Impulse Physics Example 3

Follow the full solution, then compare it with the other examples linked below.

medium

60 \text{ kg}

person jumps and lands on the ground, changing velocity from

-4 \text{ m/s}

(downward) to

0 \text{ m/s}

0.5 \text{ s}

. What average force does the ground exert?

1
Change in momentum: $\Delta p = m(v_f - v_i) = 60(0 - (-4)) = 240 \text{ kg m/s}$ .
2
Average force: $F = \frac{\Delta p}{\Delta t} = \frac{240}{0.5} = 480 \text{ N}$

Answer

F_{\text{avg}} = 480 \text{ N}

The impulse-momentum theorem states

J = F\Delta t = \Delta p

. Longer contact time means smaller force, which is why bending your knees when landing reduces impact force.

About Impulse

The product of force and time interval, equal to the resulting change in an object's momentum.

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