# Projectile motion - Kinematics

1. Jan 5, 2008

### rejz55

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

At 75% its maximum height, the speed of a projectile is four-fifths its initial speed. What is the angle of the initial velocity vector with respect to the vertical?

I really have no idea how to do this. Can you give me the idea on how to start or how to do it. (just please don't tell me the answer)

Thanks a lot!

2. Jan 5, 2008

### Shooting Star

At the max height, there is no component of velo in y dircn. vx remains constant. vy is given by the eqn vy^2 = vy_0^2 - 2gh.

Do you understand something now?

3. Jan 5, 2008

### rejz55

if i will use that equation there will be 2 unknown variables. the initial velocity of y and the height..

how can i use the given 75% its max height and the 4/5 speed of the projectile

4. Jan 5, 2008

### foxjwill

As shooting star wrote,

$${v_{y0}}^2 = 2gy_\textrm{max}$$

Try working with that equation and

$${v_{y2}}^2 = {v_{y0}}^2 - 2gy_{2}.$$

Last edited: Jan 5, 2008
5. Jan 7, 2008

### Shooting Star

You only have to find the ratio of vx/vy_0, not the absolute values.

6. Jan 7, 2008

### rejz55

Thanks!! Got it!