Teaching Physics with the Physics Suite

Edward F. Redish

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Break a leg (not!)

When jumping straight down, you can be seriously injured if you land stiff-legged. One way to avoid injury is to bend your knees upon landing to reduce the force of the impact. Suppose you have of mass m and you jump off a wall of a height h.

(a) With what speed, v, will you hit the ground? Assume you simply step off the wall so your initial y-velocity is zero. Ignore air resistance. (Express your answer in terms of the symbols given.)

(b) Suppose that the time interval starting when your feet first touch the ground until you stop is Δt. Calculate the (average) net force acting on you during that interval. (Again, express your answer in terms of the symbols given.)

(c) Suppose h = 1 m. If you land stiff-legged, the time it takes you to stop may be as short as 2 ms, while if you bend your knees, it might be as long as 0.1 s. Calculate the average net force that would act on you in the two cases.

(d) The net force on you while you are stopping includes both the force of gravity and the force of the ground pushing up. Which of these does you the injury? For the two cases in part (c), calculate the upward force the ground exerts on you.

Page last modified October 8, 2002: P&E02