Broken Ankles
Classical Mechanics
Level
1
When jumping off a ledge feet-first, why is it advisable to bend your knees on landing on the ground?
To reduce the change in your velocity
You can exert a greater force on the ground with bent knees
It increases the time over which you decelerate
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Relevant wiki: Newton's Second Law
Recall that impulse is equal to both the average force times time and the change in momentum. The force that slows you down is the normal force from the ground (and not a downward-acting force, which rules out a choice). We want to minimize this force in order to avoid being injured.
Since the change in velocity is always constant (you go from some non-zero velocity to zero velocity), and your mass always remains the same, the change in momentum is always constant. (This rules out the change in velocity choice.)
That means the only way to reduce the force your feet feel is by increasing the time your feet are in contact with the ground , which can be done by bending your knees as soon as you land on the ground. Indeed, force and time are in an inverse relationship, so an increase in time means a decrease in force, and thus a much safer jump for you.