Posted by **Danielle** on Tuesday, March 13, 2012 at 4:19pm.

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 dont 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 :)

- Physics - grade12 -
**Danielle**, Tuesday, March 13, 2012 at 4:24pm
and the answer at the back of this textbook says the acceleration should be 5.7 m/s^2.... but once again.. i dont know how they came up with this answer

- Physics - grade12 -
**bobpursley**, Tuesday, March 13, 2012 at 4:37pm
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)9.8-47*9.8sin25-47*9.8*.19= (35+47+addedmass)a

solve for a.

- Physics - grade12 -
**Danielle**, Tuesday, March 13, 2012 at 5:01pm
For the force down the plane due to gravity, why isn't it just F=mg... and thank you so much for your help!

- Physics - grade12 -
**Danielle**, Tuesday, March 13, 2012 at 5:06pm
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?

- Physics - grade12 -
**Danielle**, Tuesday, March 13, 2012 at 5:14pm
forget about that last question haha

- Physics - grade12 -
**Danielle**, Tuesday, March 13, 2012 at 5:26pm
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?

- Physics - grade12 -
**Dani**, Tuesday, March 13, 2012 at 5:49pm
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

- Physics - grade12 -
**anonymous**, Tuesday, March 13, 2012 at 6:58pm
still doesn't help with finding the acceleration...

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