# Homework Help: A static problem

1. Jan 20, 2008

### yasar1967

1. In order to hang a load of mass M1=30kg from the horizontal, flat roof of a building, a plank of length L=2.4m is placed on the roof. One hand is held in place with a chunk of concrete of mass M2=15kg, and the other supports the load M1 with a light rope. How far can the end of the plank reach without tipping over? Neglect the mass of the plank.

2. System is in static. Total torque of the system and the net force is zero.

3. I saw the solution puts the normal force at the edge of the roof, vertical-upwards. Why? why don't we put it right above M2g? -just opposite direction of weight vector mass number 2?? Isn't it where normal force of surface occurs due to the gravitational force of mass number 2? Doesn't Newton's third law state that each force creates an equal one but in opposite direction WHERE it's applied?
1. The problem statement, all variables and given/known data

2. Relevant equations

3. The attempt at a solution

2. Jan 20, 2008

### Staff: Mentor

If you treat the system as being "the plank + mass 1 + mass 2" then the only normal force you need to worry about is that between the roof and the plank.

If instead you treat the plank and masses as three separate systems, then you must include the force each mass exerts on the plank (which will equal the weight of the mass, of course).

Either way works, but you are certainly correct that between each mass and the plank is a normal force.

3. Jan 20, 2008

thank you