Energy stores and carriers
Simple demonstrations to stimulate discussion about transfers between kinetic (motion), gravitational and other forms of energy.
Apparatus and materials
Large mass or block of wood (tied up with string)
Single pulleys on clamps, 2
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 b and c require a 'trolley catcher'.
For demonstration b, 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.
a 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 energy changes as suggested in the Teaching notes.
b 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 energy changes. (See Teaching notes.) Now attach the free end of the cord to the stretched spring. When the spring is released the stored elastic (strain) energy will be used to raise the mass.
c Keep the spring clamped as in b and attach the other end to the dynamics trolley. Stretch the spring and hold it still. Then release the trolley, so that the stored energy is transferred into the kinetic energy of the trolley.
d 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.
1 Demonstrations a and b involve gravitational potential energy.
Take care in defining where is the zero of gravitational potential energy. Students are often mystified when they discover that, if you raise a mass from the floor it will gain some gravitational potential energy, but that if a trap door is opened in the floor then the mass could fall further. The mass has more gravitational potential energy with respect to the basement floor than it has with respect to the ground floor. Before you start, think carefully and avoid confusing statements regarding the gain or loss of gravitational potential energy.
2 In a the mass begins with gravitational potential energy which transfers to kinetic energy as the mass falls. When this is complete, the mass is still moving and continues to move, stretching the spring. The mass stops when all its kinetic energy is transferred into elastic (strain) energy in the spring.
The elastic (strain) energy enables the spring to pull the mass up again. It overshoots the equilibrium position, and the process repeats, until all the energy has been transferred into warming up the spring and the support. The mass then comes to rest.
You might say:
'I have lifted the mass from its rest position so its 'uphill' or gravitational potential energy 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:
'What has happened to the gravitational potential energy 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 energy transfers.
3 When you stretch the spring horizontally it gains elastic (strain) energy. Pause at this stage for students to consider where the energy to stretch the spring has come from (chemical energy (food + oxygen) stored in your muscles). When the stretched spring is attached to the mass, around the pulley, the spring transfers its elastic energy to the gravitational potential energy of the mass.
4 In demonstration c, the elastic energy of the spring is transferred to the kinetic energy of the trolley.
5 In demonstration d when the trolley is given a push, chemical energy (food and oxygen) in muscles is transferred to kinetic energy of the trolley. When the trolley moves over a rough surface it comes to rest because all its kinetic energy has been transferred to the surfaces it is in contact with. These are warmed up as a result. The thermal energy of the system has increased.
This experiment was safety-checked in November 2005