Concepts you need to remember for IGCSE Physics: 

• momentum = mass x velocity
• When there is no external force acting on the system, the momentum of the system is conserved.
• Force = change in momentum / time

Example 1

A raindrop of mass 0.000035kg falls to the group. The speed of the raindrop as it hits the ground is 8.8m/s. Calculate the momentum of the raindrop as it hits the ground.

 momentum = 0.000035 x 8.8 = 3.08 x 10^-4 kg·m/s

Example 2

A boy of mass 43.2kg runs and jumps onto a stationary skateboard. The boy lands on the skateboard with a horizontal velocity of 4.10m/s. The skateboard has a mass of 2.50kg. Using ideas about conservation of momentum, calculate the combined velocity of the boy and skateboard just after the boy lands on it

• Since momentum is conserved, momentum right before and after the boy jumps onto the skateboard should be the same.
• Initial momentum = momentum of boy = 43.2 x 4.10 = 177 kg·m/s
• Final momentum = momentum of boy and skateboard = (43.2 + 2.50) x v = 45.7v
• 177 = 45.7v
• v = 177 / 45.7 = 3.87 m/s

Example 3

The boy holds a heavy ball as he stands on a stationary skateboard. The boy throws the ball forwards while still standing on the skateboard. Explain what happens to the boy and the skateboard.

• Momentum is conserved for the system, which consists of the ball, boy and skateboard.
• Before the boy throws the ball, the momentum is 0 because the ball, boy and skateboard are all stationary.
• Therefore, the momentum after the boy throws the ball should also be 0.
• Final momentum = momentum of ball + momentum of boy and skateboard
• Since the ball is travelling forwards, the boy and skateboard should move backwards for the momentum to become 0.

Example 4

A car travels at 20m/s. The mass of the car is 1500kg. 

(a) Calculate the momentum of the car.

momentum = 1500 x 20 = 3.0 x 10⁴ kg·m/s

(b) The car crashes into a wall and stops in 0.14s. Calculate the average force on the car during the crash.

force = 3.0 x 10⁴ / 0.14 = 2.1 x 10⁵ N

Example 5

An ice skater wears soft knee pads that compress easily. Explain how the pads protect her knees when she falls on the ice.

• When compared to the ice skater falling directly on knees, falling with knee pads increases the time of impact.
• The change of momentum does not change for both cases.
• Force = change of momentum / time
• With soft knee pads, the force on the knees is reduced.

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