Galileo's rolling ball

Demonstration

Roll a ball on a curved track to explain Galileo's idea, which in turn led to Newton's First Law of Motion.

Apparatus and Materials

• Large ball bearing (or large marble)
• Retort stands, 2
• Flexible curtain rail
• Boss
• Clamp
• G-clamps, 2

Health & Safety and Technical Notes

The risks here are trivial provided the instructions are followed. If the ball is turned into a projectile, consider the use of eye protection.

Read our standard health & safety guidance

The flexible rail should be symmetrical and not too flimsy. An alternative to a curtain-rail is the flexible track used with Hot wheels toy cars.

A good way of supporting the curtain rail is to glue a 0.5 m wooden lath (1 cm square) to each end of the underside of the curtain rail. One end can be held with a retort stand and clamp, at a height of about 30 cm above the bench. The other end can be held in another retort stand or you may prefer to hold it by hand.

It is very important for the support to be rigid if the experiment is to be effective. Avoid energy dissipated by the rail moving.

Procedure

1. Hold the ball bearing at the top of one end of the curtain rail and release it so that it rolls down one side and then up the other. Release the ball from each end in turn to see if any difference occurs.
2. Tilt the curtain rail to various slopes, both equal and unequal and repeat the demonstration The experiment may also be tried with a horizontal length between the two slopes. Finish with a slope on one side and the other side horizontal.

Teaching Notes

• When the ball is at the top of the track energy is stored gravitationally. When the ball is at the bottom of the track the energy is now stored kinetically. If no energy is dissipated through friction, the ball would return to its orginal height on the far side of the track, and briefly be at rest. This remains true regardless of the shape and slope of the track. In practise, energy will be dissipated. The ball will warm up, as will the track, and also sound will be produced.
• When the steel ball rolls down the slope and along a horizontal level it continues at uniform speed (as long as there was no frictional force) because there is no force to do work on it. There is no change to the energy stored kinetically. It would go on for ever. This idea led Galileo to what we now call Newton's First Law:
'If a body is at rest it remains at rest or if it is in motion, it moves with uniform velocity until it is acted upon by a resultant force.'
• Galileo also used this experiment as a starting point for his theory of forces and motion on an inclined plane, which gave a hint towards Newton's Second Law F = ma.
• If students are genuinely willing to blame friction for the failure of the ball-bearing to reach its original height, the demonstration will have worked. If they accept the excuse because you tell them to, the demonstration is probably not worth pursuing.
• See also the collection on this site:

  Inertia and Newton's First Law

This experiment was safety-tested in November 2005