More inertia experiments

Demonstration

These are short activities, suitable for demonstration as a conjuring act.

Apparatus and Materials

• Coin
• Card (such as postcard)
• Beaker
• Thread (beraking force, approx 1 N)
• Mass, large, 1 kg or more
• Retort stand, boss, and clamp
• Pile of books
• Mass, 1 g
• Blocks, wooden, with rounded edges, smooth, 5 (approx 10 cm x 7.5 cm x 5 cm)
• Hammer or mallet
• G-clamps

Health & Safety and Technical Notes

Read our standard health & safety guidance

Step 3: You can make the large mass by Sellotaping together two 2-kg masses.

Step 8: For strong sewing cotton you will need a mass of at least 5 g.

Step 10: The blocks should be smooth blocks of wood, say 10 cm x 7.5 cm x 5 cm with their edges and corners rounded.

Procedure

Coin and tumbler
1. Place the coin on the card, and place the card over the open end of a beaker.

2. Flick the card away sharply and observe the effect of the coin's motion.
One large mass
3. Hang a large mass (1 kg or more) by thread from a strong, rigid support. Attach a second thread to the underside of the mass.
4. Pull the lower thread with a force that increases slowly until one of the threads breaks.
5. Try again with a short sharp pull.

Pile of books
6. Set up a pile of books or magazines on the bench and pull out one of the books in the middle quickly.

Horizontal snap
7. Show that the thread can support a suspended mass of about 100 g.
8. Tie the thread to the small mass. Hold the other end of the thread with the thread slack.

9. A very abrupt jerk of the thread will break it.
Pile of bricks As a reverse form of 6, push a wooden brick in to replace the bottom one at the bottom of a pile of similar wooden blocks.
10. Build a pile of four blocks.
11. Push a fifth brick quickly at the bottom brick of the pile. The fifth brick goes in and the bottom brick goes out. This is most dramatic if the fifth brick is projected along the table towards the pile by hitting it with a small mallet.

Teaching Notes

• Step 2: Pulling the cord slowly means the coin has a low acceleration, and so the frictional force between card and coin is big enough to accelerate the coin. Pulling the cord quickly requires too great a frictional force to accelerate the coin and so slipping occurs.
• Steps 4 and 5: Due to the weight of the mass, the upper thread breaks in the first case. But due to the inertia of the mass, the lower thread breaks in the second case. So by applying forces differently you can successfully predict which thread will break.
• Step 9: A slow pull just moves the mass along, but a quick pull snaps the thread because the force required for the high acceleration of the mass is greater than the thread's breaking strength.
• Steps 6 and 11: The only force that the moving book can exert on the pile of books above it is friction. If the acceleration of the moving books/blocks is large enough, there is insufficient force to make the pile above it move too.
• A variation is to pull a table cloth from beneath some unwanted crockery.

This experiment was safety-tested in March 2005