# Trolley collisions

##### Demonstration

Collisions between trolleys that result in energy being dissipated to the surroundings.

#### Apparatus and materials

Dynamics trolleys, 2

Elastic cords, 2

Large pin and cork

#### Health & Safety and Technical notes

In place of the elastic cords, a weak spiral spring of good steel wire can be used.

#### Procedure

a Place the two trolleys on the bench with one or two elastic cords held between them, secured over a wooden dowel post on each of the trolleys. Start with the trolleys close together and the elastic slack. With a hand on each trolley, give them outward motions and let go so that they move apart. The elastic will stretch and bring them to rest for an instant. At this point, hold the trolleys in that position and ask where the energy has gone.

Release the trolleys so that most of the kinetic energy is regained.

b Now start with the trolleys well apart and the elastic stretched. Release them so that the trolleys meet with a bang. Again ask where the energy has gone. Repeat with a large pin sticking out of the end of one trolley and a cork attached to the other so that the trolleys stick together on collision. (To fix the cork, it may be easier to fix a pin on each trolley and then fix a cork on one of the pins beforehand.)

c Repeat b using the trolleys with the sprung buffer-rods protruding from them. When the trolleys collide, they will now rebound apart, be stopped by the elastic, return and collide again. This process will be repeated several times before they finally come to rest. Again, discuss where the energy has gone.

#### Teaching notes

1 When the trolleys are moved apart their kinetic energy is transferred to strain (elastic) energy in the cords connecting them. Releasing the trolleys, the energy in the stretched cord is transferred back into kinetic energy of the trolleys. When the two trolleys collide they come together with a 'bang' and have no kinetic energy left. The kinetic energy is not conserved. Collisions of this type are called 'inelastic'. The energy has been carried away by sound waves and dissipated by warming up, and maybe deforming, the colliding trolleys.

2 Spring buffers between the trolleys: As the trolleys collide their kinetic energy is transferred into elastic energy of the springs as the trolleys briefly come to rest. The process reverses and the energy in the compressed spring is transferred back to the trolleys so they regain their kinetic energy. The trolleys will oscillate backwards and forwards a few times. Ultimately, the trolleys will come to rest as all their kinetic energy is transferred to the environment to warm it up.

3 In some collisions, the kinetic energy before and after is exactly the same. These are called 'elastic' collisions. Collisions between molecules in air are elastic, for example.

This experiment was safety-checked in November 2005