Collisions Practical Report

by Kobi Snow-Abeyratne


To investigate whether total momentum is conserved during a collision.


Momentum is the motion of an object, with the equation to find the momentum of an object being the mass of the object (in kg) multiplied by the velocity (in m/s). The overall measurement of momentum is kg m/s. Momentum has a law of conservation in which the momentum preceding an event is equal to the momentum succeeding that same event.


Two dynamics trolleys

Electronic balance

Metre ruler

Several 1 kg masses to add to the trolleys

Two rubber bands tied together that will stretch to 20 cm quite easily

Level bench top

Piece of A4 paper

Masking tape


  1. The piece of A4 paper was attached to the bench top with masking tape. Two parallel lines were ruled, 20cm apart.

  2. The two trolleys were linked with the rubber bands.

  3. The trolleys were pulled apart and their front ends were held on the two lines.

  4. The trolleys were released. The trolleys accelerated towards each other and collided at the same time. The distance the trolleys travelled in the given time was proportional to their relative velocities. The place where the trolleys collided were determined and the collision point was marked on the paper.

  5. The distance was measured from one line to the collision point (d1) and the same for the other line (d2). As the trolleys both collided at the same time, there was no need to measure the times as the distances were proportional to the collision speeds.

  6. Various masses to one (or both) of the trolleys were added and the experiment was repeated. Approximately five different mass combinations were tested.



m1 = 0.2kg

d1 = 0.13m

m1 x d1 = 0.026

m2 = 0.2kg

d2 = 0.07m

m2 x d2 = 0.014

(m1 x d1) - (m2 x d2) = 0.026 - 0.014 = 0.012


m1 = 0.2kg (trolley) + 0.5kg (weights) = 0.7g

d1 = 0.11m

m1 x d1 = 0.077

m2 = 0.2kg (trolley) + 0.5kg (weights) = 0.7g

d2 = 0.09m

m2 x d2 = 0.063

(m1 x d1) - (m2 x d2) = 0.077 - 0.063 = 0.014


When the trolleys are released there are a variety of factors that determine whether both trolleys will travel towards each other for the same period of time. This can occur if both trolleys have the same velocity, distance apart, weight, acceleration and release time. The magnitude of the force acting on each trolley should also be the same if they have the same acceleration and same mass, which should occur if an equal amount of weights were added and the trollies were released at the same time from the same distance.

As both trolleys were not moving at the end after the collision they would have no momentum (0 kg m/s). According to the results the initial total momentum of the system without the added mass was 0.012 kg m/s and with the added mass the ITM was 0.014 kg m/s.