# Homework Help: Stone is thrown from the top of the building.

1. Jan 29, 2014

### patelneel1994

1. The problem statement, all variables and given/known data
A stone is thrown from the top of a building with an initial velocity of 20 m/s downward. The top of the building is 60 m above the ground. How much time elapses between the instant of release and the instant of impact with the ground?

Vi = -20 m/s height = 60 m
t = ?

2. Relevant equations

d = Vi.t + 1/2 a t2

3. The attempt at a solution
Couldn't figure out. How to use height as a distance?

2. Jan 29, 2014

### BvU

It has the same dimension ! The equations are OK.

3. Jan 29, 2014

### patelneel1994

So 60 m = -20m/s . t + 1/2 a t2
60 m = -20m/s t - 4.9 m/s2 t2?
What should I do after this ...

4. Jan 29, 2014

### collinsmark

You seem to be using the convention that up is positive, which is fine. But is the stone traveling 60 m downward or upward? What does that tell you about the sign of the 60 m in your equation?

Solve for t. (Hint: quadratic equation.)