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GCSE Physics · GCSEChapter 2 · Efficiency, power and energy calculations
GCSE · Chapter 2

Efficiency, power and energy calculations

By the end of this chapter you will be able to

  • Calculate power from energy and time, and energy from power and time
  • Calculate efficiency as a percentage, and explain why no machine is 100% efficient
  • Calculate gravitational potential energy and rearrange the equation
  • Calculate kinetic energy and explain why it depends on speed squared

You should already know

  • Energy stores and wasted energy (Chapter 1)
  • Rearranging equations and using percentages (KS3 Maths)

Where this leads

  • Specific heat capacity (Chapter 3)
  • Work done and energy (Chapter 15)
  • Equations of motion (Chapter 16)

GCSE Physics. Content follows the GCSE (9-1) subject content for England (Edexcel and AQA). Content references: Calculate efficiency as useful output energy divided by total input energy, as a fraction or percentage, Calculate power as energy transferred divided by time, in watts and kilowatts, Calculate gravitational potential energy using E = mgh, Calculate kinetic energy using E = 1/2 mv squared, Use these equations in context, such as falling objects, braking and appliance ratings.

Exercise P  Do you remember?

1
FWhat is 3 x 40? Write the number.
2
FHow many seconds are there in one hour? Write the number.
3
FHow many grams are there in 5 kg? Write the number.
4
FWrite 0.25 as a percentage. Write the number.
5
FWhat is the square root of 49? Write the number.
2.1

Energy and power

GCSE
Calculate power as energy transferred divided by time, in watts and kilowatts
Video lesson: Energy and powerComing soon
Context. Holly is comparing two hairdryers in a shop. The labels say 1500 W and 1800 W. “What does W mean?” she asks. The number tells you how quickly the hairdryer transfers energy. The bigger the power, the faster the energy goes.

Power is the rate at which energy is transferred, or the energy transferred each second. Its unit is the watt (W). One watt means one joule of energy transferred each second. A kilowatt is 1000 watts. The equation is:

power (W) = energy transferred (J) / time (s), or P = E / t

QuantitySymbolUnit
energy transferredEjoule (J)
timetsecond (s)
powerPwatt (W)
massmkilogram (kg)
heighthmetre (m)
speedvmetre per second (m/s)
gravitational field strengthgnewton per kilogram (N/kg)
Symbols and units used in this chapter
AppliancePowerEnergy each secondEnergy in 60 s
LED bulb10 W10 J each second600 J
laptop60 W60 J each second3600 J
hairdryer1500 W1500 J each second90000 J
kettle2200 W2200 J each second132000 J
Each appliance transfers its power each second. Energy in 60 seconds is power x 60

Key ideas

  • Power P = E / t. Energy E = P x t. Time t = E / P.
  • 1 kW = 1000 W. Convert kilowatts to watts before you calculate.
  • 1 W = 1 J/s.
  • Energy is in joules, time in seconds, power in watts.
Worked example 2.1.1
A kettle has a power of 2200 W. How much energy does it transfer in 60 seconds? Write the number of joules.

Method. Use E = P x t.

  1. Take the numbers. Power P = 2200 W and time t = 60 s.
    P = 2200 W, t = 60 s
  2. Multiply. Energy = power x time = 2200 x 60.
    2200 x 60 = 132000
  3. Write the answer.
    132000 J

Check. 132000 / 60 = 2200 W, so the calculation is right. ✓

Worked example 2.1.2
A motor transfers 4500 J of energy in 30 seconds. What is its power? Write the number of watts.

Method. Use P = E / t.

  1. Take the numbers. Energy E = 4500 J and time t = 30 s.
    E = 4500 J, t = 30 s
  2. Divide. Power = 4500 / 30.
    4500 / 30 = 150
  3. Write the answer.
    150 W

Check. 150 W x 30 s = 4500 J. ✓

Worked example 2.1.3
A 60 W lamp is on for 50 seconds. How much energy does it transfer? Write the number of joules.

Method. Use E = P x t.

