Forces and Motion

Adding mass to a moving system

Practical Activity for 14-16 PRACTICAL PHYISCS

Class practical

Another example of an inelastic interaction.

Apparatus and Materials

For each student pair

  • Dynamics trolleys, 2
  • Runway, friction-compensated
  • Ticker-tape vibrator

Health & Safety and Technical Notes

The runways are heavy and long. They need to be handled with care to avoid damage to nearby students or equipment. Runways are best lifted into position by 2 people. Place a barrier to ensure the trolleys do not roll off the end of the bench.

Read our standard health & safety guidance

The sudden placing of the second trolley on top of the moving one requires some practice. It is probably better to tie a cradle of string around the second trolley and lower it by a single string attached to the cradle.

If a second trolley is not available, use a brick instead; but then the mass of the brick and trolley must be known.


  1. Attach ticker-tape to a trolley. Push the trolley so that it runs along the runway at constant velocity.
  2. About halfway along the track gently drop a second trolley, which has been held at rest in mid-air onto the moving trolley from just above. Use the ticker-tape to obtain estimates of speeds.
  3. Calculate the momentum before and after the addition, to see whether it is conserved.

Teaching Notes

Although the added trolley brings in and loses some vertical momentum, it contributes no horizontal momentum but only changes the mass of the moving system. Where does the vertical momentum go? It is transferred to the Earth.

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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