What is Impulse?
Impulse is the effect a force has on an object's motion when applied over time — it's why a longer contact time (like a padded landing) reduces the force felt in an impact.
Impulse equals the average force applied multiplied by the time interval: J = F·Δt, measured in N·s. Impulse also equals the change in momentum, Δp.
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Step-by-step worked examples
A force of 500 N is applied for 0.2 s. Find the impulse.
J = F·Δt J = 500 × 0.2 J = 100 N·s
A 0.45 kg ball speeds up from 0 to 20 m/s when kicked over 0.05 s. Find the average force.
J = Δp = m·v − m·0 = 0.45 × 20 = 9 kg·m/s F = J/Δt = 9/0.05 F = 180 N
A bat hits a ball with a force of 8000 N for 0.001 s. Find the impulse.
J = F·Δt J = 8000 × 0.001 J = 8 N·s
Flashcards
Quick quiz
Q1.A force of 20 N acts for 3 s. What is the impulse?
Q2.Impulse is equal to the change in…
Q3.Why does bending your knees when landing a jump reduce injury?
Q4.What is the SI unit of impulse?
The full card deck, worked steps and AI-tutor support for “What is Impulse?” are in Notek — study by hand before your exam.
Common mistakes
Thinking impulse and force are the same thing. — Correct: Force is instantaneous; impulse is force applied over a time interval.
Ignoring that impulse equals Δp, not p. — Correct: Impulse equals the change in momentum, m·Δv, not the final momentum alone.
Assuming a smaller force always means a smaller impulse. — Correct: A small force over a long time can produce a large impulse.
Forgetting impulse is a vector. — Correct: Direction matters — impulse and the force causing it point the same way.
FAQ
What is impulse in physics?
Impulse is the change in momentum caused by a force acting over time: J = F·Δt.
What is the formula for impulse?
J = F·Δt, and equivalently J = Δp (change in momentum), measured in N·s.
How do you calculate impulse with an example?
Multiply average force by time. A 100 N force for 0.5 s gives J = 100 × 0.5 = 50 N·s.
Why is impulse important?
It explains why padding, airbags, and follow-through in sports reduce peak force during impacts.