Hanging mass on a massless pulley

In summary, the conversation discusses a conceptual question about a problem involving two blocks connected by a rope over a pulley. The question involves setting the forces of friction equal and understanding the different values of tension and weight for the two blocks. The expert advises that assuming the second block's acceleration is negative is necessary for finding the correct answer and clarifies that there is only one force of friction in this scenario.
  • #1
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Homework Statement
A 3.5kg block is on a tabletop and is attached by a string to hanging block of mass 2.8 kg. The blocks are released from rest and allowed to move freely. If the table has a coefficient of friction of 0.4

A) Find the Acceleration
Relevant Equations
fnet x and fnet y
I have a conceptual question about this problem.
I can write the 3.5 kg block equation as Fnet(block 1)=(Force of tension)-(Force of friction)=m1a
I can write the 2.8 kg block as Fnet(block 2)=(Force of tension)-(Force of gravity2)=m2a

My question is this
If I set the forces of friction equal I get the wrong answer. However If I assume the fnet of block 2 is negative (because its going to fall) I get the correct answer.
Why do I need to assume the second block fnet is negative? Shouldnt the math already account for what needs to happen?
 
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  • #2
You used the same symbol ##a## for the acceleration of both blocks. That means you are assuming the two blocks have the same acceleration - including the sign of their acceleration. If you take the positive direction for block 1 to be in the direction of the tension force acting on block 1, then what must be the positive direction for block 2 (downward or upward)?
 
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  • #3
kasnay said:
I can write the 2.8 kg block as Fnet(block 2)=(Force of tension)-(Force of gravity2)=m2a

My question is this
If I set the forces of friction equal I get the wrong answer.
How can tension and weight of mass 2 have different values?
How those two compare if you screw mass 1 to the table?

You mention forces of friction, but there is only one, if mass 2 is freely hanging from the pulley.
 

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