Friction can cause energy to dissipate (spread out and become unusable) into the surroundings, causing them to heat up.

Efficiency is the fraction of energy supplied to an object or system that is usefully transferred by it.

The amount of energy usefully transferred by an object or system is the useful energy transferred by the device.

The total amount of energy transferred to an object or system is the total energy supplied to the device.

Calculating efficiency (in terms of useful energy transferred)

Physics

In the Forces make things change unit, they will build on this knowledge to learn how to combine forces in two dimensions, how to calculate acceleration from a resultant force and how to calculate forces in collisions and explosions.

In the Measuring and calculating motion unit, pupils learnt how to measure and calculate velocity and acceleration, and how to interpret displacement-time and velocity-time graphs. 

Energy of moving objects

Physical activity and equipment requiring safe use

Energy is {{}} when it is transferred to the thermal store of the surroundings.

The amount of energy that makes something useful happen.

The fraction of the energy supplied that makes something useful happen.

The fraction of the energy supplied that dissipates.

Which of the following is a correct definition for efficiency?

efficiency = useful output energy transfer × total input energy transfer

efficiency = useful output energy transfer ÷ total input energy transfer

useful output energy transfer = efficiency × total input energy transfer

total input energy transfer = useful output energy transfer ÷ efficiency

total input energy transfer = useful output energy transfer × efficiency

Which of the following equations show the correct relationship between efficiency, useful output energy transfer and total input energy transfer?

A forklift truck transfers 24 000 J of energy when lifting a box onto a high shelf.
Calculate the efficiency of the truck if 18 000 J of energy is transferred to the gravitational store. 

Aisha is investigating how the temperature of a tennis ball affects its rebound height.
Which of the following are control variables for this experiment?

An elevator has an efficiency of 0.8. When it lifts four people to the tenth floor, it uses 160 000 J of energy. 
How much energy is transferred to the people?

Which of the following energy stores does friction transfer energy into? 

All of its energy is transferred to the surroundings.

Some of its energy is transferred to the surroundings.

Earth's gravitational force is pulling the ball downwards.

The normal reaction force from the ground is less than the gravitational force.

Pushing air out of the way uses up some of the ball's energy.

Pushing air out of the way transfers some energy to the air particles.

Why is the rebound height of a ball not the same as the height that it was dropped from?

The law of conservation of energy states that energy cannot be created or ...

Match the values on the left to the equivalent values on the right.


An electric heater transfers no energy to the surroundings.

An electric heater transfers all energy usefully into thermal stores. 

An electric heater has a low power input.

An electric heater has a high power input.

Which of the following statements explains why an electric heater can be considered to be 100% efficient? 

Content guidance

Risk assessment required - equipment

Adult supervision required

This lesson will soon be taken down.

Calculating efficiency (in terms of useful energy transferred)

Lesson outcome

Content guidance