# Projectile motion of conservation of energy?

1. Oct 14, 2005

### firemanpato

Projectile motion of conservation of energy?????

I have a problem that is stumping me. A guy throws a ball and wants it to go through a hoop 20 meters away and the hoop is 2 meters taller than from where he released the ball. If the only given info is the angle at which he threw the ball (45 degrees) How do I set up this problem to determine the initial speed at which the ball was thrown to make it go through the hoop. I have no idea how to set it up.

2. Oct 14, 2005

### Pengwuino

Dang it! I just helped someone solve this exact problem a week ago.

I think the one thing that will help you the most is that the major problem here is that you really don't have the time or the initial velocity. With that in mind, one hint that should help is that you can determine the time using gravity and the height requirement!

3. Oct 14, 2005

### shanu

since you know both the X and Y at the time the projectile pass the thread you can use the eqation for the trijectory of the projectile. I solved it in that way and got answer as 14.75

y=Xtan(45)-(gX^2)/2(vcos45)^2

(refer any book for the eqation i am not so sure about it but the value 14.75 have been tested and found correct.)

4. Oct 15, 2005

### Päällikkö

Although shanu's equation seems correct, memorizing loads of equations is hardly the way to do physics. The equation can be easily derived as we know that for constant acceleration:
x = x0 + v0xt + ½axt2
y = y0 + v0yt + ½ayt2

Two equations, two unknowns (as the equations simplify quite a bit with the information given in the problem).

5. Oct 15, 2005

### hotvette

I agree 100%. In fact, I'd go one step further and say that it's even easier to start with F = ma and derive the 2 equations of motion of the projectile (but, only if you've had calculus). I had elementary physics x years ago (I won't tell you what x is ), and had long since forgotten those 2 equations, but F=ma I'll always remember.

Last edited: Oct 15, 2005
6. Oct 15, 2005

### shanu

thank you for your advice. but it is the same i do. never memeorises the equation which i could get in a moment of calculation or thinking but when it is to take a long time i memorises them anlong with the way to derive it.

ANY WAY THANK U ALL FOR UR SUGESTIONS