Calculations involving freefall

[SPH4UJS]

Homework Statement

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?

Homework Equations

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 that's not the right answer :S so I am not sure if its just that I am using a wrong equation or that I have the wrong givens I don't know exactly what I am doing wrong please point me in the right direction.

 

[AEM]

Homework Statement

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?

Homework Equations

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 that's not the right answer :S so I am not sure if its just that I am using a wrong equation or that I have the wrong givens I don't know exactly what I am doing wrong please point me in the right direction.

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

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

 

[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.

 

[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.

 

[AEM]

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.

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

 

[SPH4UJS]

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

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 what's to come especially since I want to be an engineer. Btw thank you very much for your help as well as Imperitor.