Simple energy transfers involving motion

Demonstration

Simple demonstrations to introduce the concept of energy stored kinetically.

Apparatus and Materials

• Retort stand
• Pulley, single, on clamp
• Mass hanger and slotted masses (100 g)
• Thread
• Dynamics trolley
• Spring, expendable
• G-clamp, 10 cm

Health & Safety and Technical Notes

In all of these experiments a student should act as a trolley catcher to ensure no trolleys land on toes.

Read our standard health & safety guidance

Procedure

1. Put the trolley on a bench and give it a push. Energy stored chemically (in food + oxygen) is now stored kinetically.

2. Instead of a trolley you could, more impressively, use a student on a skate board (with due consideration to student behaviour and safety).
3. Put a trolley on the bench. Fasten the pulley to the edge of the bench, running a thread over it from the trolley to a 100 g hanger. Let the load fall a short distance to the floor so that the thread falls slack, allowing the trolley to continue moving. Energy stored gravitationally is now stored kinetically.

4. Repeat the demonstration, but in reverse. Start the trolley moving with a push away from the pulley. Let it pull the thread taut and lift the load as it comes to rest. Energy stored kinetically is now stored gravitationally.
5. Before you use the expendable spring, stretch it until it is clearly an open, weak spring. Anchor one end (A) to the bench by slipping the end loop over the rod of a retort stand, which is itself clamped to the bench.

6. Fasten 1 – 2 m of thread to the other end (B) of the spring. Then stretch the spring gently by at least several centimetres. Anchor this end (B) temporarily.
7. Straighten out the thread and attach the free end to a trolley. Position the trolley so that the thread is taut. Energy stored chemically in muscles is now stored elastically in the thread.
8. Release the end B. You need to have a long enough piece of thread for the trolley to travel 30 cm or so at constant speed after the spring has fully contracted.
9. Repeat the experiment, but in reverse. Give the trolley a push so that it causes the spring to stretch. Energy stored kinetically is now stored gravitationally. If you make any compensation for friction, by tilting the board, this will have to be adjusted when you reverse the transfers.

Teaching Notes

• The concept of energy stored kinetically is very important. Energy stored in fuels can be transferred to energy stored kinetically in rockets, gas molecules and anything else which is moving.
• Before students can tackle calculations with energy stored kinetically, they need a clear picture of what energy stored kinetically is, and a good feeling for it. These simple qualitative demonstrations invite discussion. It is useful to use them before attempts to measure and calculate values for energy stored kinetically.

This experiment was safety-tested in November 2005