Conservation of Momentum

OCR

GCSE

The principle of conservation of momentum states that in a closed system, the total momentum before an event is equal to the total momentum after the event. This fundamental law applies to all interactions, including collisions and explosions, necessitating the treatment of momentum as a vector quantity where direction is indicated by sign. Candidates must apply the equation p=mv to calculate unknown velocities or masses, recognizing that while momentum is always conserved in a closed system, kinetic energy is not conserved in inelastic collisions. Mastery requires linking this concept to Newton's Third Law, demonstrating how equal and opposite forces result in equal and opposite changes in momentum.

Objectives

Exam Tips

Pitfalls

Key Terms

Mark Points

What You Need to Demonstrate

Key skills and knowledge for this topic

Award 1 mark for explicitly stating that total momentum before equals total momentum after in a closed system
Credit the correct calculation of initial momentum for each object using p = m × v
Award marks for the correct algebraic rearrangement to isolate the unknown velocity
Deduct credit if the vector nature of velocity is ignored (e.g., failing to assign a negative sign to a trolley moving in the opposite direction)
For explosion scenarios, award credit for equating the sum of final momenta to zero

Example Examiner Feedback

Real feedback patterns examiners use when marking

"You correctly calculated the momentum, but check your signs — did you account for the object moving in the opposite direction?"
"Excellent use of the conservation principle. To secure full marks, explicitly state 'total momentum before = total momentum after'."
"Remember to convert grams to kilograms at the start of your calculation to avoid errors later."
"For this explosion question, start by stating that the initial momentum is zero."

Marking Points

Key points examiners look for in your answers

Award 1 mark for explicitly stating that total momentum before equals total momentum after in a closed system
Credit the correct calculation of initial momentum for each object using p = m × v
Award marks for the correct algebraic rearrangement to isolate the unknown velocity
Deduct credit if the vector nature of velocity is ignored (e.g., failing to assign a negative sign to a trolley moving in the opposite direction)
For explosion scenarios, award credit for equating the sum of final momenta to zero

Examiner Tips

Expert advice for maximising your marks

💡Always sketch a 'Before' and 'After' diagram, clearly labeling the positive direction to avoid sign errors
💡For explosion questions (recoil), remember the initial total momentum is zero; set the sum of final momenta to zero
💡When asked to explain safety features (crumple zones), link the change in momentum to the increase in collision time and reduced force (F = Δp / Δt)

Common Mistakes

Pitfalls to avoid in your exam answers

Neglecting the vector nature of momentum by treating velocity as a scalar, resulting in addition rather than subtraction of momenta for opposing objects
Failing to convert mass from grams to kilograms before calculating momentum, leading to order-of-magnitude errors
Confusing the conservation of momentum with the conservation of kinetic energy; incorrectly assuming kinetic energy is conserved in inelastic collisions

Study Guide Available

Comprehensive revision notes & examples

Read Study Guide

Key Terminology

Essential terms to know

Conservation in closed systems

Vector nature of momentum (direction and sign)

Elastic versus inelastic collisions

Explosions and recoil effects

Relationship to Newton's Second and Third Laws

Likely Command Words

How questions on this topic are typically asked

Calculate

Explain

Describe

Show

Determine

Practical Links

Related required practicals

•{"code":"PAG P3","title":"Investigation of collisions","relevance":"Using light gates and trolleys to verify conservation of momentum"}

Ready to test yourself?

Practice questions tailored to this topic