# Homework Help: Projectile motion

1. Sep 18, 2007

### flash21

some body help me...it is very simple...somebody throws two rocks from the top of a hill, one above the horizontal line and the other rock below the horizontal line, at an angle of 30 degrees for both rocks and the inicial speed is 13m/s for both rocks as well...the quiestion is how far does the rock 1 is from rock two when they both touch the ground..(rock 1 is the one below horizontal line...rock 2 over the hor....line)...wht i do not know is if wheter rock one(going down) has an inicial speed of 6.5m/s or 13m/s....by the way 6.5m/s comes from the inicial vertical velocity= (sin30)(13m/s)...

2. Sep 18, 2007

### Stevedye56

Try drawing a picture of the problem, it may help you visualize what is going on better.

3. Sep 18, 2007

### flash21

i have the picture in my book..just tellme if you know how to calculate the inical speed when a body is thrown downward at an angle of 30 degrees (below the horizontal line) with an initial speed of 13m/s?is it the same speed as the inicial vertical velocity or it is only the inicial speed given..?

4. Sep 18, 2007

### learningphysics

the initial horizontal speed is 13cos(30). The initial vertical speed is 13sin(-30) = -13sin(30). in other words the initial vertical speed is 13sin(30) downwards.

5. Sep 18, 2007

### flash21

i suppose that it is -6.5 then?

6. Sep 18, 2007

### learningphysics

Yes, the one thrown downward at 30 degrees below the horizontal has initial vertical velocity -6.5m/s.

The one thrown above has initial vertical velocity 6.5m/s.