Collisions (AQA GCSE Physics)

• Examples of momentum in an event are collisions
• Objects will either:
  • Collide and move in opposite directions - this is an elastic colision
  • Collide and move in the same direction together - this is an inelastic collision

• When the objects move in opposite directions:
  • Each object will have a different velocity depending on its mass and initial momentum of the system

• When the objects move in the same direction together:
  • They will have a combined mass and velocity

• Momentum is always conserved in a collision

Types of collisions

• Calculations involving collisions use the conservation of momentum to determine the velocity of an object (or objects) before or after the collision
• This means the momentum before the collision must equal the momentum after the collision for momentum to be conserved

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