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Mechanics · Quick Reference
Impulse Cheat Sheet
In one line: impulse connects force and time to the change in an object's momentum — explaining why airbags, padded gloves, and bending your knees when landing all reduce injury.
Key Ideas
1Impulse Definition. Impulse = force x time = F*t, measuring the overall effect of a force applied over a duration.
2Impulse-Momentum Theorem. Impulse equals the change in momentum: F*t = (change in momentum) = m*(v_final - v_initial).
3Units of Impulse. Measured in newton-seconds (N*s), which is equivalent to kg*m/s (the same units as momentum).
4Extending Collision Time to Reduce Force. Since impulse (F*t) for a given momentum change is fixed, increasing the time of impact (like with an airbag) decreases the force experienced.
5Impulse from a Force-Time Graph. The area under a force-time graph represents the total impulse delivered, even for forces that change over time.
Worked Examples
A 10 N force acts on an object for 3 seconds. Find the impulse delivered.
30 N*s
A 2 kg ball changes velocity from 3 m/s to 9 m/s due to a force applied for 4 seconds. Find the average force.
3 N
Explain why bending your knees when landing from a jump reduces the force on your body.
Bending your knees increases the stopping time, which reduces the average force for the same momentum change
Formulas
Impulse = F*t = m*(v_final - v_initial)
Practice Yourself
A 5 N force acts for 6s. Find impulse.
30 N*s
A 1 kg object's velocity changes from 2 to 8 m/s in 3s. Find the average force.
F*3=1*6=6, F=2N
What theorem relates impulse to change in momentum?
The impulse-momentum theorem