Physics  grade12
posted by Danielle on .
In this problem they show a diagram of a pulley system with an inclined plane (25 degree incline). There is a box called m1=47kg resting on the incline plane and m1=35kg hanging on the vertical line. The question reads: The coefficient of friction between m1 and the surfvace of the inclined plane are Ustatic=0.42 and Ukinetic=0.19.
a)if the masses are held in place and then released , will they start to move? > I know the answer is NO.
b)how much mass would you have to add to m2 to cause the masses to begin to move? > The answer should be 2.8kg but I don't know how the book got this answer.
c)If you said no to a) and added the mass that you calculated in b), what would be the acceleration of the masses?
ANY help with this problem would be greatly appreciated!! THANK YOU SO MUCH in advance :)

and the answer at the back of this textbook says the acceleration should be 5.7 m/s^2.... but once again.. i don't know how they came up with this answer

well, look at m1. Force down the plane due to gravity: mgSin25
friction opposing motion: mg*mu*cos25
so the force on the m2 side (hanging mass) should be equal to the above sum.
47*9.8(sin25+.42cos25)=35*9.8+addedmass*9.8
your added mass should be the amount it takes to get it started.
Now, acceleration:
Net force= totalmass*a
(35+addedmass)9.847*9.8sin2547*9.8*.19= (35+47+addedmass)a
solve for a. 
For the force down the plane due to gravity, why isn't it just F=mg... and thank you so much for your help!

and which u do I use? should I use the static coefficient of friction since it isn't moving yet or the kinetic one since we want to know what will make the masses move?

forget about that last question haha

the formulas you gave didn't give the right answer, or at least not the same answer as they gave in the back of my textbook... any idea why this may be?

for some reason by multiplying the answer that I got for the "mass added" by 35kg, this gave me the answer at the back of the book

still doesn't help with finding the acceleration...