# Homework Help: Projectile motion of half a parabola

1. Oct 14, 2012

### weesieman

1. I have a ball that is thrown off a 3m high cliff at 20 m/s horizontally, what is the displacement on the X axis, Vf, and tair? (assuming no air resistance)

2. Relevant equations
d=Vi*t+(1/2a*t2

3. The attempt at a solution
When I tried to solve this problem I couldn't really find an equation that worked. I'm confused about how to find the time in the air with the given information, do you guys know any other formulas that would be helpful for this problem?

2. Oct 14, 2012

### mishek

The formula you wrote would work. Do you know what are the parameters in it?

3. Oct 14, 2012

### hotvette

You need to think in terms of separate x and y components. The equation you've stated is for the y-component. What is Vi (y direction) in this case? The problem statement tells you. From this you can find t.

Next, you need a kinematic equation for the x direction. It's pretty simple. Think of the forces acting in the x direction. Any guess?

4. Oct 14, 2012

### weesieman

I don't think I can use it because all I really know in that formula is Vi and Acceleration

5. Oct 14, 2012

### weesieman

Vi for Y is 0, so I would need to use the formula to find the time, which is .77 seconds, so with this I can find Dx right?

6. Oct 14, 2012

### hotvette

Yes, but what's the distance equation in the x-direction. Like I said, pretty simple.

7. Oct 14, 2012

### mishek

I think you know one more thing. Check the known data once more.