  1. Take the numbers. Power = 60 W and time = 50 s.
    P = 60 W, t = 50 s
  2. Multiply. Energy = 60 x 50 = 3000.
    60 x 50 = 3000
  3. Write the answer.
    3000 J

Check. 3000 / 50 = 60 W. ✓

Watch out

Watch out: convert the units first. 2 kW is 2000 W, and 2 minutes is 120 s. Wrong units are the most common mistake in power questions.

Exam tip

Exam tip. Write the equation, substitute the numbers with units, then calculate. Give the unit in your answer.

Exercise 2A  Power and energy transferred

1
FA device transfers 600 J of energy in 20 seconds. What is its power? Write the number of watts.
2
FA heater has a power of 1500 W and is on for 4 seconds. How much energy does it transfer? Write the number of joules.
3
FA device transfers 3600 J of energy in 60 seconds. What is its power? Write the number of watts.
4
FA device has a power of 30 W. How long does it take to transfer 1800 J? Write the number of seconds.
5
FConvert 2 kW into watts. Write the number of watts.
6
FConvert 0.5 kW into watts. Write the number of watts.
7
FA kettle has a power of 2200 W. How much energy does it transfer in 3 minutes? Write the number of joules.
8
FWhich appliance in the table has the largest power? Write the name.
9
PSA lamp transfers 5000 J in 10 seconds. A second lamp transfers 5000 J in 25 seconds. Which lamp has the larger power? Write the number 1 or 2.
10
PSA 100 W lamp is on for 10 seconds. A 40 W lamp is on for 30 seconds. Which transfers more energy? Write the number 1 or 2 (1 for the 100 W lamp).
11
PSA machine transfers 25 J every second. What is its power? Write the number of watts.
12
PSTwo kettles, each rated at 2200 W, are run for 60 seconds. How much energy does one kettle transfer? Write the number of joules.
13
RExplain what it means that a lamp has a power of 1 W, in one sentence. Accept any sentence that says 1 joule of energy is transferred each second.
14
PSA heater has a power of 2 kW. Calculate the energy it transfers in 1 minute. Write the number of joules.
15
RA pupil says “a 2000 W heater uses 2000 J of energy every minute.” Explain whether this is correct. Accept any answer that says the heater transfers 2000 J each second, which is 120000 J each minute.
2.2

Efficiency

GCSE
Calculate efficiency as useful output energy divided by total input energy, as a fraction or percentage, Explain why no machine is 100% efficient
Video lesson: EfficiencyComing soon
Context. Leo is building a model crane that lifts a small load. The motor takes in a lot of energy, but only some of it lifts the load. “How good is the crane?” he asks. We use efficiency to answer this question.

Efficiency compares the useful energy you get out of a device with the total energy you put in. It can be given as a fraction, a decimal or a percentage. A machine is never 100% efficient, because some energy is always dissipated to the surroundings.

efficiency = useful energy output / total energy input

A motor: 500 J in, 400 J useful (example)input 500 Juseful: lifts the load: 400 Jwasted as heat and sound: 100 J
A motor with 500 J in and 400 J useful: the efficiency is 400 / 500

Key ideas

  • Efficiency = useful output energy / total input energy.
  • Multiply by 100 to give a percentage.
  • Efficiency is never more than 1 (or 100%).
  • Efficiency can also be found from powers: efficiency = useful power output / power input.
Worked example 2.2.1
A motor has 500 J of energy in and 400 J of useful output. Find its efficiency as a percentage. Write the number of percent.

Method. Use efficiency = useful output / total input, then multiply by 100.

  1. Divide. Efficiency = 400 / 500 = 0.8.
    400 / 500 = 0.8
  2. Percentage. Multiply by 100 to give a percentage: 0.8 x 100 = 80%.
    0.8 x 100 = 80
  3. Write the answer.
    80%

Check. 80% of 500 J is 400 J. ✓

Worked example 2.2.2
A motor takes 2000 J of energy and gives 500 J of useful output. Find its efficiency as a fraction in its lowest terms.

