- #1

- 15

- 0

## Homework Statement

A ball is thrown leftward from the left edge of the roof, at height [tex]h[/tex] above the ground. The ball hits

the ground [tex]1.50s[/tex] later, at distance [tex]d=25.0m[/tex] from the building and at angle [tex]\theta =60^

{\circ}[/tex] with the horizontal. (a) Find h. What are the (b) Magnitude and (c) angle relative to the horizontal

of the velocity at which the ball is thrown? (d) Is the angle above or below the horizontal?

http://lcphr3ak.is-a-geek.com/fig49.png [Broken]

[tex]d=25.0m[/tex]

[tex]\theta=60^{\circ}[/tex]

The answers in the back are:

a)[tex]32.3m[/tex]

b)[tex]21.9\frac{m}{s}[/tex]

c)[tex]40.4^{\circ}[/tex]

d)below

## Homework Equations

[tex]x - x_{\circ} = v_{\circ} t - \frac{1}{2} a t^2[/tex]

[tex]v_{\circ x} = v_{\circ} \cos{\theta _{\circ}}[/tex]

[tex]v_{\circ y} = v_{\circ} \sin{\theta _{\circ}}[/tex]

## The Attempt at a Solution

I'm missing an observation here, because I don't see enough information to solve this. This problem is suppose to

be solved with projectile motion. So first I attempt to find [tex]v_{fx}[/tex] (which is the [tex]x[/tex] component

of the vector at [tex]\theta[/tex]): [tex]x - x_{\circ} = v_{fx} t - \frac{1}{2} a t^2[/tex] which is [tex]-25 =

v_{fx} (1.50)[/tex]. This gives me [tex]v_{fx} = -16.667[/tex]. Using this, I take [tex]v_{fx} = v_f \cos{\theta}

[/tex], which is: [tex]-16.667 = v_f \cos{60}[/tex], so [tex]v_f = -33.32[/tex]. Using this, I find [tex]v_{fy}

[/tex] by taking [tex]v_{fy} = v_f \sin{\theta}[/tex] and doing [tex]v_{fy} = (-33.32)\sin{60}[/tex] so [tex]v_{fy}

= -28.856[/tex]. So, to find [tex]h[/tex], we solve for [tex]y[/tex]. [tex]y = v_{fy} t - \frac{1}{2} a t^2[/tex]

becomes [tex]y = (-28.856)(1.50) - \frac{1}{2} (9.8) (1.50)^2[/tex] which says [tex]y = -54.309[/tex]. Now, the

absolute value of [tex]y[/tex] should be the height, but this is waaaaay off from the answer. I'm practically out of

ideas.

Could anyone explain what I'm doing wrong? If I didn't provide enough info, just let me know and I'll try to

provide more.

Thanks!

Last edited by a moderator: