What is the tension in the cord connecting the two blocks at this instant?

AI Thread Summary
The discussion revolves around calculating the tension in a cord connecting two blocks with different coefficients of static friction. Block A has a friction coefficient of 0.45, while Block B has a coefficient of 0.30, both with a mass of 2.0 kg. The initial calculation for tension was incorrectly determined to be 8.82 N, based on the friction force needed to move Block A. The correct approach involves understanding the forces at play when a horizontal force is applied to Block B, leading to a required force of 15.092 N to initiate movement. The conversation concludes with the realization that the initial tension calculation was incorrect, but the correct answer was later confirmed by the instructor.
huybinhs
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Homework Statement



The coefficient of static friction between block A and a horizontal floor is 0.45 and the coefficient of static friction between block B and the floor is 0.30. The mass of each block is 2.0 kg and they are connected together by a cord.

What is the tension in the cord connecting the two blocks at this instant?
Answer: N


Homework Equations

and

The Attempt at a Solution



as the friction force on the block A is 8.82 N, for just to move it the force is 8.82 N and this is applied by the cord only. Hence the tension in the cord is 8.82 N

T = 0.45 * 9.8 * 2 = 8.82 N => INCORRECT. Why? Please help!
 
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Hi huybinhs! :smile:

At what instant? You haven't said what is happening to the blocks. :confused:
 
tiny-tim said:
Hi huybinhs! :smile:

At what instant? You haven't said what is happening to the blocks. :confused:

Part a for this problem is "If a horizontal force F pulling on block B is slowly increased until it is barely enough to make the two blocks start moving, what is the magnitude of F at the instant they start to slide?"

I already figured it out that the answer is 15.092 N.

I don't know how to do part 2. Please help!
 
huybinhs said:
Part a for this problem is "If a horizontal force F pulling on block B is slowly increased until it is barely enough to make the two blocks start moving, what is the magnitude of F at the instant they start to slide?"

ahhh! :rolleyes:

ok then, the acceleration of the second block is zero, so just balance the forces on the second block. :smile:

(or the first block … same result … just takes a bit longer :wink:)
 
Ok. Thanks! My answer is CORRECT, my instructor just fixed it. Thanks anyway ;)
 

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