# Homework Help: Projectile Motion boulder

1. Oct 14, 2008

### jackleyt

1. A 76.0 kg boulder is rolling horizontally at the top of a vertical cliff that is 20.0 m above the surface of a lake, as shown in figure below. (Intro 1 figure) The top of the vertical face of a dam is located 100 m from the foot of the cliff, with the top of the dam level with the surface of the water in the lake. A level plain is 25.0 m below the top of the dam.

1)What must the minimum speed of the rock be just as it leaves the cliff so that it will travel to the plain without striking the dam?

2)How far from the foot of the dam does the rock hit the plain?

2. Relevant equations
Equations of Motion, Projectile Motion

3. The attempt at a solution
I don't know how to incorporate the mass of the rock in the equation. I don't know where to start!

2. Oct 14, 2008

### hage567

Perhaps you don't need the mass? How would you approach the problem if you didn't know what the mass was?

Sometimes you don't always use all the information given to you in the question.