Impulse
Change in momentum from a force over time.
Formula
Variables
Example
A 5 N force for 2 s changes momentum by J = 5·2 = 10 kg·m/s.
How it works
Impulse tells you how much a force changes an object’s momentum over a time interval. It works because a net force causes acceleration, and when that force acts for some time, the acceleration changes the velocity, so the momentum changes by the same amount. A larger force or a longer time gives a bigger impulse, which means a bigger change in motion.
When to use it
Use this formula when a force acts over a short but noticeable time, such as in collisions, catching a ball, kicking a football, or airbags slowing a passenger. It is also useful anytime you know the force and duration and want the change in momentum.
Worked example, step by step
A hockey stick applies an average force of 120 N to a puck for 0.15 s. Use J = FΔt J = 120 × 0.15 J = 18 N·s Since 1 N·s = 1 kg·m/s, the change in momentum is 18 kg·m/s.
Common mistakes to avoid
- Using the object's mass instead of the force in the formula
- Forgetting that the time must be in seconds
- Confusing impulse with momentum itself instead of the change in momentum
Did You Know?
Airbags work by extending the time of impact, which lowers the force for the same change in momentum.
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