Calculations involving freefall

1. Mar 3, 2009

SPH4UJS

1. The problem statement, all variables and given/known data
A baseball pitcher throws a ball vertically upward and catches it at the same level 4.2s later

a) with what velocity did the pitcher throw the ball?

2. Relevant equations

3. The attempt at a solution

I put this into givens

t = 4.2 s
displacement = 0 m if the ball goes up and then comes back at the same level I figured it has a displacement of 0

acceleration = -9.81 m/s

Vf = 0 m/s since it goes up then reaches a climax and it was already thrown with an initial velocity that you are trying to find

so I decided to use the vf = vi + a(t)

so.. then I rearranged it so that it was

vf - ( a ) ( t ) = vi

which gave me

-(-9.81)(4.2) = vi

and then I got 41.2 but thats not the right answer :S so im not sure if its just that im using a wrong equation or that I have the wrong givens I don't know exactly what im doing wrong please point me in the right direction.

2. Mar 3, 2009

AEM

With y representing the vertical direction you might want to take a look at

$$y = y_o + v_o t + \frac{1}{2} a t^2$$

3. Mar 3, 2009

Imperitor

Remember that your using vf as 0 which is at its climax. The time you used is for the total time (up and down) not just the time to get to the climax.

Last edited: Mar 3, 2009
4. Mar 3, 2009

SPH4UJS

Alright I got the answer thank you very much. Also I have a test tommorow on this whole kinematics unit and so far I know ab out half what would be my best bet? Should I just keep doing questions until I know them all and asking for help if I can't figure it out myself? I really want to do well as I did so poorly on the quiz :S.

5. Mar 3, 2009

AEM

50 years of experience has taught me that the best bet is NOT to put off studying until the night before the exam!

6. Mar 3, 2009

SPH4UJS

Yeah, it's not the best thing to do :s but I have no idea where my priorities are well..that and my time management skills I have put off doing physics homework until today :S. Hopefully all my questions can be answered on the forums by tomorrow. Its just I feel that I don't have the trained mindset for physics just yet, and actually not learning kinematics scares me for whats to come especially since I want to be an engineer. Btw thank you very much for your help as well as Imperitor.