# Homework Help: Projectile Motion - Horizontal Launch

1. Oct 22, 2008

### bkoiskewl1

1. The problem statement, all variables and given/known data
A stone is thrown horizontally at 5.0 m/s from the top of a cliff 78.4 m high.
a. How long does it take the stone to reach the bottom of the cliff?
b. How far from the base of the cliff does the stone strike the ground?
c. What are the horizontal and vertical components of the velocity of the stone just before it h its the ground?

2. Relevant equations

a. yf = yi + vyi t + (1/2)at^2
b. xf = xi + vxi t
c. ?????

3. The attempt at a solution
a. 0 = 78.4 + 0 - 4.9t^2
78.4 + 0 - 4.9t^2
-4.9t^2 = -78.4
t= 4 sec...is this correct?

b. xf = 0 + 5m/s(4 sec)
= 20 m..is this correct??

2. Oct 22, 2008

### heth

Do you know any equations that involve velocity and acceleration?

3. Oct 22, 2008

### bkoiskewl1

i don't know what the question is asking for

4. Oct 22, 2008

### heth

Can you explain a bit more about exactly what it is you don't get? Is it the meanings of some of the words, or something else?

5. Oct 22, 2008

### bkoiskewl1

I guess its the meaning of the words...what kind of answer are they asking for..?

6. Oct 22, 2008

### heth

Do you know what a "component" is? Also called a "component vector" sometimes? If not, it's probably an idea to read about them in your textbook, then come back to the question and see if it makes more sense.

(You already used some components in your solutions to the earlier parts of the question, but probably without calling them that yourself! It's well worth the time spent, as you'll be using them all the time later on in your course.)

7. Oct 22, 2008

### bkoiskewl1

Could you tell me what it is...? I don't have my book with me..

8. Oct 23, 2008

### heth

Google is your friend. This is the first hit when you search for "component vector"

http://id.mind.net/~zona/mstm/physics/mechanics/vectors/findingComponents/findingComponents.htm [Broken]

Last edited by a moderator: May 3, 2017