Racing Jam Jars

Race Jam Jars down a slope and find out that all things don't allways fall at the same speed.

What you need

3 identical jars

What to Do

Make a slope 1-2m long that drops about 2cm every metre.

Race the three jars against one another.

Be very careful to release them in a fair way, sorepeat the exeriment and swap over the jars in case you are being unfair.

In order to reduce the amount of jam on the floor, if your floor is hard you may wish to pad the bottom of the track with some jumpers or something to stop the jars gently.

What may Happen

What is going on?

We were all taught that things should fall under gravity at the same weight no matter how heavy they are, because the heavier something is the more force it needs to accellerate, but the more it is pulled towards the earth. So what is going on?

The jars are not just falling they are rolling and this is a different process. If you think about the centre of a jar  as it rolls it goes in a straight line.

However if you draw a dot on the edge of the jar it will go on a much longer curved path.

So to roll at the same speed a jar that has all its weight at the edge is moving faster than one with weight at the centre. It will take more kinetic energy for the jar with its weight at the edge to go at teh same speed as the jar with weight at the center. This kinetic energy can only come from the jar falling, so the jar has to fall further to go at the same speed, so it accelerates more slowly.

Written by Dave Ansell