Home · IGCSE 0625 · Motion, forces, energy · Impulse and force · Practice
Extended · Practice questions · Impulse and force

Force is a rate.

Six original Cambridge-style questions on force as the rate of change of momentum, calculating force from a momentum change, rebounds, and explaining cushioning safety features.

Original questions All questions on this page are original work, written in the Cambridge IGCSE style. They are not from past papers. They test the same concepts and skills the syllabus rewards.
One equation, many uses

F = Δp ÷ t.

01
Analysis
[2 marks]

State what the resultant force on an object is equal to, and write the equation linking force, change in momentum and time.

  • The resultant force equals the rate of change of momentum. ✓
  • F = Δp ÷ t (= (mv − mu) ÷ t). ✓
02
Calculation
[2 marks]

The momentum of an object changes by 24 kg m/s in a time of 0.40 s. Calculate the average resultant force.

F = Δp ÷ t = 24 ÷ 0.40

F = 60 N

03
Calculation
[3 marks]

A 2.0 kg trolley speeds up from 3.0 m/s to 8.0 m/s in 2.0 s. Calculate the resultant force acting on it.

Δp = m(v − u) = 2.0 × (8.0 − 3.0) = 10 kg m/s F = Δp ÷ t = 10 ÷ 2.0

F = 5.0 N

04
Analysis
[3 marks]

Explain, using the idea of change of momentum, how the crumple zone of a car reduces the force on the passengers in a crash.

  • The change in momentum is fixed by the mass and speed of the car. ✓
  • The crumple zone folds, increasing the time over which the car stops. ✓
  • Since F = Δp ÷ t, a longer time gives a smaller force on the passengers. ✓
05
Calculation
[3 marks]

A 0.20 kg ball hits the floor moving downward at 5.0 m/s and bounces straight up at 3.0 m/s. The contact with the floor lasts 0.10 s. Taking upward as positive, calculate the average force the floor exerts on the ball.

Δp = m(v − u) = 0.20 × (3.0 − (−5.0)) Δp = 0.20 × 8.0 = 1.6 kg m/s F = 1.6 ÷ 0.10

F = 16 N upward

downward velocity is −5.0, so the change is 8.0, not 2.0

06
Analysis
[2 marks]

Two identical eggs are dropped from the same height. One lands on a hard tiled floor and breaks; the other lands on a soft cushion and survives. Explain the difference in terms of force and time.

  • Both eggs have the same change in momentum on landing. ✓
  • The cushion increases the stopping time, so F = Δp ÷ t gives a smaller force, low enough not to break the egg. ✓