# Physics help average acceleration

So here is the question I'm stuck on.

A woman is reported to have fallen 112 ft from a building, landing on a metal ventilator box, which she crushed to a depth of 24 in. She suffered only minor injuries.

Calculate the magnitude of her average acceleration while in contact with the box.

I know average accel. = (delta v) / (delta t)

I calculated the speed of the woman just before she collided with the box to be 84.664 ft/s.

Although I'm a bit lost on how to calculate the avg. acceleration because I'm not sure what equation I should use to calculate (t).

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Try using one of the equations of motion which doesn't involve time, that is,

v^2 = u^2 + 2as

where v is the final velocity, u is the initial velocity, a is the average acceleration and s is the displacement between when vel = u and vel = v

Shooting Star
Homework Helper
DELETED: gave the wrong formula by mistake. Rudipoo is right.

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You need to know the accleration to find the time using the s = ut - 0/5at^2 equation, so that's no good surely, because that's ultimatley what you're trying to find. Use your initial velocity as the one you calculated, and write down what the final velocity must be. You know the displacement while in contact with the box, and thus you can find the acceleration using equation I gave above.