# Find initial speed?

1. Jan 24, 2014

### patelneel1994

1. The problem statement, all variables and given/known data
A ball thrown vertically from ground level is caught 3.0 s later by a person on a balcony which is 14 m above the ground. Determine the initial speed of the ball.

t = 3.0 s, d = 14 m, Vi = ?

2. Relevant equations
d = Vi.t + 1/2 a.t^2

3. The attempt at a solution
I have no idea how to get this problem. I don't know if the equation is right or not?

Thank You

2. Jan 24, 2014

### Staff: Mentor

You can certainly use that equation. What would you use for the acceleration? (Careful with signs.)

3. Jan 24, 2014

### patelneel1994

How do I find acceleration?

4. Jan 24, 2014

### Staff: Mentor

It's a projectile. You should know the acceleration.

5. Jan 24, 2014

### patelneel1994

Is it 3 ms-2?

6. Jan 24, 2014

### Staff: Mentor

No. Where in the world did that come from?

7. Jan 24, 2014

### patelneel1994

I coudn't think of any. I thought time and acceleration some how are related, which are not.
Could we use gravity, which is 9.79 m/s2?

8. Jan 24, 2014

### Staff: Mentor

The acceleration is constant.

Of course you should use the acceleration due to gravity!

9. Jan 24, 2014

### patelneel1994

So if we solve using acceleration, then a = 0 right!
How can I solve using gravity? Is there any different formula?

Thank you

10. Jan 24, 2014

### Staff: Mentor

No. Why would you think that?

You have the right formula. For the acceleration, use the acceleration due to gravity.

11. Jan 24, 2014

### patelneel1994

Sorry a = 9.78 m/s^2
I've tried that didn't ge the right answer.
14 = Vi . 3 s + [4.9 m/s^2 . 9 s^2]
14 - 44.1 / 3 s = Vi

12. Jan 24, 2014

### Staff: Mentor

What's the sign of the acceleration? Which way does gravity act?

13. Jan 24, 2014

### patelneel1994

Gravity pull an object down towards the ground. a = 9.78 ms-2!

14. Jan 24, 2014

### Staff: Mentor

Use a proper sign convention in your equation. Let 'up' be positive and 'down' be negative.

15. Jan 24, 2014

### patelneel1994

Tha gives me the right answer. So whenever its down - and up +
a = -9.78 ms-2
Thank you