Homework Help: Kinematics stone fall problem

1. Sep 10, 2008

joe215

1. The problem statement, all variables and given/known data

A falling stone takes 0.28s to travel past a window 2.2m tall. From what height above the top of the window did the stone fall?

3. The attempt at a solution

Vo=?, the velocity at the top of the window
a= -9.8m/s^2
t=.28s
d=2.2m
v=?

Vo=(1/2at^2-d)/-t

[1/2(-9.8)(.28)^2-2.2]/-.28=9.2m/s

Vo now becomes V although it still represents the velocity at the top of the window.

t=?
d=?
a=9.8 m/s^2
Vo=0
V=9.2m/s

Solve for "D"

D=(V^2-Vo^2)/2a

(9.2^2 - 0)/ (2*-9.8)= -4.3m

The ball is dropped from 4.3m.

Am I right?

Thanks,

Joe

2. Sep 11, 2008

alphysicist

Re: Kinematics

Hi joe215,

This value cannot be right; the object is moving downwards so the velocity must be negative.

The equation before this last one is right, but when you plugged in the numbers you made an error. Do you see what it is?

3. Sep 11, 2008

joe215

Re: Kinematics

I don't see the error?

4. Sep 11, 2008

alphysicist

Re: Kinematics

d is the displacement; since the stone is falling downwards, d is equal to -2.2m.