# Collisions

## Introduction

A collision is an event in which two or more bodies exert forces on each other for a relatively short time. Although the most common colloquial use of the word "collision" refers to accidents in which two or more objects collide, the scientific use of the word "collision" implies nothing about the magnitude of the forces.

## 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

## Methods

1. Attach the piece of A4 paper to the benchtop with masking tape. Rule two parallel lines on the paper, 20 cm apart.
2. Link the two trolleys with the rubber bands.
3. Pull the trolleys apart and hold them with their front ends on the two lines.
4. Release the trolleys. The trolleys will accelerate towards each other and collide at the same time. How far the trolleys travel in a given time is proportional to their relative velocities. Determine where the trolleys collide and mark the collision point on the paper.
5. Measure the distance from one line to the collision point ( d1 ) and the same for the other line ( d2 ). Because the trolleys both collide at the same time, there is no need to measure the times because the distances are proportional to the collision speeds.
6. Add various masses to one (or both) of the trolleys and repeat the experiment. Test approximately five different mass combinations.

## Results

• m1 = 0.2kg
• d1 = 0.1m
• m1 × d1 = 0.02
• m2 = 0.2kg
• d2 = 0.1m
• m2 × d2 = 0.02
• ( m1 × d1 ) – ( m2 × d2 ) = 0kg m/s

## Discussion

• When the trolleys are released, do they travel towards each other for the same period of time? Explain.
• Yes. Because they have the same weight.

• Is the magnitude of the force acting on each trolley the same? Explain.
• Yes. Because they have the same mass and acceleration.

• If both trolleys come to a stop after the collision, what was the final total momentum of the ‘system’? Explain.
• 0. Because the momentum before an action and after the action is the same.

• According to the last column of results, what was the initial total momentum of the ‘system’?
• 0kg m/s

## Conclusion

This experiment demonstrates that the total momentum before and after a collision stays the same because of the law of conservation