# Question on motion in two dimensions:

1. Oct 5, 2006

### integra2k20

OK, ive been trying to get problems like these where you are given the angle (but not the velocity) of the direction in which a projectile is fired. I have tried as many ways as i could conceive using kinematics, but i keep getting stuck. If anyone could just give me an idea how to go about getting a little more info so i can complete the problem, it would be greatly appreciated.

Barry Bonds hits a home run so that the baseball just clears the top row of bleachers, 23.6 m high, located 130 m from the home plate. The ball is hit at an angle of 42° to the horizontal, and air resistance is negligible. Assume that the ball is hit at a height of 1.00 m above the ground. Find the following values.
(a) the initial speed of the ball
(m/s)

(b) the time at which the ball reaches the cheap seats

(s)

(c) the velocity of the ball when it passes over the top row
(____i+____j m/s)

(d)the speed of the ball when it passes over the top row

(m/s)

2. Oct 5, 2006

### valueduser

I think a good first step would be to use some trig to figure out the vertical distance the ball travels

3. Oct 5, 2006

After drawing a diagram, write down the equations of displacement of the ball for the x and y direction separately. What do you know about the initial position of the ball? What do you know about the final position of the ball? Eliminate the variable t from one of the equations, to get the initial velocity. The rest should be easy.

4. Oct 5, 2006

### integra2k20

how would you do this, since the path would be parabolic?

EDIT: the total displacement is 131.9498, still dont get how im gunna figure it out tho, i drew everything out but cant seem to get it still...

Last edited: Oct 5, 2006
5. Oct 5, 2006

### integra2k20

i tried what radou said, and it (seemed) to work. i got the values for Vxi and Vyi, then found Vi from that. it all worked out the way it should have an di checked my answer. these are the values i got:

Vxi = 174.9323 m/s
Vyi = 33.7752 m/s
Vi = 235.3946 m/s

t = .7431 seconds

with these values, i was able to calculate that the ball was at the following points at t= .7431: x = 129.9922 (which is right, its supposed to be 130, just some round off error). y = 23.3926 (should be 23.6, again, roundoff error). Since those values are so close, webassign should have no problem accepting those answers, but it says theyre wrong...