Momentum and Impulse – IGCSE Physics 0625
1. What is Impulse?
Impulse is the “push” you give an object over a period of time.
Think of a soccer ball being kicked: the striker’s foot applies a force F for a short time Δt.
The total effect of that push is the impulse, which changes the ball’s momentum.
2. Impulse Formula
The mathematical relationship is:
\$\$
\text{Impulse} = F\,\Delta t = \Delta(mv)
\$\$
where
- F = average force (N)
- Δt = time interval (s)
- Δ(mv) = change in momentum (kg·m/s)
3. Quick Example
A 0.5 kg bowling ball is moving at 4 m/s.
It is hit by a bat that applies a force of 200 N for 0.02 s.
What is the new velocity of the ball?
- Impulse = 200 N × 0.02 s = 4 N·s
- Δ(mv) = 4 N·s → mΔv = 4 N·s
- Δv = 4 N·s ÷ 0.5 kg = 8 m/s
- New velocity = 4 m/s + 8 m/s = 12 m/s
4. Exam Tips 📚
- Remember the units: Force (N) × Time (s) = kg·m/s.
- Check the sign: Δ(mv) can be positive or negative depending on direction.
- Use the correct formula: If you’re given force and time, use FΔt; if you’re given masses and velocities, use Δ(mv).
- Draw a diagram: Show the direction of forces and motion to avoid sign errors.
- Practice unit conversions: 1 N = 1 kg·m/s².
5. Summary Table
| Scenario | Given | Impulse Formula | Result |
|---|
| Force over time | F = 150 N, Δt = 0.05 s | Impulse = FΔt | 7.5 N·s |
| Change in momentum | m = 2 kg, vi = 3 m/s, vf = 7 m/s | Δ(mv) = m(vf – vi) | 8 kg·m/s |