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Mass on incline will it slip or stick?

  • Thread starter charan1
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  • #1
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Homework Statement


Figure shows a block of mass m resting on a 20* slope. The block has coefficients of friction uS=0.80 and uK=0.50 with the surface. It is connected via a massless string over a massless, frictionless pulley to a hanging block of mass 2.0 kg.

Part A:
What is the minimum mass m that will stick and not slip?

Part B:
If this minimum mass is nudged ever so slightly, it will start being pulled up the incline. What acceleration will it have?

Homework Equations


Fnet=ma
us=F/N

The Attempt at a Solution



I really have no idea how to do Part B, but I tried part A and was real confident with my answer but it turned out wrong. This is what I did.

(2kg)(9.8)=19.6N for the hanging mass

assuming that the Hanging mass = to the tension force on the block with mass m I then did this...

19.6N=m(9.8)cos20*(.8)
m=2.66kg and this is wrong

Were did I go wrong?

Thanks
 

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Answers and Replies

  • #2
tiny-tim
Science Advisor
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Welcome to PF!

Figure shows a block of mass m resting on a 20* slope. The block has coefficients of friction uS=0.80 and uK=0.50 with the surface. It is connected via a massless string over a massless, frictionless pulley to a hanging block of mass 2.0 kg.

Part A:
What is the minimum mass m that will stick and not slip?

Part B:
If this minimum mass is nudged ever so slightly, it will start being pulled up the incline. What acceleration will it have?

(2kg)(9.8)=19.6N for the hanging mass

19.6N=m(9.8)cos20*(.8)
m=2.66kg and this is wrong
Hi charan1! Welcome to PF! :smile:

(Useful tip: don't bother to multiply everything by 9.8 … all the terms have 9.8, so just call it g, and cancel it at the end. :wink:)

(oh … and have a mu: µ :smile:)


I take it you were applying Newton's second law in the direction of the slope.

That means that you must include all the forces … you left out the force due to gravity. :smile:

(For part B, use µK instead of µS)
 

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