Inertia on a low friction surface
Practical Activity for 14-16
This seemingly simple activity makes fundamental points about force and related quantities.
Apparatus and Materials
- Smooth surface with frame, e.g. mounted glass plate
- Mass, 1 kg
- Mass, 0.5 kg
- Gas jar lid
- Steel ball bearings, small, to to 2,000
Health & Safety and Technical Notes
It would be best to avoid using polystyrene beads as an alternative to ball bearings. These are now classified as a dangerous substance because of the pressure generated when they expand.
If the glass plate is one metre square, it will require two persons to lift it onto the bench.
Ensure that the beads are not scattered on the floor.
A mounted glass plate provides a low friction surface, but other materials will serve. The surface must have a frame to contain the ball bearings. (Strictly these are the balls used to make ball bearings.
The number of ball bearings required depends on the size of the surface. There should be enough to spread quite thinly on the surface, so that the mass on the gas jar lid can move with little frictional resistance.
The ball bearings should be about 2 mm in diameter.
- Make a layer of ball bearings quite thinly spread on the flat surface, to reduce the frictional forces.
- Stand the 0.5 kg mass on the gas jar lid and rest it on the ball bearings. A little blu-tack may help it to stick.
- Push the mass gently.
- Push the mass harder and note the difference in behaviour.
- Replace the 0.5 kg mass with the 1.0 kg mass. Repeat the actions and note the differences in how it feels and how it behaves.
- Try pushes on moving masses.
- Do this as a participative demonstration. Ask your students to move the masses.
- This simple activity makes fundamental points about force and related quantities. The points are relevant both at introductory and at advanced levels, and can be summarized as:
- Force, when it is not
balancedby other forces, is what produces change in motion (acceleration)
- Mass is what resists change in motion (acceleration)
- The tendency of a body to resist acceleration is called its inertia.
- Thus, all in one go, you have working definitions of both force and inertial mass. (There is an additional
paralleldefinition of mass: Gravitational mass is what allows a body to exert and experience gravitational force. To see how the two definitions of mass relate to each other you would have to consider Einstein's theory of general relativity.)
- Emphasize that it is not just the
startof motion that is resisted by masses, but
anychange in motion, including slowing down and stopping, or changing direction.
- The activity provides a suitable introduction to Newton's first law.
This experiment was safety-tested in March 2005