work by lifting their shopping bag 10 cm off the ground. The bag has a weight of 60 N. Determine the work done by the person.
Easy
42.A father pushes his daughter on a sled across the snow on flat ground. He pushes with a force of 400 N for a distance of 20 m. Calculate the energy transferred.
Medium
43.Complete the sentence about the unit of work.
Easy
One joule of work is done when a force of one ____ causes a displacement of one ____.
44.What is the name of the force that opposes the motion of an object moving through the air?
Medium
45.A mass of 200 g is lifted 90 cm by a motor. Taking g = 9.8 N/kg, calculate the work done by the motor in joules.
Medium
46.A girl and sledge of combined mass 50 kg descend a slope of vertical height 25 m while travelling 200 m at constant speed. Taking g = 9.8 N/kg, calculate the average frictional force acting on the sledge in newtons.
Hard
47.An electric motor with efficiency 80% wastes 100 J of energy while lifting a load. Calculate the useful work done by the motor in joules.
Medium
48.Complete the statements about work done and energy.
Medium
Work done is measured in ____, and the energy transferred is equal to the ____ done.
49.A sledge accelerates down a smooth slope at first but later travels at constant speed. Give the name of the force that balances the component of weight down the slope once the speed is constant.
Medium
50.A car of mass 1500 kg travelling at 15 m/s is stopped by brakes. Calculate the kinetic energy of the car just before the brakes are applied, in joules.
Medium
51.Complete the sentence about work done against friction.
Easy
When friction is present, energy is transferred by ____ and the temperature of the object and its surroundings ____.
52.Give the name of the energy transfer pathway involved when work is done by a force moving an object.
Medium
53.A student rubs their hands together on a cold day. The friction between the surfaces of the student's hands causes them to get warmer. Describe the energy transfers taking place as the student rubs their hands together.
Medium
54.A penny falls from the top of a 158 m tower. Air resistance transfers some of the penny's energy to the thermal store of the air. Explain how you would determine the work done against air resistance and hence the speed of the penny just before it hits the ground.
Hard
55.Evaluate how the design of the station increases the efficiency of the trains. Use ideas about work done in your answer.
Hard
56.A builder pushes a crate up a slope with a force of 200 N over 15 m, but the crate only gains 2450 J of gravitational potential energy. Explain why the work done by the builder is greater than the gain in gravitational potential energy.
Hard
57.A penny falls from the top of a 158 m tower. Air resistance transfers some of the penny's energy to the thermal store of the air. Explain how you would determine the work done against air resistance.
Hard
58.A person does work by lifting their shopping bag 10 cm off the ground. The bag has a weight of 60 N. Determine the work done by the person.
Easy
- A8 N m
- B6 N m
- C7 N m
- D5 N m
59.A father pushes his daughter on a sled across the snow on flat ground. He pushes with a force of 400 N for a distance of 20 m. Calculate the energy transferred.
Medium
- A8800 J
- B7200 J
- C9600 J
- D8000 J
60.A mass of 200 g is lifted 90 cm by a motor. Taking g = 9.8 N/kg, calculate the work done by the motor in joules.
Medium
- A3.76 J
- B1.76 J
- C2.76 J
- D0.76 J
61.A girl and sledge of combined mass 50 kg descend a slope of vertical height 25 m while travelling 200 m at constant speed. Taking g = 9.8 N/kg, calculate the average frictional force acting on the sledge in newtons.
Hard
- A73.51 N
- B61.25 N
- C67.38 N
- D55.12 N
62.An electric motor with efficiency 80% wastes 100 J of energy while lifting a load. Calculate the useful work done by the motor in joules.
Medium
- A480 J
- B440 J
- C400 J
- D360 J
63.A car of mass 1500 kg travelling at 15 m/s is stopped by brakes. Calculate the kinetic energy of the car just before the brakes are applied, in joules.
Medium
- A151875 J
- B168750 J
- C202500 J
- D185625 J

Cartes mémo

When is work done on an object?

Work is done when a force moves an object over a distance, with the force acting in the direction of the object's displacement.

What happens if a force is applied to an object but the object does not move?

No work is done, because there is no displacement in the direction of the force.

State the equation linking work done, force and distance.

W = F × s, where W is work done, F is force and s is distance moved along the line of action of the force.

What are the units of work done?

Joules (J) or newton-metres (N m).

What are the units of force and distance in the work done equation?

Force is in newtons (N) and distance is in metres (m).

How many joules are equal to one newton-metre?

1 J = 1 N m, so the number of joules equals the number of newton-metres.

Define one joule of work.

One joule of work is done when a force of one newton causes a displacement of one metre.

How is work done related to energy transferred?

Whenever work is done, energy is transferred from one store to another, and the energy transferred in joules equals the work done in joules.

What is the energy transfer pathway when work is done mechanically?

Mechanical working (or electrical working) is the energy transfer pathway.

What happens to an object's energy when the force acts in the direction of motion?

The object gains energy — energy is transferred to its kinetic store.

What happens to energy when the force acts in the opposite direction to the motion?

The object loses energy — energy is transferred away to the thermal store of the object and its surroundings.

A car's brakes apply a force of 500 N and stop it in 23 m. Calculate the work done by the brakes.

Work = F × s = 500 × 23 = 11 500 J.

What is friction?

Friction is a force that works in opposition to the motion of an object, slowing it down.

How does friction transfer energy?

Work done against frictional forces transfers energy by heating, raising the temperature of the object and its surroundings.

Why does rubbing your hands together warm them up?

Work is done against friction between the hands, transferring energy by heating to the thermal store of the hands and surroundings.

What is air resistance?

Air resistance is a type of friction that slows an object moving through the air, transferring energy by heating as work is done against it.

Why does a rocket's return module heat up as it travels through the atmosphere?

Work is done by air resistance over the distance travelled, transferring energy to the thermal store of the module.

What does a change in an object's speed relate to?

A change in speed is related to kinetic energy.

What does a change in an object's height relate to?

A change in height is related to gravitational potential energy.

What does a change in the shape of a material relate to?

A change in shape is related to elastic potential energy.

A 10 kg bucket is pulled up a 15 m well. What equation gives the work done against its weight?

Work = m × g × h, which is the same as the gravitational potential energy equation.

A 10 kg bucket is raised 15 m (g = 9.8 N/kg). Calculate the energy transferred.

Work = 10 × 9.8 × 15 = 1470 N m, so 1470 J of energy is transferred to the bucket's gravitational potential store.

Why is no work done when carrying an object horizontally at constant speed?

The upward force supporting the object acts perpendicular to the horizontal motion, so there is no displacement along the line of action of that force.

What is the difference between work and power?

Work is the energy transferred when a force moves an object a distance; power is the rate at which work is done (work done ÷ time).

What is the common mistake when calculating work done along a slope?

Using the total distance travelled instead of the distance moved in the direction of the force.

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