# Ball projectile physics

1. Mar 7, 2005

### carltonblues

1)A ball is thrown upward with an initial velocity of v = 20 m/s. How long is the ball in the air? What is the greatest height of the ball? When is the ball 15 m
above the ground?

1) I have figured out

t total = 4.08s
greatest height = 20.4m

For the last bit (Finding the time when x=15m), I am having trouble. I know the answer but cannot work it through.

Do I use s = ut+1/2at^2 for it?

If I do, it wont work out because the 20.4 m has to come into it, doesnt it?

2. Mar 7, 2005

### dextercioby

The equation giving height (let's label it through 'y') as function of time is
$$y(t)=v_{0}t-\frac{1}{2}gt^{2}$$

Now set "y" to 15m and solve for "t"...

Daniel.

3. Mar 7, 2005

Cheers mate

4. Mar 9, 2005