Calculate Initial Speed of Ball Thrown Vertically 14m in 3s

[patelneel1994]

Homework Statement

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 = ?

Homework Equations

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

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

 

[Doc Al]

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

 

[patelneel1994]

How do I find acceleration?

 

[Doc Al]

How do I find acceleration?

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

 

[patelneel1994]

Is it 3 ms-2?

 

[Doc Al]

Is it 3 ms-2?

No. Where in the world did that come from?

 

[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?

 

[Doc Al]

I thought time and acceleration some how are related, which are not.

The acceleration is constant.

Could we use gravity, which is 9.79 m/s2?

Of course you should use the acceleration due to gravity!

 

[patelneel1994]

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

Thank you

 

[Doc Al]

So if we solve using acceleration, then a = 0 right!

No. Why would you think that?

How can I solve using gravity? Is there any different formula?

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

 

[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

 

[Doc Al]

Sorry a = 9.78 m/s^2

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

 

[patelneel1994]

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

 

[Doc Al]

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

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

 

[patelneel1994]

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