Energy stores and transfers GCSE physics lesson: Ek = ½mv², Ep = mgh and efficiency

Energy stores and transfers: practice

Show your working on paper. Use g = 9.8 N/kg. Give energies in joules (J); turn grams into kilograms first. The stretch is optional.

Diagnostic question: choose and explain

1. A pendulum is let go 10 cm above its lowest point. Each swing is a little lower, and a few minutes later it hangs still. What has happened to the energy it started with?
   1. It has been used up by the swinging
   2. It has moved to the thermal store of the pendulum and the air around it
   3. It is still in the pendulum's gravitational store
   4. It has turned into air resistance

Practice questions

1. A spring is stretched and held. Which store holds the energy you put in?
   1. Kinetic
   2. Elastic potential
   3. Thermal
   4. Gravitational potential
2. A torch is switched on. Energy moves from the ___ store of its battery, is transferred ___ to the bulb, then away by ___ and by heating, ending in the ___ store of the surroundings.
3. A 3.0 kg bag is lifted 2.0 m onto a shelf. How much energy goes into its gravitational store, in joules?
4. A 60 kg runner moves at 5.0 m/s. How much energy is in her kinetic store, in joules?
5. A 900 g ball rolls at 4.0 m/s. How much energy is in its kinetic store, in joules?
6. A 0.50 kg ball has 49 J in its gravitational store, measured from the ground. How high is it, in metres?
7. A stone is dropped from rest and falls 2.5 m. Ignoring air resistance, how fast is it going when it has fallen that far, in m/s?
8. A 1 kg ball and a 4 kg ball roll from rest down the same smooth slope, 5 m high. Which is faster at the bottom?
   1. The 4 kg ball: it has four times the energy
   2. The 1 kg ball: it is easier to speed up
   3. Neither: they reach the same speed
9. An electric motor is given 1200 J. It lifts a 15 kg load through 6.0 m. What is its efficiency, as a percentage?
10. A light bulb is given 60 J each second. Of that, 51 J heats the bulb and the room around it. What is its efficiency, as a decimal?
11. Stretch: a 0.20 kg ball is thrown straight up at 14 m/s. With no air resistance it would rise 10 m; with air resistance it rises only 9.0 m. How much energy, in joules, went to the thermal store of the air and the ball on the way up?

Support: For each question, write three things before any numbers: the store at the start, the way it moves, the store at the end. Then the formula, then the numbers in kilograms, metres and metres per second.

Energy stores: exit

Use g = 9.8 N/kg. Write the store at the top and the store at the bottom first.

1. A 40 kg child sits at the top of a slide 3.0 m high. How much energy is in her gravitational store, measured from the bottom of the slide, in joules?
2. She starts from rest and reaches the bottom at 6.0 m/s. How much energy, in joules, was wasted on the way down?

Exit check

A gravitational store at the top and a kinetic store at the bottom show who can follow energy, and say where the rest went.

A 40 kg child at the top of a slide 3.0 m high, starting from rest. At the bottom she moves at 6.0 m/s.

Energy stores and transfers

Use g = 9.8 N/kg. For each change, write the store at the start and the store at the end before any numbers.