# Why the magnetic truck oscillates?

1. May 15, 2006

Hi

Saw this in a Science Centre -

There truck on an inclined slope. At the head of the truck is a magnet. the end of the track is a wire with electric current (DC i think). As the truck is released, the wire moves a little away from the truck. The truck gets near the wire but not touching it and then moves backwards. the motion repeats.

Why? I think it has something to do with the wire becoming a magenet and repel the truck-but how the whole thing work?

truck (magnet) ----> wire attached to battery.

Thanks for the explanation.

Cheers!

2. May 15, 2006

### Andrew Mason

Your explanation could be clearer. The truck must be pointed down the slope, magnet in front and the wire must be further down slope from the magnet.

When the truck is released, gravity pulls the truck down until the magnetic repulsion between the wire (which has a magnetic field because it is carrying current) and the magnet reaches the force of gravity and the truck stops accelerating downward.

When the magnetic and gravitational forces are balanced, the truck has downward speed, so it takes some time to stop. When it actually stops, it is even closer to the wire where the magnetic repulsion exceeds the force of gravity. So the truck accelerates back up the slope. When the force of gravity and magnetic force are equal, the truck is still moving and goes a little higher where the force of gravity is greater than the magnetic repulsion.

The interesting part of your question, though (and it is a good one) is: why does the truck keep going back up and down, without eventually stopping? Where does the energy come from to replace the energy lost by friction?

AM