# Collisions between balls

##### Demonstration

Further collisions similar to snooker balls colliding.

#### Apparatus and materials

Flexible curtain-rail

Steel balls (or glass marbles), 6

#### Health & Safety & Technical notes

Support the curtain-rail by gluing a 70 cm wooden lath to each end of the underside of it. One end is conveniently held with a retort stand and clamp.

Set up the plastic curtain-rail with its main section horizontal but one end inclined slightly.

To make a Newton's cradle, hang each steel ball by a bifilar suspension. Each ball must have two small hooks soldered to it and the suspensions must be adjusted so that the balls all hang in line at the same level. If any ball is displaced laterally from the rest, or is too high or too low, the demonstration fails. The apparatus is a delight to play with, but much play is apt to upset the adjustment.

#### Procedure

a Let one of the balls roll down the slope to hit another ball on the horizontal section and observe what happens.

b Repeat with several balls on the horizontal section.

c As an alternative demonstration, use a line of steel balls as in Newton’s cradles. Use a commercially-produced version if at all possible, as home-made versions are notoriously tricky to adjust successfully.

d Students might try a simple form of this with a line of pennies, but that does not deserve much time. If they have some form of runway for common marbles, they can try this experiment on their own with marbles.

#### Teaching notes

1 The rolling ball might upset the simple collision story because of its angular momentum.

2 Behind the simple story of the momentum of the impinging ball travelling through a line of balls and sending the front ball of the line forward there is a complicated story of the propagation of a compression wave through a ball. The complete transfer of momentum from one ball to the next is even more surprising when one thinks of it in terms of compression waves.