# Homework Help: Finding inital velocity

1. Oct 29, 2004

### UrbanXrisis

A ball is kicked horizontally off a cliff 40m high. After 3seconds, the person that is on top of the cliff hears the thump of the ball hitting the floor. Assuming speed of sound travles 343m/s in air, what is the initial velocity that the ball was kicked at?

Please check if I did it correct....

Find time it takes for the ball to hit the floor:
d=.5at^2
40m=.5(9.8)(t^2)
t=2.857142857 s

Find time it takes for the sound to travel back to person on cliff:
3-2.857142857=.1428571429

Find the distance away from cliff:
d=vt
d=343*.1428571429
d=49

Find the distance travled in the x-direction:
49^2=40^2+x^2
x=28.3019434m

Find the initial speed:
v=d/t
v=28.3019434m/2.857142857s
v=9.9m/s

2. Oct 29, 2004

### vsage

Since the ball was kicked at an initial velocity $$v_1$$, $$d = v_1t + \frac {1}{2}at^2$$
You're solving for $$v_1$$ eventually. The calculation you made for v after that was the average velocity which isn't entirely right.

3. Oct 30, 2004

### UrbanXrisis

shoot, you're right

how should I go about solving this?

4. Oct 30, 2004

### UrbanXrisis

I was solving for the time it took the ball to drop. It has no initial velocity.

That's exactly what I did.

5. Oct 30, 2004

### Staff: Mentor

Looks good to me.

6. Oct 30, 2004

### Skomatth

I got 9.9 also.

7. Oct 30, 2004

### Galileo

The initial velocity was in the x-direction, while the acceleration is in the y-direction. So this isn't the way to get the distance d.

UrbanXrisis' solution looks good to me.

8. Oct 30, 2004

### vsage

Sorry I totally didn't see that it was horizontally kicked just that it was kicked.