Exam Tips: If an exam question asks you to analyse a collision, follow these tips for full marks:
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 collisionStep 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/sStep 3: Determine the momentum after the collision
The combined mass is now 1 + 2 = 3 kg
pafter = mv = 3 × vStep 4: Substitute values into conservation equation
pbefore = pafter
3 = 3 × vStep 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!
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