Momentum & Impulse
Ball vs wall vs catcher. Builds on Newton's Three Laws.
An egg is thrown at you. You can catch it with stiff hands (stopping it quickly) or with soft, yielding hands (slowing it gradually).
Which technique is less likely to break the egg?
Soft hands! The impulse (momentum change) is the same either way: Δp = mv. But impulse = F × Δt. By extending the time Δt, you reduce the force F. This is the ENTIRE principle behind airbags, crumple zones, and bungee cords.
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Throw a ball at a wall vs. a cushion. Same momentum change, very different force peaks.
J = FΔt = Δp
This is just Newton's 2nd law (F=ma) rewritten: F = m(Δv/Δt) = Δ(mv)/Δt = Δp/Δt. Same physics, different perspective.
Cars are designed to crumple in a crash — not to be weak, but to extend the collision time from 0.01s to 0.1s, reducing force on passengers by 10×.
Boxing gloves don't reduce the punch's momentum — they extend the impact time. The force is spread over a longer duration, reducing peak force on the skull.
Following through keeps the racket in contact with the ball longer, increasing the impulse (FΔt) and therefore the momentum given to the ball.
“Momentum is what matters in collisions. Force is how fast you change it. Extend the time, reduce the force. Every safety device in existence is based on this one idea.”
Impulse Lab
Throw objects at different surfaces — hard wall, foam, spring. Compare the force-time curves and see why soft landings save lives.
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Throw the ball at the hard wall, then the foam pad. Same ball, same speed — but watch the force peak drop by 5× with the foam!
The area under the Force-Time graph is always the same (= impulse = Δp). But the SHAPE changes: tall and narrow for hard surfaces, short and wide for soft ones.