5.2 Work Done & Energy Transfer

Figure 1 shows a cross-section of a Tube station on the London Underground.

Figure 1

A group of students are learning about the station in a physics lesson before they visit it on a school trip.

When the students exit the station, they will ascend to street level in an elevator.

Student A says that the passengers in the lift will gain gravitational potential energy as they travel in the lift.

Student B says that they will not gain gravitational potential energy, because they are below ground level.

State which student is correct and explain your answer.

Higher Tier Only

The Tube station was designed to maximise efficiency.

The station has an upward-sloping track at its entrance and a downward-sloping track at its exit.

The driver uses breaks to stop the train and uses a motor to make the train move away.

Explain how the design of the station increases the efficiency of the trains. Use ideas about work done in your answer.

Which of the following is the correct efficiency equation in terms of work done?

Tick (✓) one box.

The efficiency of an electric motor is 80 %.

When lifting a load, the amount of energy wasted is 100 J.

Calculate the useful work done (W) by the motor.

A girl rides a sledge down a smooth snowy slope, as shown in Figure 2.

Figure 2

To begin with the sledge accelerates, but after a while it starts to travel at a constant speed.

Explain why.

Whilst travelling at a constant speed of 5 m/s, the girl travels a distance of 200 m and descends a vertical height of 25 m.

The girl and sledge have a combined mass of 50 kg.

Calculate the change in the gravitational potential energy of the girl and sledge.

Gravitational field strength = 9.8 N/kg

As the sledge descends the slope friction does work against the sledge.

Calculate the average frictional force acting on the sledge.

Calculate the kinetic energy of the girl and the sledge.

Eventually the slope levels out and the sledge slows down and comes to a halt.

Assuming that the frictional force on the sledge remains the same as that calculated in part (c), calculate the distance it will take the sledge to come to a halt.

A student carries out an experiment which involves using an electric motor to lift some masses, as shown in Figure 3.

Figure 3

At one point, a mass of 200 g is lifted.

Calculate the weight of this mass.

Gravitational field strength = 9.8 N/kg

Give your answer to 2 significant figures

Calculate the work done by the motor when it lifts the above mass a height of 90 cm.

The student estimates that the efficiency of the motor is about 20%.

Calculate the amount of total energy supplied to the motor.

A car is travelling at a steady speed of 15 m/s when the driver suddenly has to brake.

The car has a total mass of 1500 kg.

Calculate the kinetic energy of the car, just before the brakes are applied.

State the work done by the brakes in stopping the car.

The brakes apply a force of 12 000 N on the car.

Calculate the braking distance of the car.

Give your answer to 2 significant figures.

Explain what happens to the kinetic energy of the car as it slows down and the effect that it will have on the brakes.

Electric cars are fitted with regenerative braking systems.

These systems consist of generators attached to the car’s wheels.

Electric cars are also fitted with standard brakes alongside the regenerative ones.

Explain the advantages of both regenerative brakes and standard brakes.