Method. Divide the useful output by the input.

  1. Write the fraction. Efficiency = 500 / 2000.
    500 / 2000
  2. Simplify. Divide the top and bottom by 500: 1/4.
    500 / 2000 = 1/4
  3. Write the answer.
    1/4

Check. 1/4 of 2000 J is 500 J. ✓

Worked example 2.2.3
A device has a power input of 1000 W and a power output of 250 W. What is its efficiency as a percentage? Write the number of percent.

Method. Use the powers in the same way as energies.

  1. Divide. Efficiency = 250 / 1000 = 0.25.
    250 / 1000 = 0.25
  2. Percentage. Multiply by 100: 0.25 x 100 = 25%.
    0.25 x 100 = 25
  3. Write the answer.
    25%

Check. 25% of 1000 W is 250 W. ✓

Watch out

Watch out: efficiency uses the useful output, not the total. If a question gives you the wasted energy, find the useful energy first: useful = input - wasted.

Exam tip

Exam tip. Efficiency has no units. Give your answer as a percentage if the question asks for one.

Exercise 2B  Calculating efficiency

1
FA machine has 600 J in and 300 J useful output. What is its efficiency? Write the number as a percentage.
2
FA machine has 100 J in and 90 J useful output. What is its efficiency as a percentage? Write the number of percent.
3
FA machine has 200 J in and 40 J useful output. What is its efficiency as a percentage? Write the number of percent.
4
FA lamp is supplied with 400 W and gives out 120 W of useful light. What is its efficiency as a percentage? Write the number of percent.
5
FA machine has 100 J in and 75 J useful. What is its efficiency as a fraction in its lowest terms? Write the fraction.
6
PSA machine has 75 J useful and 25 J wasted. What is its efficiency as a percentage? Write the number of percent.
7
PSA machine is 60% efficient and has 500 J of energy in. How much useful energy does it give out? Write the number of joules.
8
PSA machine is 25% efficient and gives out 50 J of useful energy. How much energy goes in? Write the number of joules.
9
PSA motor uses 1200 J of energy and gives 360 J of useful output. Find its efficiency as a percentage. Write the number of percent.
10
PSAn LED is 85% efficient and an old lamp is 30% efficient. Which is the better device for lighting? Write LED or lamp.
11
RA pupil says “a machine can be 120% efficient because it gives out more than it takes in.” Explain why this is wrong. Accept any answer that says the useful output cannot be more than the energy input, because energy is conserved.
12
PSA lift motor takes 800 J and lifts a load with 600 J of useful energy. Give the energy wasted and the efficiency. Write two values: the number of joules wasted, then the percentage efficiency, separated by a comma.
13
RDescribe one way to make a machine more efficient, in one sentence. Accept any sentence that names a method such as lubrication, insulation, or reducing friction.
2.3

Gravitational potential energy

GCSE
Calculate gravitational potential energy using E = mgh
Video lesson: Gravitational potential energyComing soon
Context. A builder lifts a bag of cement onto a wall. The bag gains energy because it is higher above the ground. How much energy does it gain? We use the equation for gravitational potential energy.

An object raised above the ground has energy in its gravitational potential store. The energy depends on three things: its mass m in kilograms, the height h in metres, and the gravitational field strength g, which is 9.8 N/kg near the Earth. The equation is:

Ep = m x g x h, with g = 9.8 N/kg

hmass mG = 9.8 N/kgnear Earthweight
A block raised to a height h. Its gravitational potential energy depends on m, g and h

Key ideas

  • Gravitational potential energy E = m x g x h.
  • g = 9.8 N/kg near the Earth. Some questions give g as 10 N/kg; use the value given.
  • Height is measured from the level it would fall to (usually the ground).
  • Rearrange to find m = E / (g x h) or h = E / (m x g).
Worked example 2.3.1
A 2 kg bag is lifted through a height of 5 m. Calculate its gravitational potential energy. Write the number of joules.

Method. Use E = m x g x h with g = 9.8 N/kg.

