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Forces and momentum

Relate resultant force to motion and explore momentum, impulse and conservation in simple interactions.

12 free questions · Notes · Worked solutions

Practise this topic

What you will practise

  • Use resultant force and Newton's second law.
  • Distinguish mass from weight and calculate weight using g=10 N/kg.
  • Calculate momentum and impulse.
  • Apply conservation of momentum to simple one-dimensional collisions.

Revision notes

Resultant force

The resultant force is the vector sum of forces. Newton's second law is F=ma for constant mass; acceleration is in the direction of the resultant force. If resultant force is zero, velocity is constant.

Mass and weight

Mass is measured in kilograms. Weight is a gravitational force: W=mg. In these questions use g=10 N/kg when stated; weight is measured in newtons.

Momentum and impulse

Momentum p=mv. Impulse equals change in momentum and also force×time for a constant resultant force. In a closed system, total momentum is conserved.

Common checks

Include direction or sign in one-dimensional momentum calculations. A larger force does not necessarily mean a larger acceleration if mass also changes.

A worked example

A 2 kg trolley moving at 3 m/s collides with a stationary 1 kg trolley. They join together. Find their common speed, assuming momentum is conserved.

  1. Initial momentum=2×3+1×0=6 kg m/s.
  2. Combined mass=2+1=3 kg.
  3. Final speed=6÷3=2 m/s.

2 m/s

Try it yourself

These are original practice questions, not official past-paper questions. Practice points count correct final answers; working is not automatically assessed.

Choose a question to begin.
Question 1 · Standard1 practice point

A 4 kg object accelerates at 3 m/s². Find the resultant force.

Need a hint?

Use F=ma.

Worked solution
  1. F=4×3=12 N.
Question 2 · Standard1 practice point

What is the weight of a 5 kg object when g=10 N/kg?

Need a hint?

Weight=mass×gravitational field strength.

Worked solution
  1. W=5×10=50 N.
Question 3 · Standard1 practice point

Find the momentum of a 3 kg ball moving at 4 m/s.

Need a hint?

Momentum=mass×velocity.

Worked solution
  1. p=3×4=12 kg m/s.
Question 4 · Standard1 practice point

A 2 N force acts for 5 s. What impulse does it provide?

Need a hint?

Impulse=force×time.

Worked solution
  1. Impulse=2×5=10 N s.
Question 5 · Foundation2 practice points

A 6 kg box is pushed with a resultant force of 18 N. Find its acceleration.

Need a hint?

Rearrange F=ma.

Worked solution
  1. a=F/m=18/6.
  2. a=3 m/s².
Question 6 · Foundation1 practice point

Find the weight of a 7.2 kg bag using g=10 N/kg.

Need a hint?

Use W=mg.

Worked solution
  1. W=7.2×10.
  2. W=72 N.
Question 7 · Standard2 practice points

A 0.5 kg ball moves at 12 m/s. Find its momentum.

Need a hint?

Momentum equals mass times velocity.

Worked solution
  1. p=mv=0.5×12.
  2. p=6 kg m/s.
Question 8 · Stretch3 practice points

A 10 N force acts on a stationary 2 kg cart for 3 s. Find the cart's final speed.

Need a hint?

Find acceleration, then use change in velocity=at.

Worked solution
  1. a=F/m=10/2=5 m/s².
  2. v=u+at=0+5×3.
  3. v=15 m/s.
Question 9 · Foundation1 practice point

A 1.5 kg object moves at 8 m/s. Find its momentum.

Need a hint?

Multiply mass by velocity.

Worked solution
  1. p=1.5×8.
  2. p=12 kg m/s.
Question 10 · Stretch3 practice points

A 0.2 kg ball changes velocity from 10 m/s forwards to 4 m/s forwards. Find its change in momentum.

Need a hint?

Take final momentum minus initial momentum; forward is positive.

Worked solution
  1. Initial p=0.2×10=2 kg m/s.
  2. Final p=0.2×4=0.8 kg m/s.
  3. Change=0.8−2=−1.2 kg m/s.
Question 11 · Stretch3 practice points

A 2 kg trolley moving at 5 m/s collides with a stationary 3 kg trolley. They stick together. Find their final speed, assuming momentum conservation.

Need a hint?

Initial momentum is shared by the combined 5 kg mass.

Worked solution
  1. Initial momentum=2×5+3×0=10 kg m/s.
  2. Combined mass=5 kg.
  3. Final speed=10/5=2 m/s.
Question 12 · Standard2 practice points

A resultant force of 24 N produces acceleration 4 m/s². Find the object's mass.

Need a hint?

Rearrange F=ma to find m.

Worked solution
  1. m=F/a=24/4.
  2. m=6 kg.

Coverage and sources

Selected IGCSE concepts, with a mix of introductory and more demanding questions. Check your own specification. AI drafted; independent teacher review is pending. Editorial details.

Cambridge IGCSE Physics 0625 syllabus

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