How Can I Solve This Newton's Forces Problem for My Homework?

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The discussion revolves around solving a Newton's forces problem involving two blocks and friction. One participant expresses confusion about calculating the acceleration and the role of static and kinetic friction. Another contributor clarifies that the tension in the system is not simply 10g and emphasizes the importance of performing a force balance on the second block to understand the teacher's approach. The conversation highlights the need to consider both blocks' masses in calculations and the distinction between static and kinetic friction. Overall, the thread addresses common misunderstandings in applying Newton's laws to multi-body problems.
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Homework Statement


ScreenShot2014-05-04at33239PM_zps8bf4c472.png


Homework Equations





The Attempt at a Solution


A)
I am not sure how to go about solving this. I would say no you can't find Us. Can you just say that you do not know when the object will move?
B) First I found acceleration x= 1/2 at^2, 18= .5a(9), a= 4.
Then I used the free body diagram of m1. Fk pointing to the left, Tension 2 pointing to the right.
T2- Fk= ma
10g - 5g Uk = (5)(4), but my teacher uses the mass of both the blocks, 5+ 10. I am not sure why...
 
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I don't understand how there can be static AND kinetic friction between M1 and the surface..
 
mrnike992 said:
I don't understand how there can be static AND kinetic friction between M1 and the surface..

Not at the same time, obviously...
 
Your mistake is that T2 is not 10g. Just do a force balance on M2 and you will see what it really is, and you will also find out why your teacher used (10 + 5).

Chet
 
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