When a child falls on a cement floor, his body comes to rest instantly. But F X ∆t = change in momentum = constant. As time of stopping ∆t decreases, therefore F increases and hence child feel more pain. When she falls on a soft muddy ground in the garden the time of stopping increases and hence F decreases and she feel lesser pain.
Why does a child feel more pain when she falls down on a hard cement floor, than when she falls on the soft muddy ground in the garden?