# Help with projectiles!

1. Jan 14, 2008

### erico004

baseball player hit a home run that was estimated to have landed 180 m from home plate and to have reached a height of 21.3 m.

a. how long was it in the air?
b. what was the initial vertical velocity?
c. what was the initial horizontal velocity?
d. what was the initial velocity of the ball? ( speed and direction)

??? anyone??

2. Jan 14, 2008

### Integral

Staff Emeritus
You need to show us something that you have tried.

You can start by finding how long it would take something to fall 21.3m.

3. Jan 14, 2008

### Davidrdguez

hahah the physics homework

you have to think in the movement in two phases:

vertical and horizontal.

with the vertical one you use the formule y=vo*t+..., there are acceleration

with the horizontal one you use s=v*t, there is constant the velocity

4. Jan 14, 2008

### rohanprabhu

I agree with Integral above that you should do something yourself before asking for help and as Moonbear puts it:

However, to get you started, I am gonna solve the first part for you.

i. time of ascent = time of descent

This follows by simple symmetry. It can also be proved using algebra/calculus and geometry, but i'll skip that here.

Let us consider the motion of the ball after it has reached the topmost point [initial position] along the y-axis only. From here, it falls to the ground [the final position]. At the initial position, it has a velocity = 0 [in the y-direction]. An acceleration in the y-direction = 9.8 m/s² causes it to come down.

http://img150.imageshack.us/img150/7938/projectilefbd1fs1.jpg [Broken]

So, we have:
$$s = -21.3~m$$
$$a = -9.8~ms^{-2}$$
$$u = 0$$
$$t = ?$$

Using $s = ut + \frac{at^2}{2}$, you get

$$t = 2.085~s$$

This is the time of descent. The total time will be twice of this i.e. 4.16s as it shall include the time of ascent also.

Do note that all calculations are made along the y-axis only.

Last edited by a moderator: May 3, 2017
5. Jan 14, 2008

### jake119

yeah, rohanprabhu is right, you need to show some work first, at least make an attempt on doing it yourself before you ask for help. How are you going to pass your test if you rely on everyone else to do your work for you? We won't be there for you doing your test, so make an attempt first kid :]

6. Jan 15, 2008

### Distortion

Eric, this uses the same techniques that I showed you yesterday in your horizontal velocity question.