AQA 7408 A Level Physics · Mechanics and Materials
Newton’s Laws, Momentum and Impulse
Define the system before applying momentum conservation and keep resultant force distinct from an individual interaction force.
Newton’s laws in models
Use F = ma for the resultant force on one object. Newton’s third-law forces act on different objects, are the same type of interaction and cannot cancel on a single free-body diagram.
Momentum conservation
For an isolated system, total momentum before an interaction equals total momentum after it. Choose a positive direction and retain velocity signs. Kinetic energy need not be conserved in an inelastic collision.
Impulse
Impulse equals change in momentum: FΔt = Δp for constant force. For a varying force, the area under a force–time graph gives impulse. Increasing collision time reduces the average force for the same momentum change.