  1. Take the numbers. m = 2 kg, h = 5 m, g = 9.8 N/kg.
    m = 2, h = 5, g = 9.8
  2. Multiply. E = 2 x 9.8 x 5 = 98 J.
    2 x 9.8 x 5 = 98
  3. Write the answer.
    98 J

Check. 98 / (9.8 x 5) = 2 kg. ✓

Worked example 2.3.2
A 3 kg box is lifted to a height of 4 m. Calculate its gravitational potential energy. Write the number of joules.

Method. Use the same equation with the new numbers.

  1. Take the numbers. m = 3 kg, h = 4 m.
    m = 3, h = 4
  2. Multiply. E = 3 x 9.8 x 4 = 117.6 J.
    3 x 9.8 x 4 = 117.6
  3. Write the answer.
    117.6 J

Check. 117.6 / (3 x 9.8) = 4 m. ✓

Worked example 2.3.3
An object of mass 5 kg has 490 J of gravitational potential energy. How high is it? Write the number of metres.

Method. Rearrange E = m x g x h to h = E / (m x g).

  1. Rearrange. h = E / (m x g) = 490 / (5 x 9.8).
    h = 490 / 49
  2. Divide. h = 490 / 49 = 10 m.
    490 / 49 = 10
  3. Write the answer.
    10 m

Check. 5 x 9.8 x 10 = 490 J. ✓

Watch out

Watch out: height means the height above the ground, in metres. A height of 150 cm is 1.5 m before you use the equation.

Exam tip

Exam tip. Write g = 9.8 N/kg at the start of the calculation, even if the question does not give it.

Exercise 2C  Gravitational potential energy

1
FA 4 kg object is lifted through 10 m. Calculate the gravitational potential energy gained. Write the number of joules.
2
FA 2 kg object is lifted through 3 m. Calculate the gravitational potential energy. Write the number of joules.
3
FA 10 kg load is lifted 2 m. Calculate the gravitational potential energy. Write the number of joules.
4
FAn object has 980 J of gravitational potential energy and a mass of 10 kg. What is its height? Write the number of metres.
5
FAn object has 294 J of gravitational potential energy when it is 3 m high. What is its mass? Write the number of kilograms.
6
PSConvert 150 cm into metres, then calculate the gravitational potential energy of a 1 kg object raised by 150 cm. Write the number of joules.
7
PSWhich object has more gravitational potential energy: 5 kg at 4 m or 2 kg at 12 m? Write the mass and height of that object, for example “5 kg at 4 m”.
8
PSA stone of mass 0.2 kg is held at the top of a 20 m cliff. Calculate its gravitational potential energy. Write the number of joules.
9
PSA 60 kg person has a weight of W = m x g. Calculate the weight. Write the number of newtons.
10
PSA 4 kg object is lifted onto a shelf 2.5 m above the floor. Calculate the gravitational potential energy it gains. Write the number of joules.
11
RExplain, in one sentence, why an object on the Moon would have less gravitational potential energy than on Earth at the same height. Accept any sentence that says the Moon has a smaller gravitational field strength.
12
RA pupil doubles the height of an object but keeps the mass the same. Explain what happens to the gravitational potential energy. Accept any sentence that says it doubles, because E is proportional to h.
2.4

Kinetic energy

GCSE
Calculate kinetic energy using E = 1/2 mv squared
Video lesson: Kinetic energyComing soon
Context. Ahmed is driving a car on a road. He sees a sign for a speed camera. “Why does speed matter so much?” he thinks. The answer is in the equation for kinetic energy, which depends on the speed squared.

A moving object has energy in its kinetic store. The kinetic energy depends on the mass m in kilograms and the speed v in metres per second. The equation is:

Ek = 1/2 x m x v2

Because the speed is squared, doubling the speed makes the kinetic energy four times bigger.

vspeed vspeed 2v: four times the KE
Doubling the speed of the same car makes its kinetic energy four times bigger

Key ideas

  • Kinetic energy E = 1/2 x m x v squared.
  • Mass is in kilograms, speed in metres per second, energy in joules.
  • Doubling the speed multiplies the kinetic energy by 4.
  • Rearrange to find v = square root of (2E / m).
Worked example 2.4.1
A 1200 kg car travels at 15 m/s. Calculate its kinetic energy. Write the number of joules.

