A-Level Physics Revision — Momentum

Revise Momentum for A-Level Physics with a topic explanation, worked example and common mistakes. Check the board notes for specification differences.

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Topic explanation

What is Momentum?

Momentum is a measure of an object's motion, defined as the product of its mass and velocity. This topic introduces the concept of linear momentum and the crucial principle of conservation of momentum, which states that the total momentum of an isolated system remains constant. You will also study impulse, which is the change in momentum of an object and is equal to the product of the force and the time for which it acts.

Board notes: Momentum is a key topic in all A-Level Physics specifications (AQA, Edexcel, OCR). All boards require a thorough understanding of the conservation of momentum in one and two dimensions, as well as the concepts of impulse and its relation to force-time graphs. The distinction between elastic and inelastic collisions is also a common focus.

Step-by-step explanation

Worked examples

Worked example

A 1000 kg car travelling at 20 m/s collides with a stationary 1500 kg car. They stick together after the collision. To find their common velocity, we use conservation of momentum: (m1u1) + (m2u2) = (m1+m2)v. So, (1000 * 20) + (1500 * 0) = (1000 + 1500)v. This gives 20000 = 2500v, so v = 20000 / 2500 = 8 m/s. Their common velocity after the collision is 8 m/s.

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Common mistakes

1Forgetting that momentum is a vector quantity. When solving problems, especially in two dimensions, momentum must be resolved into components, and the direction is crucial.
2Confusing elastic and inelastic collisions. In an elastic collision, both momentum and kinetic energy are conserved. In an inelastic collision, momentum is conserved, but kinetic energy is not (it is converted into other forms like heat and sound).
3Misinterpreting force-time graphs. The area under a force-time graph represents the impulse (change in momentum), not the final momentum or the force itself.

Momentum exam questions

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Frequently asked questions

What is impulse?
Impulse is the change in momentum of an object. It is calculated as the product of the force acting on the object and the time interval over which the force acts (Impulse = FΔt). It is also the area under a force-time graph.
What is the difference between an elastic and an inelastic collision?
In both types of collisions, momentum is conserved. However, in an elastic collision, kinetic energy is also conserved. In an inelastic collision, some kinetic energy is lost to other forms, such as heat or sound.

What is Momentum?

Step-by-step explanation

Worked examples

Worked example

Common mistakes

1Forgetting that momentum is a vector quantity. When solving problems, especially in two dimensions, momentum must be resolved into components, and the direction is crucial.

2Confusing elastic and inelastic collisions. In an elastic collision, both momentum and kinetic energy are conserved. In an inelastic collision, momentum is conserved, but kinetic energy is not (it is converted into other forms like heat and sound).

3Misinterpreting force-time graphs. The area under a force-time graph represents the impulse (change in momentum), not the final momentum or the force itself.

Frequently asked questions

What is impulse?

Impulse is the change in momentum of an object. It is calculated as the product of the force acting on the object and the time interval over which the force acts (Impulse = FΔt). It is also the area under a force-time graph.

What is the difference between an elastic and an inelastic collision?

In both types of collisions, momentum is conserved. However, in an elastic collision, kinetic energy is also conserved. In an inelastic collision, some kinetic energy is lost to other forms, such as heat or sound.