# Initial velocity of a Ball thrown up

1. Sep 30, 2009

### simpleee

Can someone one show me a step by step example of how to work out a problem where a ball is thrown up and it is a catched after lets say 10.5s, so how would I find out the initial velocity?

2. Sep 30, 2009

### tiny-tim

Welcome to PF!

Hi simpleee! Welcome to PF!

You have s t and a, and you want to find u …

which of the usual constant acceleration equations do you think will help here?

3. Sep 30, 2009

### simpleee

truthfully, im not quiet sure.
I need help understanding which one to use and why exactly I should use it.
s = ut + 1/2 at squared

4. Sep 30, 2009

### simpleee

Oh and thank you for the welcome. =]

5. Sep 30, 2009

### tiny-tim

(try using the X2 tag just above the Reply box )

ok … there's only three constant acceleration equations, each with four variables, so you just chose the equation with the four variables that you're interested in.

In this case, you have s t and a, and you want to find u, so you choose s = ut + 1/2 at2 because it has all of them.

6. Sep 30, 2009

### simpleee

but i only have 10.5s and 9.8 for gravity.
So how would i know the rest?

7. Sep 30, 2009

### tiny-tim

u = 10.5, a = -9.8, and s = 0 (I'm assuming that the person catches it at the same height that (s)he throws it from ).

8. Sep 30, 2009

### simpleee

0 = 10.5 t + 1/2 9.8 t2
0 = 10.5 - 4.9 t2

9. Sep 30, 2009

### tiny-tim

You mean 0 = 10.5 - 4.9 t …

yes, that's right.

10. Sep 30, 2009

### simpleee

Do I get the t by itself now?
I think I might have posted this on the wrong section. .-.

11. Sep 30, 2009

### tiny-tim

Yes … t = 10.5/4.9.
That happens quite a lot here!

12. Sep 30, 2009

### simpleee

Why did it turn into 10.5/4.9?
Is there any rules on how many times we can ask for help? o.o

13. Sep 30, 2009

### tiny-tim

0 = 10.5 t - 1/2 9.8 t2

so 0 = 10.5 - 4.9 t

so 10.5 = 4.9t

so t = 10.5/4.9
You have to keep asking until you understand it!

14. Sep 30, 2009

### simpleee

Good, good.
All the better! ^_^
2.1m/s?

15. Nov 10, 2009

### Sobeita

I guess this problem depends on how strong your esophagus is.