Collisions (AQA GCSE Physics): Revision Note

If you are asked to analyse a collision, make sure to consider:

• The motion of each object before and after the collision

• The speed and direction of each object

An object of mass 1 kg is travelling at 3 m/s when it collides with a heavier object of mass 2 kg. The two objects stick together and travel off as one. Calculate the combined velocity of the objects after the collision.

Answer:

Step 1: Draw a diagram

Step 2: State the principle of conservation of momentum

The total momentum before a collision = The total momentum after a collision

Step 3: Calculate the momentum before the collision

• Before the collision, only the 1 kg object has any momentum

p_before = mv = 1 × 3 = 3 kg m/s

Step 3: Determine the momentum after the collision

• The combined mass is now 1 + 2 = 3 kg

p_after = mv = 3 × v

Step 4: Substitute values into conservation equation

p_before = p_after

3 = 3 × v

Step 5: Rearrange for the combined velocity v

v = 3 ÷ 3 = 1 m/s

Always double-check the signs (positive or negative) for the velocity in your answers, as this is the most common type of calculation error!