Grade 12 physics - one mass on incline and other hanging

[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.

 

[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.

 

[an_mui]

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

 

[an_mui]

urgent... please help!

 

[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?

Show your working out please.

 

[an_mui]

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

Thanks I got parts b and c now

 

[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.