Constant and varying forces between trucks

Demonstration

This is an interesting way to show some balanced and unbalanced forces and the effects on motion.

Apparatus and Materials

• Model railway track (1.5 m)
• Flat trucks, 4
• Magnets, 2, horseshoe or flat-faced ceramic (suitably mounted)
• blocks, wooden, with spring buffers attached, 2
• Rubber cord (or extending spring)

Health & Safety and Technical Notes

Consider fitting buffer stops to the ends of the track if there is a risk that loaded trucks may fall onto observers.

Read our standard health & safety guidance

In advance of the lesson, mount the magnets on the trucks, such as to repel each other. It may be necessary to counterbalance the magnets with masses taped to the other ends of the trucks.

Attach buffer springs to the wooden block, for example with sticky tape. They should be short compressible springs. Each of them should have a disc attached, so that they can make reasonably "clean" contact with each other. The blocks can then be attached to the trucks using sticky tape or rubber bands.

Alternatively, the demonstration can be done using dynamics trolleys, but the low mass and low friction of the trucks produces more effective results.

Procedure

1. Push one of the trucks with a mounted magnet towards the other one, which should be stationary to start with. Release it. A collision can be made to take place without contact.
2. Repeat this with both trucks being given an initial push.
3. Now repeat the actions, using the pair of trucks with the buffer springs.
4. Compare these varying forces acting on the trucks with a situation in which a force is constant. Such a constant force can be applied to a truck by pulling it with a cord (or spring) which is carefully held with a constant extension.

Teaching Notes

• A truck moving along a level track experiences only a small force of friction. This is quite similar to balanced forces, and acceleration (or deceleration) is small. If the wheels and track were perfectly smooth, the truck's motion would not change at all.
• As the magnets approach each other (or the springs contact each other) the varying forces produce varying changes in motion. There is both acceleration and deceleration (or negative acceleration).
• The trucks decelerate as they approach and accelerate again when they move apart. The velocity is zero at maximum compression of the spring. This can be used as an analogy for the magnetic case.
• An entertaining variant involves hiding one of the trucks, initially stationary, behind a large screen. This initially stationary truck can be first of all free to move and then fixed. This produces different effects on the pushed truck.
• For older students or able younger students it is worth inviting comparison of the observations with magnets with what we imagine must take place when a pair of repelling particles, such as electrons, collide. As with the trucks and their magnets and springs, the sizes of the forces depend on their distance apart. More sophisticated again are interactions between atoms, where there is longer range attraction and shorter range, strong repulsion.
• If the trucks have the same mass it is useful to point out what happens when the moving one hits the stationary one. (The moving one stops and the stationary one moves off at the same velocity).

This experiment was safety-tested in October 2004