Collisions | Physics for Grade 10 PDF Download

Introduction

• 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

Exam Tips: 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

Calculations Involving 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

Solved Example

Example: 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.

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
pbefore = mv = 1 × 3 = 3 kg m/s

Step 3: Determine the momentum after the collision
The combined mass is now 1 + 2 = 3 kg
pafter = mv = 3 × v

Step 4: Substitute values into conservation equation
pbefore = pafter
3 = 3 × v

Step 5: Rearrange for the combined velocity v
v = 3 ÷ 3 = 1 m/s

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