# Homework Help: Help find initial velocity

1. Oct 4, 2012

### gsg822

1. The problem statement, all variables and given/known data
I need help finding the x and y components of initial velocity being given the angle the trajectory was fired at and the distance the projectile traveled. For this example I'm using an angle of 15° and the projectile travels 0.534m. I'm not sure if I'm going in the right direction in trying to solve this problem.

2. Relevant equations
Vx = Vcos(angle)
Vy = Vsin(angle)

x -x initial = Vx * t

3. The attempt at a solution
Solved those to get: Vx = Vcos(15) and Vy = Vsin(15).

After pluggin Vx into the equation, I'm having trouble figuring out where to go from here. Any help is appreciated.

2. Oct 4, 2012

### tiny-tim

welcome to pf!

hi gsg822! welcome to pf!

(try using the X2 button just above the Reply box )
use the x equation to find t (as a function of V), then use that value of t in the y equation …

show us what you get

3. Oct 4, 2012

### gsg822

When you say to use the y equation are you referring to y = y0 + voy * t - 0.5 * g * t2. I've tried using this equation and I get stuck when I try and simplify it down.

I solved the x equation for time and got t = 0.534m / (V * sin(15))

4. Oct 4, 2012

### tiny-tim

yup!
if i'm reading the question correctly, y = yo

5. Oct 4, 2012

### gsg822

I have two variables, V and Voy.

0 = Voy(0.534m / Vsin(15)) - (1/2)(9.8m/s2)(0.534m / Vsin(15))2

(1/2)(9.8m/s2)(0.285m / (Vsin(15)2) = Voy(0.534m / Vsin(15))

2.79m2/s2 / ( Vsin(15)2) = 2Voy(0.534m / Vsin(15))

2.79m2/s2 / ( Vsin(15)) = 2Voy(0.534m)

I get stuck here and don't know where to take it from here. I'm also not really sure if this work I did is even right. Am I heading in the right direction?

6. Oct 4, 2012

### tiny-tim

Voy = Vcos15°

(and you could have divided the whole of that equation by t, since you had 0 on the LHS)

and now i'm off to bed :zzz:

7. Oct 5, 2012

### gsg822

I forgot that I could substitue Vsin(15) for Voy.

I think I may have gotten the answer.

Vsin(15) = 5.2 / 2Vcos(15)

Rearranged that to V2 = 5.2 / (2sin(15)cos(15))

V = sqrt(5.2 / (2sin(15)cos(15)))

V = 3.22 m/s

Plug that value back into the Vox and Voy equations to get:

Vox = 3.11 /s
Voy = 0.833 m/s

Could you verify if this is correct?

8. Oct 9, 2012

### tiny-tim

hi gsg822!

(i'm sorry for the delay )

where did this come from? …