Storing energy and transferring energy

Demonstration

Simple demonstrations to stimulate discussion about the different ways in which energy can be stored: gravitationally, elastically, kinetically.

Apparatus and Materials

• Large mass or block of wood (tied up with string)
• Single pulleys on clamps, 2
• Large spring
• Dynamics trolley
• G-clamps, 5 cm, 2
• Retort stands, bosses and clamps, 2
• Rough cloth, piece approx 30 cm x 30 cm

Health & Safety and Technical Notes

The retort stand should be clamped to the bench. Any attempt to attach a pulley to the ceiling requires two persons: one to hold the ladder or steps. Ensure no one walks beneath the suspended object.

Steps 2 and 3 require a trolley catcher.

Read our standard health & safety guidance

For demonstration 2, a better alternative to the pulley attached to a retort stand is a hook fixed in the ceiling. A single pulley can be attached to the hook.

Procedure

1. Hang the spring from a retort stand and attach the large mass or block of wood to the lower end. Support the mass or block with your hand raised from the equilibrium position, release it and then catch it at its lowest point. Discuss the changes in the way that energy is stored - as suggested in the teaching notes.
2. Clamp one end of the spring to the end of the table with a G-clamp. The two pulleys are clamped to the retort stand, which itself will need clamping to the bench. The cord is run round the two pulleys as shown in the diagram, but without attaching the free end to the spring. Stretch the spring horizontally and hold it in the stretched position while you discuss the way in which energy is stored. (See teaching notes.) Now attach the free end of the cord to the stretched spring. When the spring is released the unbalanced force from the spring accelerates the mass upwards; in energy terms, the energy stored elastically decreases (as the spring contracts) and the energy stored gravitationally increases (as the mass rises).

3. Keep the spring clamped as in 2 and attach the other end to the dynamics trolley. Stretch the spring and hold it still. Then release the trolley. The unbalanced force will accelerate the trolley. In energy terms, the energy stored elastically decreases (as the spring contracts) and the energy stored kinetically (kinetic energy) increases (as the trolley speeds up). What is more, the increase in energy stored kinetically is the same as the decrease in the energy stored elastically. Energy is conserved.

4. Lay the cloth on the bench as in the diagram. Give the trolley a push so that when it runs over the rough patch of cloth it slows down and stops. You can discuss the transfer of energy to the cloth as suggested in the Teaching notes.

Teaching Notes

• Demonstrations 1 and 2 involve energy stored gravitationally.
• Take care in defining where is the zero of gravitational energy. It is best to refer to changes in energy stored gravitationally rather than absolute values. Students are often mystified when they discover that, if you raise a mass from the floor the energy stored gravitationally will increase, but that if a trap door is opened in the floor then the mass could fall further and the energy stored gravitationally can decrease. When the mass is on the floor, the system stores more energy gravitationally with respect to the basement but less with respect to the ceiling. Before you start, think carefully and avoid confusing statements regarding the gain or loss of gravitational potential energy.
• In 1 the system is storing energy gravitationally at the start. Although the spring is pulling up on the mass, to begin with the gravitational force downwards is bigger than the upwards force from the spring. The net downwards force causes the mass to accelerate downwards. As it moves downwards, the spring will begin to stretch.
• In energy terms, the energy stored gravitationally decreases; the energy stored kinetically increases and the energy stored elastically increases.
• At the equilibrium point, the spring's force balances the gravitational force. Beyond that point, the upwards force from the spring is greater than the downwards gravitaitonal force. So the mass slows down (it is accelerating upwards whilst moving downwards). As it does so, the spring is stretched more. In energy terms, the energy stored kinetically is decreasing (as is the energy stored gravitaionally) as the energy stored elastically is increasing. When it reaches the bottom, the mass is no longer moving and the system is storing energy elastically.
• The stretched spring is still pulling upwards on the mass with a force bigger than the gravtiational force. So there is still a net upwards force on the mass. Therefore it begins to accelerate upwards. It overshoots the equilibrium position, and the process repeats.
• In each cycle, the stretching and compressing of bonds in the spring will result in its temperature going up a little. And, in turn, the spring will raise the temperature of the surroundings (by heating). Therefore, in each cycle, it does not go quite so high because some of the energy has been transferred to the surroundings and there is less energy to be stored gravitationally.
• Over time, the spring will come to rest. At this point, the energy stored thermally by the surroundings has increased by an amount equal to the energy stored gravitationally by the system before the mass was released.
• Initially, you might say: "I have lifted the mass from its rest position so the energy stored gravitationally has increased. What will happen to this energy when I let it drop?"
• When you catch the mass at its lowest point you might ask: "How is the energy stored now? Will the mass stay here when I let it go? If not, why not?" You should then allow the mass to oscillate for a few cycles. Ask again for an explanation of the changes in the way in which energy is stored in the system.
• When you stretch the spring horizontally the energy stored elastically increases. Pause at this stage for students to consider where and how that energy was stored before (stored chemically (food + oxygen) in your muscles).
• When the stretched spring is attached to the mass, around the pulley, the force from the spring lifts the mass. In energy terms, the energy stored elastically has gone down and the energy stored gravitationally has increased.
• In demonstration 3, the energy stored elastically decreases and the energy stored kinetically (due to the moving trolley) increases.
• In demonstration 4 when the trolley is given a push, the energy stored chemically (food and oxygen) in muscles decreases and the energy stored kinetically increases.
• When the trolley moves over a rough surface it is slowed down by friction. There is a very slight rise in temperature as the sufaces rub over each other. When the trolley comes to rest the energy stored thermally by the surroundings has increased and the energy stored kinetically - due to the moving trolley - has decreased.

This experiment was safety-tested in November 2005