# More help needed

lunarskull
this problem deals with freely falling bodies

A woman is reported to have fallen 144 ft from the 17th floor of a building, landing on a metal ventillation box, which she crushed to a depth of 18.0 in. she suffered only minor injuries. Neglecting air resistance, calculate (a) the speed of the woman just before she collided with the ventillator, (b) her average acceleration while in contact with the box, and (c) the time it took to crush the box.

wow I am so lost. how do u use the 17th floor anf dent made in the ventillator? can someone start all 3 parts for me?

Homework Helper
a) For uniform acceleration:
$$x = x_0 + v_0t + \frac{1}{2}at^2$$
and $$v = v_0 + at$$
Or you can use conservation of energy.
b) How is average acceleration defined?
c) see a)

lunarskull
still lost...

i plugged in: (18in)=(1728in)(<---converted to inches)+$$v_0$$+(1/2)(9.8m/s^2)(t)

what now?

Homework Helper
You missed two t's.
Calculate the speed just before she collides. That means x = 0.
From the fisrt equation, solve for time. Hint: $v_0$ = 0. You know what a is, what is it?

lunarskull
a=-9.8m/s correct?

Homework Helper
That is correct.

lunarskull
what do u mean i missed 2 ts?

lunarskull
also, when you plug into the equation, both x and $$v_0$$=0 correct?

Staff Emeritus
Gold Member
Notice that you are given distances in inches and feet, if you want to use 9.8 m/s^2 as g, you need to convert all distances to meters.

lunarskull
ok, i got a right. now how do u do b? i no that the equation is deltav/delta t

Homework Helper
lunarskull said:
what do u mean i missed 2 ts?
(18in)=(1728in)(<---converted to inches)+$v_0$+(1/2)(9.8m/s^2)(t)
Instead of $v_0$ you should have $v_0t$ and instead of (t) you should have $t^2$.

b) Well actually, I suppose this is an easier approach:

$$x = x_0 + v_0t + \frac{1}{2}at^2$$
$$v = v_0 + at$$
Can you see it?