# Kinematics ball throw problem

Tags:
1. Jul 30, 2016

### Erenjaeger

1. The problem statement, all variables and given/known data
Concept Simulation 2.3 provides some background for this problem. A ball is thrown vertically upward, which is the positive direction. A little later it returns to its point of release. The ball is in the air for a total time of 8.21 s. What is its initial velocity? Neglect air resistance.

2. Relevant equations
xf=xo+vo+1/2at2

3. The attempt at a solution
known values
xf = 0 because it comes back to where it was released from
xo = 0 because initial displacement is going to be 0
a = -9.8m/s2
t = 8.21 s
so couldn't you solve by using the kinematic formula xf=xo+vo+1/2at2
and then solving for vo
leaving you with -vo=xo+1/2at2-xf
and because both the initial displacement and final displacement are 0
-vo = 1/2 at2 ????
so -vo=-9.8⋅8.212
which gives me -330.62 which would be just 330.62m/s but it is incorrect. pls helpp

2. Jul 30, 2016

### TSny

Can you spot the mistake in this equation?

3. Jul 30, 2016

### Erenjaeger

a
ahhhh i havent put in vo t
because the equation is really xf=xo+vot+1/2at2 right?

4. Jul 30, 2016

### TSny

Yes, good.

5. Jul 30, 2016

### Erenjaeger

so by just dividing my answer by t (8.21) ill get the vo right?

6. Jul 30, 2016

### TSny

Yes. But be careful with the signs. In your first post you wrote
But you have a negative sign on both sides, so this will give a positive value for v0.

7. Jul 30, 2016

### Erenjaeger

yeah okay, thank you!

Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted