Physics · Grades 9–12
Impulse and stopping time
Relate changes in momentum to average forces over a time interval.
Learn the idea
Impulse
Net impulse equals the change in momentum. It is not necessarily the final momentum.
Average force
For a known interval, divide impulse by elapsed time to find average net force.
Longer contact
For the same momentum change, a longer stopping time reduces average force. The maximum force needs more information.
Units
A newton-second is equivalent to a kilogram metre per second. Convert milliseconds before dividing.
Worked examples
A 2 kg object changes velocity from +3 to −2 m/s. Find its net impulse.
2 × (−2 − 3) = −10 N·s.
-10 N·s
An impulse of 18 N·s acts over 0.3 s. Find average force.
18/0.3 = 60 N.
60 N
Check your understanding
For equal momentum change, doubling contact time does what to average force magnitude?
- Doubles it
- Halves it
- Leaves it fixed
Convert a contact time of 40 ms to seconds.
Why does knowing average force not determine the largest force during contact?
Answers and marking notes
Answers and marking notes
- Halves it
Average force magnitude is impulse magnitude divided by time.
- 0.04 s
40/1000 = 0.04 s.
- Different force-time histories can have the same impulse and average but different peaks.
Credit peak versus average rather than assuming a rectangular force pulse.
Further reading and review references
These are reference links, not copied exercises, institutional endorsement or proof of independent review.
Independent educational review required. Grade bands are approximate; these are not officially aligned curricula.