# Homework Help: 2D kinematics question

1. Oct 30, 2009

### CocoonOHorror

1. The problem statement, all variables and given/known data

A softball is hit over a third baseman's head with some speed v-sub0 at an angle theta above the horizontal. Immediately after the ball is hit, the third baseman turns around and begins to run at a constant velocity V = 7.00m/s. He catches the ball t=2.00sec later at the same height at which it left the bat. The third baseman was originally standing L=18.0 m from the location at which the ball was hit. Find v-sub0 Use g=9.81 m/s^2 for the magnitude of the acceleration due to gravity. also find theta.

2. Relevant equations

x =( initial velocity times cos theta)* time
y = initial velocity times sin theta)* time - .5GT^2

3. The attempt at a solution

i dont understand how to solve an equation with 4 variables and i only have values for 2 of them. i have time = 2 seconds, and distance = 32 meters. how do i get an angle and an initial velocity?

2. Oct 30, 2009

### Staff: Mentor

There are two equations and two unknowns. Hint: What's y, measured from the starting height?

3. Oct 30, 2009

### CocoonOHorror

is it 9.8 meters?

4. Oct 30, 2009

### Staff: Mentor

No. How does the initial height compare to the final height?

5. Oct 30, 2009

### CocoonOHorror

it seems like you just use the dude running at 7 m/s for 2 seconds to get the total distance (18m + 14m) but i guess thats not the right way to go about it?

i really wish my physics prof spoke english!

6. Oct 30, 2009

### CocoonOHorror

they are the same.

7. Oct 30, 2009

### Staff: Mentor

No, that's perfectly correct. That gives you the value for x you'll need in the first equation.

8. Oct 30, 2009

### Staff: Mentor

Yes! So, if you measure from the starting point, what's y? (What's the change in height?)

9. Oct 30, 2009

### CocoonOHorror

zero?

10. Oct 30, 2009

### Staff: Mentor

Yes, y = 0 when t = 2.

11. Oct 30, 2009

### CocoonOHorror

so i have:
32=(initial velocity*cos theta)*time
and
0= (initial velocity*sin theta)*time - .5GT^2
right?
but now what?

12. Oct 30, 2009

### CocoonOHorror

wait, i have 16 = initial velocity * cos theta
and 9.8 = initial velocity * sin theta (or is it 4.9/2 = initial Velocity * sin theta ?)

(i think)
now what?

13. Oct 30, 2009

### Staff: Mentor

Play around with those equations and see if you can isolate one of the variables. There are several ways to go. (Try division.)

14. Oct 30, 2009

### CocoonOHorror

OK, maybe 16/cos theta = 9.8/sin theta?

im so lost....

15. Oct 30, 2009

### CocoonOHorror

am i warm?

16. Oct 30, 2009

### CocoonOHorror

is it an easy Trigonometric solution? im in Trig/precalc algebra in one class and we just started the Trig half, so maybe im not familiar with a function to equate Sin/Cos?

17. Oct 30, 2009

### Staff: Mentor

You're not that lost. Multiply both sides by sin theta. (What other trig function appears?)

18. Oct 30, 2009

### CocoonOHorror

does Tan theta = 9.8/16 ?
am i on the right track?

19. Oct 30, 2009

### CocoonOHorror

holy mackeral!!! thanks Doc Al, i really appreciate the help!

20. Oct 30, 2009

You got it.