# Homework Help: Two objects in free fall

1. Mar 30, 2016

### Ab17

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

A stone is dropped from rest from the roof of a 100-m high building; 2.00 s after that, a second stone is thrown straight down with initial speed v0, and the second stone passes the first stone 21.6 m above ground. (a) When are the stones at the same height? (b) What was the initial speed, v0, of the second stone, in m/s? (c) What are the speeds of the two stones, in m/s, just before they hit the ground?

h= 100m
h(meet)= 26.1m
V1i= 0m/s
2. Relevant equations
Xf=Xi +Vt +0.5at^2

3. The attempt at a solution
(a). Xf1= -4.9t^2
Xf2 = Vo(t-2) -4.9(t-2)^2
-73.1 = -4.9t^2

(b). -73.1 = Vo(1.86) -4.9(1.86)^2
Vo = 30.19 m/s

(c). Vf = Vi + at
Vf = 0 -9.8(3.86)
Vf = 37.83 m/s

Vf= 30.19 -9.8(1.86)
Vf= 48.42m/s

Am I right? Not sure if I am

2. Mar 30, 2016

### SammyS

Staff Emeritus
You don't give the time, although the next entry seems to have used the correct time.

You calculated the speed of the both stones at 21.6 m above the ground.

Last edited: Mar 30, 2016
3. Mar 30, 2016

### ramzerimar

I could be wrong, but I found slightly different values. For time, t = 4s, and initial speed V0 = 29,4. Are you sure about your results?

4. Mar 30, 2016

### SammyS

Staff Emeritus
I think you're right.

I didn't even think to check the distances OP came up with.

100 - 21.6 = 78.4, not 73.1

If it's supposed to be 26.1,
then 100 - 26.1 = 73.9, not 73.1

5. Mar 30, 2016

### haruspex

6. Mar 30, 2016

### rickz02

@Ab17 double check your computation in (a).