# Homework Help: A boy throws a ball

1. Sep 22, 2009

### k2var2002

1. A boy 11.0m above the ground in a tree throws a ball for his dog, who is standing
right below the tree and starts running the instant the ball is thrown.
If the boy throws the ball upward at 40.0° above the horizontal, at 7.50m/s .
What is velocity (m/s)? and distance (m)?

2. y=yo+voy*t-1/2gt^2

y= yo+sin$$\theta$$*t-1/2gt^2

1/2gt/yo= voy

R= Voy*t=Vosin$$\theta$$*t

t= $$\sqrt{}$$(2y/g)^2

3. I plugged the known variables into the equations i listed above. First for time, I got t=1.5s. Then I used the angle and time found to find distance, R=(7.5)(sin40)=4.82 m. Which doesn't make sense to me. Any help is appreciated. Thanks!

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

2. Relevant equations

3. The attempt at a solution

2. Sep 22, 2009

### tiny-tim

Welcome to PF!

Hi Kyle! Welcome to PF!

(have a square-root: √ and try using the X2 tag just above the Reply box )
How did you get t= √(2y/g)^2 from that?

3. Sep 22, 2009

### k2var2002

t= √(2y/g)^2 is just a derivation of some sort that my professor had given us to figure time in 2d motion.

4. Sep 22, 2009

### k2var2002

Would t=Vo/-a work from the base equation Vy=Vo+at?

5. Sep 23, 2009

### tiny-tim

That only works if v0 = 0 … just look at the original equation.

Stop looking for shortcuts!