Welcome to practical physicsPracticle physics - practical activities designed for use in the classroom with 11 to 19 year olds

Collisions with coins

Class practical

A simple experiment to illustrate the conservation of momentum.

Apparatus and materials

Paper or thin cardboard

Coins, 2 identical

Health & Safety and Technical notes

Read our standard health & safety guidance

Make a launching ramp from an exercise book cover or some other thin sheet of cardboard. Prop it up against some books on the table to make a curving ramp, of smaller and smaller slope until it becomes horizontal when it reaches the table. 



a Choose a standard starting-place at the top of the ramp and allow a coin to slide down the ramp and out along the level table. It will travel some distance, decelerating before it is brought to rest by friction. Note the horizontal distance it travels along the table before coming to a stop. 

b Repeat the experiment with another coin placed at the bottom of the ramp. There is a collision and the two coins move along the table until friction brings them to a stop. Again measure the distance travelled.

Teaching notes

1 Students could do this experiment at home. 

2 Measurements of distance travelled are used to indicate the velocities just before and after collision. However the velocities are not directly proportional to those distances. You could pose the relationship between velocity and distance as a problem. 
3 If the coins have rotational momentum, a simple calculation of linear momentum will not illustrate momentum conservation.