Edexcel IAL Physics · Unit: Mechanics
Momentum & Impulse
Big idea: When things push on each other — a collision, an explosion, a tennis racket hitting a ball — the total "quantity of motion" (momentum) in the system never changes, even though it can move between objects. A force acting over time is what causes that momentum to shift, and we call that push "impulse."
Summary — What This Chapter Covers
Impulse
Force × time = change in momentum. Explains "follow through" in sport, crumple zones, airbags.
Core Practical 9
Trolley + light gates experiment to verify impulse = change in momentum (mgt = ΔP).
Conservation of Momentum
Total momentum before = total momentum after, in 1D and 2D (resolve into components!).
Core Practical 10
Using Tracker software to film and analyse 2D ball-bearing collisions.
Elastic & Inelastic Collisions
Momentum is ALWAYS conserved; kinetic energy is only conserved in elastic collisions.
Energy–Momentum Relation
Eₖ = p²/2m — links kinetic energy directly to momentum. Useful for particles.
Topic 1: Impulse
1 What is momentum, really?
Before we even touch impulse, let's be crystal clear on momentum. Momentum (symbol p) is a measure of "how hard it would be to stop something." A heavy truck moving slowly and a small car moving fast might have similar momentum — both would be a pain to stop suddenly.
Momentum is measured in kg m s⁻¹, and — critically — it's a vector. That means direction matters. If you take "moving right" as positive, then something moving left has negative momentum. This single fact trips up more students than anything else in this chapter, so keep it front of mind.
2 Force is the rate of change of momentum
Newton's second law, in its most general form, isn't actually F = ma — it's this:
Think about it this way: if you're catching a fast cricket ball, the ball's momentum has to drop to zero. That change in momentum, Δp, is fixed — it depends only on the ball's mass and speed. But how you bring it to zero is up to you. If you snatch it rigidly, Δt is tiny, so F must be huge (it stings!). If you let your hands "give" and move backward with the ball, you stretch out Δt, which means F is much smaller. Same Δp, different force, because you changed the time.
3 Defining Impulse
Rearranging that force equation gives us the definition of impulse — the effect a force has when it acts for a certain amount of time.
Notice the units: N s and kg m s⁻¹ are actually the same unit written two different ways — that's a nice sanity check that the equation is dimensionally consistent.
This equation (F = constant version) tells us something really elegant: a small force acting for a long time has exactly the same effect as a large force acting for a short time, as long as FΔt comes out the same. This is why:
- Airbags and crumple zones in cars increase the collision time, which reduces the force on the passengers (same Δp, but spread out).
- A boxer "rolling with the punch" extends the contact time to reduce the force of impact.
- Karate experts do the opposite — they want to break a board, so they aim for a very short, sharp Δt to maximise force.
Since impulse is proportional to force, it's also a vector — it points in the same direction as the force that caused it.