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Tension in a rope on frictionless pulley 
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#1
Dec1807, 05:21 PM

P: 15

1. The problem statement, all variables and given/known data
A 4kg object is palced on a table with a coefficient of friction of 0.2, and attached to a rope. This rope is run through a frictionless pulley, and attached to an 8kg weight. What is the tension in the rope? 2. Relevant equations F=ma F of friction = (coefficient of friction)(Force normal) 3. The attempt at a solution Force normal of weight = 8(9.8) = 78.4 Force normal of object = 4(9.8) = 39.2 Force of friction on object = 0.2(39.2) I don't know where to go from here, and I have an exam tomorrow. Help would be greatly appreciated, obviously :P Thanks, I hear you guys are the best for quick physics help ;) 


#2
Dec1807, 06:10 PM

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Apply Newton's 2nd law to each mass, then combine the two equations.



#3
Dec1807, 06:16 PM

P: 15

So add both forces from F=ma together?



#4
Dec1807, 06:22 PM

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P: 41,482

Tension in a rope on frictionless pulley
What forces act on each mass? 


#5
Dec1807, 06:25 PM

P: 15




#6
Dec1807, 06:29 PM

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What pulls on the object is the tension in the string. (Of course that tension also pulls up on the weight.)



#7
Dec1807, 06:34 PM

P: 15

Ahh, I think I get it (but physics has confused me from the start, so bear with me). The tension is essentially a net force?



#8
Dec1807, 06:42 PM

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No. "Net force" just means the total force on an object. The tension is just one of several forces.



#9
Dec1807, 07:03 PM

P: 15

Alright, I'll be honest, I still don't get it :(
This was a quick question I came up with to demonstrate what I need to know. What would be the tension? Then I could figure out how to actually get it. 


#10
Dec1807, 07:14 PM

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P: 41,482

Start by applying Newton's 2nd law to the hanging mass. What forces act on it?



#11
Dec1807, 07:20 PM

P: 15

Gravity times its own weight, and the object on the table (which I presume can act as friction). Anything I'm missing?
The object on the table is also prone to friction, and the weight that is hanging pulls back on it. 


#12
Dec1807, 07:27 PM

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#13
Dec1807, 07:30 PM

P: 15

I can't think of any, unless you mean the rope/tension itself.



#14
Dec1807, 07:33 PM

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There are two forces acting on the hanging mass: The rope tension, which acts up; the weight, which acts down. Now apply Newton's 2nd law. 


#15
Dec1807, 07:37 PM

P: 15

Haha, thought that would be too easy :P
The force down = 8(9.8) = 78.4 N Not sure how to deal with the rope tension, tbh... 


#16
Dec1807, 07:40 PM

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P: 41,482

The rope tension is unknown, so just call it T. (You'll end up solving for it.)



#17
Dec1807, 07:42 PM

P: 15

Oh, ok.
So the net force is F down  T 


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