Impulsa

Meet Impulsa, an egg who survives a fall by landing on something soft.

Newton's second law can be written as force times time equals change in momentum: FΔt = Δp. The product FΔt is called impulse. To stop a moving object you must take away all its momentum, and that change is fixed by its mass and speed. But you can choose how long the stop takes.

Example: an egg of about 60 grams falling at 4 m/s has 0.24 kg·m/s of momentum. Stopping it in 1 millisecond on a hard floor needs an average force of 240 newtons, enough to crack it. Stopping it over 50 milliseconds on a cushion needs only about 5 newtons.

That is the idea behind airbags, crumple zones in cars, bicycle helmets, gymnastics mats and bending your knees when you land. In each case the stop is stretched out so the force is smaller. It also works the other way: following through on a golf or tennis swing extends contact to increase the impulse.

F here is the average force. The real force varies during the impact and can peak higher than the average.