Ben Craven's Gravity Demo

What you need

  • A jar (plastic is safer).
  • Small circle of wood, about twice the width of the jar.
  • A drill to make holes in the wood.
  • Strong string.
  • Water.
  • A handle (e.g. small hand weight, coins in a toilet roll in a sock, lead weight (!)).

What you do

  1. Make a platform for your jar by drilling three holes at even intervals around the edge of the circle of wood.
  2. Cut three 60cm long pieces of string. Attach the string through each hole tying them securely. Then tie them together at around 40cm above your platform leaving the remaining string to attach to your handle. Attach the three strings to your handle - it needs to be secure otherwise a flying handle could take your head off, and spill your water!
  3. Put your jar on the platform and fill with water.
  4. Hold the equipment by the handle and start easily by simply spinning the jar / water above your head. As you gain more confidence you can experiment with letting go and, for the very advanced, catching it again.

What's going on?

This is an advanced example of the spinning bucket of water. As a bucket of water is spun the bucket experiences a force towards the centre, provided by the person at the centre (if the person lets go, the force vanishes and bucket flies away in a straight line as dictated by Newton's First Law). This unbalanced force creates an acceleration towards the centre.

The faster the bucket swings the more force the swinger has to exert to keep the bucket along its path and the greater the acceleration.

At the top of the motion the water in the bucket would like to fall out under the influence of gravity. Luckily the bucket, pulled by the person holding the handle, is accelerating towards the centre of the motion faster than gravity is making the water accelerate out of the bucket. As the water tries to fall out, the bucket catches up and the water stays firmly inside. If the bucket is moving too slowly the water will start to fall under gravity and will cause an embarrassing accident.

In the case of the jar of water sitting on a wooden platform, the person holding the strings is providing the wooden platform with a force towards the centre. This force creates the acceleration of the wooden platform which, when greater than the acceleration due to gravity, keeps the water and the jar on the platform.

With the jar of water spun in the air, the person is replaced by a heavy handle. When thrown into the air with a spin, the handle is trying to go one way and the jar of water is trying to go the other - in a straight line according to Newton's First Law. But because they are attached they can't just fly off and are locked in a circular motion with the string tense between them. As long as this tension - the force on the string - is maintained, the acceleration of the wooden platform should be greater than that of the water and the water stays in place.

Special Safety advice

You will be spinning a jar of water, lump of wood and heavy weight into the air and attempting to catch it. Only adults should do this, preferably with head protection.