# 1 projectile question

1. Oct 16, 2005

### kevinlikesphysics

how do i solve for this i only know how to do it with a degree and an Vi

the question is

a football is kicked at 45 degrees and travels 120 m before hitting the ground

and i need

Vi
time in air
and max hight

how do i get vcos45 without a velocity?

thanks

2. Oct 16, 2005

### Fermat

If you did know Vi and an angle, could you work out how far it travelled ?
How would you do that ?

3. Oct 16, 2005

### kevinlikesphysics

i dont have Vi

edit: could you work it out for me like with just equations

Last edited: Oct 16, 2005
4. Oct 16, 2005

### Tide

I think the rules here are that you cannot get direct help on a homework problem until you have at least shown some of your own work.

5. Oct 17, 2005

### Fermat

You have to find Vi, yes,
But, if if you did have a value for Vi, and also had an angle, could you then work out the distance travelled.
Just use symbols and show us some working.

6. Oct 17, 2005

### Beam me down

V(y)=vsin45
V(x)=vcos45

So t=(2vsin45)/g

d(x)=120

d(x)=v(x)t
120=(vcos45)((2vsin45)/g)
120g=2vcos45vsin45
v^2=(120g)/(2cos45sin45)
v=sqrt{(120g)/(2cos45sin45)}

7. Oct 17, 2005

### lawtonfogle

I think that constitutes giving him to much help. I think there is a equation that makes it simpilar, I'm looking now.
Ok, found it. Now, lets hope I remember how to layTex.
$$R = (V^2_0 \cdot 2 \cdot sin \theta) / g$$
this is the formula you use to find $$V_0$$ or as some say, $$V_i$$

Last edited: Oct 17, 2005
8. Oct 17, 2005

### lawtonfogle

I kinda remember. I want it to put the multipling dots, but instead it puts dots on top. Hmmm.... Guess I will go find out by looking at other tex.

9. Oct 17, 2005

### NoMeGusta

is R the distance ?

10. Oct 17, 2005

### lawtonfogle

Yes, it is the distance the object has traveled.

11. Oct 17, 2005

### Tide

It is the horizontal distance the object has traveled. :)