Method. Use E = 1/2 x m x v squared.

  1. Square the speed. v squared = 15 x 15 = 225.
    15 squared = 225
  2. Multiply. E = 0.5 x 1200 x 225 = 135000 J.
    0.5 x 1200 x 225 = 135000
  3. Write the answer.
    135000 J

Check. 135000 / 0.5 = 270000, and 270000 / 1200 = 225 = 15 squared. ✓

Worked example 2.4.2
A 0.5 kg ball moves at 4 m/s. Calculate its kinetic energy. Write the number of joules.

Method. Use the equation with the numbers.

  1. Square the speed. 4 squared = 16.
    16
  2. Multiply. E = 0.5 x 0.5 x 16 = 4 J.
    0.5 x 0.5 x 16 = 4
  3. Write the answer.
    4 J

Check. 4 / 0.5 = 8, and 8 / 2 = 4 m/s. ✓

Worked example 2.4.3
An object of mass 4 kg has 800 J of kinetic energy. Find its speed. Write the number of metres per second.

Method. Rearrange to v = square root of (2E / m).

  1. Rearrange. v squared = 2 x 800 / 4 = 400.
    2 x 800 / 4 = 400
  2. Square root. v = square root of 400 = 20 m/s.
    v = 20
  3. Write the answer.
    20 m/s

Check. 0.5 x 4 x 20 squared = 800 J. ✓

Watch out

Watch out: do not forget the 1/2 at the start, and square the speed before you multiply. Doubling the speed does not double the kinetic energy; it makes it four times bigger.

Exam tip

Exam tip. Square the speed first, then multiply by the mass, then by 0.5. Write each step on its own line.

Exercise 2D  Kinetic energy

1
FA 2 kg object moves at 5 m/s. Calculate its kinetic energy. Write the number of joules.
2
FA 0.1 kg ball moves at 10 m/s. Calculate its kinetic energy. Write the number of joules.
3
FA 1000 kg lorry moves at 20 m/s. Calculate its kinetic energy. Write the number of joules.
4
FA 3 kg object moves at 4 m/s. Calculate its kinetic energy. Write the number of joules.
5
FAn object has 100 J of kinetic energy and a mass of 2 kg. Find its speed. Write the number of metres per second.
6
PSObject A has a mass of 1 kg and moves at 10 m/s. Object B has a mass of 2 kg and moves at 5 m/s. Which has more kinetic energy? Write A or B.
7
PSA 1000 kg car moves at 25 m/s. Calculate its kinetic energy. Write the number of joules.
8
PSA car has 135000 J of kinetic energy and a mass of 1200 kg. Find its speed. Write the number of metres per second.
9
PSA 5 kg object falls 10 m. All its gravitational potential energy changes to kinetic energy. What is its kinetic energy at the bottom? Write the number of joules.
10
RExplain in one sentence why a car at 40 m/s has four times the kinetic energy of a car at 20 m/s, if the mass is the same. Accept any sentence that says the speed is squared, so doubling the speed quadruples the energy.
11
RExplain why a speed limit is important, in terms of kinetic energy. Accept any sentence that says a faster car has much more kinetic energy, which must be dissipated when it stops, so a crash does more harm.
12
PSA 0.2 kg ball is thrown and has 4 J of kinetic energy. Find its speed. Write the number of metres per second.
2.5

Using the equations in context

GCSE
Use these equations in context, such as falling objects, braking and appliance ratings
Video lesson: Using the equations in contextComing soon
Context. A motor lifts a small crate of tools up a building site. The crate gains gravitational potential energy, but the motor does not transfer all its energy to the crate. We use efficiency, power and gravitational potential energy together.

