Part 03 · Work, energy and momentum · Card 8 of 9
Impulse
The change in momentum caused by a force acting over time: F times time equals change in p.
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Analogy
A longer, softer stop means a smaller force.
Example
Catching a ball
F = change in p / timeF = 6 / 0.01F = 600 N
In practice
Stopping a 0.15 kg ball at 40 m/s in 0.01 s takes about 600 N.
Practice
Flashcards
- How is impulse related to momentum?Impulse (force times time) equals the change in momentum.
Interview questions
- Explain why airbags reduce injury using impulse.
- Same change in momentum
- Longer stopping time
- Smaller average force
Questions people ask
Why do you bend your knees when you land?
Bending your knees makes the stopping time longer. Your momentum must still drop to zero, so the impulse stays the same. But force equals impulse divided by time, so a longer time means a smaller average force on your bones and joints.
What is the area under a force-time graph?
It equals the impulse, which is the change in momentum. For a constant force it is a rectangle with area force times time. For a changing force, like a bat hitting a ball, you find the area under the curve, which may need adding shapes.