Question

bending your knees as you hit the ground after jumping from a roof
multiple choice
extends the stopping time, which reduces the momentum.
reduces the stopping time, which reduces the force.
reduces the momentum by slowing your velocity
extends the stopping time, which reduces the force.

Explanation:

According to the impulse - momentum theorem $J = F\Delta t=\Delta p$. When you bend your knees, you increase the time $\Delta t$ it takes to stop. Since the change in momentum $\Delta p$ is fixed (determined by your mass and the initial velocity before hitting the ground), increasing $\Delta t$ will decrease the average force $F$ exerted on your body. Momentum is not reduced by bending knees as the initial momentum is determined by mass and velocity before impact. Also, bending knees does not reduce the stopping time but increases it.

Answer:

D. extends the stopping time, which reduces the force.