# Projectile Motion

1. Oct 20, 2009

### Londoncalling

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

You jump off a diving board with a horizontal velocity of 1.81 and land in the water 1.4 seconds later. Gravity=9.8. How high is this platform?

2. Relevant equations
x=2.534, if that helps.

3. The attempt at a solution
I got 7.07 but this is not right...

2. Oct 20, 2009

### rl.bhat

Hi Londoncalling, welcome to PF.
What is the velocity in the vertical direction? What is the kinematic equation, which relates the initial velocity, time , acceleration and displacement?

3. Oct 20, 2009

### Londoncalling

y=vit-.5gt2 right?
I'm not sure though, we haven't learned it yet.

4. Oct 20, 2009

### rl.bhat

In the vertical direction the initial velocity is ....?
Your equation is right. But put -y for the displacement because it is also in the downward direction like g.

5. Oct 20, 2009

### Londoncalling

Okay so -y=vi -.5*9.8*1.42.
Then -y=vi*1.4-(4.9*1.96)
-y=vi*1.4-(9.604)
But there are 2 variables and only one equation.... I thought you couldn't do that...?

6. Oct 20, 2009

### Londoncalling

Or is your horizontal v of 1.81 the initial velocity?

7. Oct 20, 2009

### rl.bhat

The vertical velocity is zero.

8. Oct 20, 2009

### Londoncalling

So it's 9.604 m high then?
Edit: Yes, I got it right! Thank you SO much, you have no idea how relieved I am. Would you be able to help me with another problem (it's in a different post)....?