# Homework Help: Projectile Motion questions

1. Oct 24, 2005

### 404

I thought that was surprisingly easy, just use (sin30)*60*4, but I ended up with 207.84m, but the answer book said it was 212m? What did I do wrong?

I figured out the vertical component of the initial speed to be 28 m/s, but how can you get the horizontal? I think you may be able to get the horizontal distance with the speed of sound?

2. Oct 24, 2005

### whozum

You're looking for the horizontal component of distance.

edit: unless there is more given information, your answer is correct. I think you accidentally wrote sin instead of cos.

To solve this one you'll need to realize that the total time (3 sec) is the time it takes for the rock to fall, hit the water, then let the soundwave travel back up to you. Don't worry about horizontal and vertical components, treat it as if he dropped the rock.

3. Oct 24, 2005

### Astronuc

Staff Emeritus
One has to figure the time that the rock falls 40 m. Assume it starts at rest and drops, accelerating with gravity. That is tf or time falling.

Then the sound take time, ts to return, and

tf + ts = 3 sec

Find the relationship between time and distance in the vertical direction.

The distance the sound travelled = L = $\sqrt{{40m}^2+x^2}$ where x is the horizontal distance traveled.

Assume constant velocity, vx in x-direction.

4. Oct 24, 2005

### 404

Got it. Thank you.