Impulse
Newton's Laws & Momentum - OCR A-Level Physics
Key Definition
Impulse
The product of force and the time for which it acts. Equal to the change in momentum. Measured in N s or kg $m s^{-1}$.
The product of force and the time for which it acts. Equal to the change in momentum. Measured in N s or kg $m s^{-1}$.
$$F\Delta t = \Delta p$$
$$\begin{aligned}
\text{impulseThe product of force and the time for which it acts. Equal to the change in momentum.} &= F\Delta t \\
&= \Delta p \\
&= mv - mu
\end{aligned}$$
- ImpulseThe product of force and the time for which it acts. Equal to the change in momentum. is a vectorA quantity with both magnitude and direction (e.g. force, velocity, displacement). quantity, in the direction of the force
- The area under a force-time graph equals the impulseThe product of force and the time for which it acts. Equal to the change in momentum. (and hence the change in momentum)
- For a constant force: impulse = $F \times$t (rectangle under the graph)
- For a varying force: find the area under the curve (counting squares, trapezium rule or integration)
- Extending collision time reduces the peak force: $same \Delta$p, but $larger \Delta$t means smaller F
Common Mistake
MEDIUM
Students often: A typical mistake is to read the peak force from a force-time graph and multiply by the total time to find impulse.
Instead: Find the AREA under the graph, not peak force times total time. For a triangular pulse: $area = 1/2 x base x height$.
Instead: Find the AREA under the graph, not peak force times total time. For a triangular pulse: $area = 1/2 x base x height$.