Edexcel IAL · Mechanics 1
Momentum, Impulse
& Collisions
Moving objects carry momentum; forces change it through impulse — and when objects collide, the total momentum of the system always stays the same.
2 Main Topics
4 Worked Examples
12-Point Checklist
6 Exam Tips
Chapter Overview
- Momentum (p = mv) — a vector quantity equal to mass × velocity, measured in kg m s⁻¹. Direction matters.
- Impulse (I = Ft) — the effect of a force acting over time, also measured in N s or kg m s⁻¹.
- Impulse-Momentum Principle (I = mv − mu) — impulse equals the change in momentum.
- Newton's Third Law in collisions — two objects in contact exert equal and opposite impulses on each other.
- Direct collisions — two objects travelling along the same line; may travel same or opposite directions, or coalesce.
- Conservation of Momentum — in the absence of external forces, total momentum before = total momentum after.
- Explosions — treated like a reverse collision; a stationary object splits into two parts moving apart.
Topic 1
Momentum & Impulse
What is Momentum?
Think of momentum as a measure of how difficult it is to stop something that's moving. A slow lorry and a fast tennis ball might have similar momenta — one has huge mass, the other has huge speed. It's the combination of both that counts.
Momentum is a vector: it has both size (magnitude) and direction. If you choose rightward as positive and an object moves leftward, its momentum is negative. This sign convention is crucial in collision problems.
Analogy: Imagine trying to stop a 2 kg bowling ball rolling at 5 m s⁻¹ vs. a 5 kg ball rolling at 2 m s⁻¹. Both have momentum 10 kg m s⁻¹ — equally difficult to stop!
| Quantity |
Formula |
Units |
Scalar / Vector? |
| Momentum |
p = mv |
kg m s⁻¹ |
Vector |
Worked Example 1
A dog of mass 15 kg is running with speed 6 m s⁻¹. Find the momentum of the dog.
Identify known values
Mass m = 15 kg | Velocity v = 6 m s⁻¹
Apply formula
p = mv = 15 × 6 = 90
Practice Question 1
A ball of mass 0.4 kg is thrown horizontally at 15 m s⁻¹. Another ball of mass 1.2 kg rolls along the ground at 5 m s⁻¹ in the same direction.
(a) Find the momentum of each ball.
(b) Which ball is harder to stop?
What is Impulse?
A force doesn't just have magnitude — it acts for a period of time. Impulse captures this: it's the total "push" delivered over that time. A large force over a short time can equal a small force over a long time — same impulse, same change in momentum.
These two formulas are linked by Newton's Second Law — they always give the same answer. The Impulse-Momentum Principle (I = mv − mu) is the one examiners use most, because it connects the force's effect to the resulting change in velocity.
Topic 2