In real questions you often need more than one equation. Make a list of what you know, decide which quantity you need, and then choose the equation. For a falling object with no losses, the gravitational potential energy at the top becomes kinetic energy at the bottom. In a real situation some energy is dissipated, so the kinetic energy is less.

Efficiency of four devices (example)020406080100LEDlampmotorenginedevicepercent
Efficiency of four devices (example numbers)

Key ideas

  • Work out the energy first, then use efficiency or power.
  • If there are no losses, GPE at the top equals KE at the bottom.
  • With losses, the KE at the bottom is less than the GPE at the top.
  • Check that the efficiency is less than 100%.
Worked example 2.5.1
A motor lifts a 10 kg load through 2 m. Its efficiency is 50%. How much energy must the motor take in? Write the number of joules.

Method. Find the useful energy, then divide by the efficiency.

  1. Useful energy. GPE = 10 x 9.8 x 2 = 196 J.
    196 J
  2. Input. Input = useful / efficiency = 196 / 0.5 = 392 J.
    196 / 0.5 = 392
  3. Write the answer.
    392 J

Check. 50% of 392 J is 196 J. ✓

Worked example 2.5.2
A 3 kg object falls from a height of 8 m. There are no losses. What is its kinetic energy just before it lands? Write the number of joules.

Method. With no losses, the gravitational potential energy at the top is the kinetic energy at the bottom.

  1. Gravitational potential energy. E = 3 x 9.8 x 8 = 235.2 J.
    235.2 J
  2. Kinetic energy. With no losses, KE = 235.2 J.
    235.2 J
  3. Write the answer.
    235.2 J

Check. 235.2 J = 0.5 x 3 x v squared, so v is about 12.5 m/s. ✓

Worked example 2.5.3
A hairdryer has a power of 1500 W. How much energy does it transfer in 2 minutes? Write the number of joules.

Method. Convert minutes to seconds, then use E = P x t.

  1. Convert. 2 minutes = 120 s.
    120 s
  2. Multiply. E = 1500 x 120 = 180000 J.
    1500 x 120 = 180000
  3. Write the answer.
    180000 J

Check. 180000 / 120 = 1500 W. ✓

Watch out

Watch out: a falling object with air resistance has less kinetic energy at the bottom than the gravitational potential energy at the top. The missing energy has been dissipated.

Exam tip

Exam tip. In a multi-step question, write a short list of the quantities first: energy, time, mass, height, speed, efficiency.

Exercise 2E  Calculations in context

1
FA motor has 80 J of useful output from 100 J of input. What is its efficiency as a percentage? Write the number of percent.
2
FA 5 kg load is raised 4 m. Its gravitational potential energy is 196 J. The motor takes in 280 J. What is the efficiency as a percentage? Write the number of percent.
3
FA machine takes in 1000 J and is 40% efficient. How much useful energy does it give? Write the number of joules.
4
FA machine transfers 3000 J of energy in 12 seconds. What is its power? Write the number of watts.
5
PSA heater has a power of 4 kW. How much energy does it transfer in 30 minutes? Write the number of joules.
6
PSA 60 W lamp is on for 2 hours. How much energy does it transfer? Write the number of joules.
7
PSA 3 kg object falls from 8 m with no losses. Give its kinetic energy when it lands. Write the number of joules.
8
PSThe same 3 kg object falls from 8 m, but 20% of the energy is dissipated by air resistance. Calculate its kinetic energy when it lands. Write the number of joules.
9
PSA 600 kg car is raised through 20 m by a crane. Calculate the gravitational potential energy gained. Write the number of joules.
10
PSA lift motor takes in 980 J and is 60% efficient. The load of 3 kg is lifted. Find the height of the lift. Write the number of metres.
11
RA pupil says a 100 W motor is always more efficient than a 50 W motor. Explain why this is not necessarily true. Accept any answer that says efficiency needs the useful output and the input, not just the power rating.
12
RTwo kettles are both 2200 W. One is 90% efficient and one is 70% efficient. Which kettle heats the same water in less time? Write 90% or 70%.
13
RA machine is 60% efficient and takes in 800 J of energy. It lifts a load through 6 m. Find the mass of the load. Write the number of kilograms.

