Grade 12 physics - one mass on incline and other hanging

1. Nov 1, 2005

an_mui

Body B weighs 440 N and body A weighs 140N. The coefficients of friction between B and the incline are Us = 0.56 and Uk = 0.25.
a) Find the acceleration of the system if B is initially at rest
b) Find the acceleration of the system if B is moving up the incline
c) Find the acceleration of the system if B is moving down the incline.

(diagram: mass B is on the incline, and a is hanging)

For a, my teacher showed us the following solution:
a) mass b (g)(sin theta) ?> mass a (g)
a = ((mass b)(g sin theta) - (mass a)(g) - Ffriction) / (mass B + mass A)
= 0

Could someone explain this solution to me? I think I will be able to figure out parts b and c once i understand part a.

Thanks in advance and sorry I couldn't scan the diagram.

Last edited: Nov 2, 2005
2. Nov 1, 2005

daniel_i_l

You find the components of each force acting on B in the direction of the plane (tension , gravity ,friction) then use f = ma.

3. Nov 2, 2005

an_mui

hm.. sorry i tried that and i still couldn't figure out how to do this question

4. Nov 2, 2005

5. Nov 2, 2005

verty

First you calculate the resultant force, then you calculate the acceleration.

The resultant force is: m_b*g*sin(theta) - m_a*g +/- friction. I put +/- there because the friction always opposes motion. What don't you understand?

Last edited: Nov 2, 2005
6. Nov 2, 2005

an_mui

I don't understand why the answer to part a is 0...

Thanks I got parts b and c now

7. Nov 2, 2005

verty

The answer to part a will be 0 if friction overcomes the stronger of the applied force and gravity. You haven't given the angle of the incline so I don't know if it does or not. I imagine m_b*g*cos(theta)*Us > m_b*g*sin(theta) - m_a*g, which would mean a = 0.