Momentum
Forces and Motion

Collisions on an air-track

Practical Activity for 14-16 PRACTICAL PHYISCS

Alternative experiments to show momentum interchanges.

Apparatus and Materials

  • Air-track and accessories
  • Air blower
  • Multiflash equipment
  • Xenon strobe or regularly flashing light
  • Camera
  • Background

Health & Safety and Technical Notes

Avoid flash frequencies between 15-20 Hz and avoid red flickering light, which makes some people feel unwell. Rarely, some people can experience photosensitive epilepsy.

Read our standard health & safety guidance


A good matt-black background is essential.

The air-track comes with an accessories kit and an instruction booklet. Many momentum experiments done with trolleys can be done with an air-track.

The track should be carefully levelled and tested by running the vehicle along the track.

The air-track vehicles are lighter than trolleys; in some experiments this is a critical factor. Ticker-tape is not suitable, but multiflash photography or light gates can be used to measure the time intervals.

For multiflash photography, shine the xenon strobe along the track and place the camera halfway along the track pointing at right angles to it. See the guidance note:

Multiflash photography


With light gates, a cardboard mask will need to be attached to the vehicle to measure the time, t, for which the light is cut off. Using the length of the mask, s, then the speed of the vehicle can be calculated as v = s/t.

An aluminium foil-covered rod needs to be attached vertically to a vehicle to reflect the light so that the multiflash photograph can mark the position of the vehicle at time intervals decided by the strobe.

An alternative method for illuminating the vehicle is to build a flashing light system and attach it to the vehicle. Then take photographs with an open camera.

Procedure

  1. Show what happens when vehicles of equal mass collide
    • One initially at rest
    • Travelling at the same speed but in opposite directions
  2. Show what happens in a collision when one vehicle is more massive than the other.
Momentum
appears in the relation p=mv F=dp/dt λ=h/p ΔxΔp>ℏ/2
has the special case Angular Momentum
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