Chapter summary

SkillMethod
Calculate powerP = E / t; convert kW to W and minutes to seconds first
Calculate efficiencyefficiency = useful output / total input, x 100 for a percentage
Calculate GPEE = m x g x h, with g = 9.8 N/kg
Calculate kinetic energyE = 1/2 x m x v squared; doubling v multiplies E by 4
Use in contextFind useful energy first, then divide by efficiency; use GPE at top = KE at bottom only with no losses

Can you…?

  • I can calculate power from energy and time
  • I can convert kilowatts to watts and minutes to seconds
  • I can calculate efficiency as a percentage
  • I can calculate gravitational potential energy with E = mgh
  • I can calculate kinetic energy and explain why speed is squared

Key terms

power
the rate at which energy is transferred, measured in watts (joules per second)
watt (W)
the unit of power: one joule per second
efficiency
useful energy output divided by total energy input
gravitational potential energy
energy stored by an object because of its height above the ground
kinetic energy
energy stored by an object because it is moving
gravitational field strength (g)
the force per kilogram near the Earth, 9.8 N/kg

Exercise 2Z  Check-up

1
A device transfers 1200 J in 40 seconds. What is its power? Write the number of watts.
2
Convert 3 kW into watts. Write the number of watts.
3
A machine has 400 J of input and 100 J of useful output. What is its efficiency as a percentage? Write the number of percent.
4
Calculate the gravitational potential energy of a 6 kg object raised 5 m. Write the number of joules.
5
Calculate the kinetic energy of a 2 kg object moving at 6 m/s. Write the number of joules.
6
A 100 J input machine gives 70 J of useful output. How much energy is wasted? Write the number of joules.
7
Doubling the speed of an object multiplies its kinetic energy by how much? Write the number.
8
A 1 kW heater runs for 10 minutes. How much energy does it transfer? Write the number of joules.
9
An object of mass 2 kg has 196 J of gravitational potential energy. What is its height? Write the number of metres.
10
A pupil says “a machine that gives out 100 J of useful energy from 90 J of input is 100% efficient.” Is the pupil right? Write yes or no.

Total 26 marks. Allow about 26 minutes.

Exercise 2X  Exam-style questions

1
FA kettle has a power of 2000 W. Calculate the energy it transfers in 2 minutes. Write the number of joules.[2 marks]
2
FA machine has 500 J of input and 350 J of useful output. Calculate its efficiency as a percentage. Write the number of percent.[2 marks]
3
FA 2 kg ball is lifted to a height of 15 m. Calculate its gravitational potential energy. Write the number of joules.[2 marks]
4
FA car of mass 800 kg travels at 20 m/s. Calculate its kinetic energy. Write the number of joules.[2 marks]
5
PSA motor is 40% efficient and takes in 1500 J. Calculate the useful energy output. Write the number of joules.[2 marks]
6
PSA 1500 W hairdryer is used for 90 seconds. Calculate the energy transferred. Write the number of joules.[2 marks]
7
PSA 20 kg crate is raised by a lift through 6 m. The lift motor is 80% efficient. Calculate the energy the motor must take in. Write the number of joules.[3 marks]
8
PSA 0.5 kg ball is dropped from a height of 20 m. Assume no energy is lost. Calculate its speed just before it lands. Give your answer to 1 decimal place. Write the number of metres per second.[3 marks]
9
RA pupil says that a 3 kg object falling from 8 m has 235.2 J of kinetic energy at the bottom because the energy is conserved. Explain why this is only true if no energy is dissipated, and give the energy transferred to the surroundings if the KE at the bottom is 200 J. Write the number of joules dissipated.[4 marks]
10
PSA motor lifts a 12 kg load through 5 m in 10 seconds. The motor transfers 735 J of energy to the load in total. Calculate the efficiency of the motor as a percentage. Give the working. Write the number of percent.[4 marks]
← Chapter 1: Energy stores and transfersChapter 3: Specific heat capacity →

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