Collisions (AQA GCSE Physics)

If an exam question asks you to analyse a collision, follow these tips for full marks:

• Always consider the motion before and after the collision and state:

  • The velocities of each object

  • The direction each object moves

• State whether the collision was elastic or inelastic and explain your reasoning

  • In a perfectly elastic collision, the kinetic energy is the same before and after

  • In a perfectly inelastic collision, the two objects stick together after colliding

• Describe any energy transfers that occur if kinetic energy is not conserved

  • For example, it may be converted into heat, sound, elastic potential energy etc

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!