# Collisions

## Aim

To investigate whether total momentum is conserved during a collision.

## Materials

Two dynamics trolleys Electronic balance Metre ruler
Ruler

Several 1 kg masses to add to the trolleys

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

Level benchtop Piece of A4 paper Masking tape

## Method

1. A piece of A4 paper was attached to the benchtop with masking tape. Two parallel lines were ruled on the paper, 20 cm apart.

2. Two trolleys were linked with the rubber bands.

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

4. The trolleys were released. The trolleys accelerated towards each other and collided at the same time. How far the trolleys traveled in a given time is proportional to their relative velocities. Where the trolleys collided was determined and the collision point on the paper was mark.

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

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

## Discussion

1 When the trolleys are released, do they travel towards each other for the same period of time? Explain.

2 Is the magnitude of the force acting on each trolley the same? Explain.

3 If both trolleys come to a stop after the collision, what was the final total momentum of the ‘system’? Explain.

4 According to the last column of results, what was the initial total momentum of the ‘system’?

## Results

• m1 = mass of trolley 1 plus any 1 kg masses added

(total in kg)

• d1 (in m)

• md1

• m2 = mass of trolley 2 plus any 1 kg masses added

(total in kg)

• d2 (in m)

• md2

• (m1 × d1) – (m2 × d2)

The final column gives a measure of the total momentum of the two trolleys just prior to the collision. The minus sign is used because the two trolleys are travelling in opposite directions and therefore one momentum is negative.