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QUESTION: What is the maximum mass of a rubber block that can be pulled by a rope whose maximum tension is 3500N? The block is sitting on dry asphalt.

FT = Fg = FN = 3500N

1.2 is the value of the coefficient of static friction (Ás) that is given in my textbook.

Ff(static) = (Ás)(FN)
         = (1.2)(3500N)
         = 4200N

So there is a force of 3500N pulling on the rope to move the block, however the force of static friction is 4200N, which appears to be greater than the force of motion. So right now my calculations show that the block cannot be moved unless the force pulling the rope is greater than 4200N. I do not think this makes sense because apparently the block is supposed to be able to move, but I do not know the right calculations to prove that.

ANSWER: I don't do homework questions, as it says in my profile very clearly, but since you started it I'll give you a hint.  You put the weight of the block equal to the tension in the rope for no apparent reason.  The weight is unknown.  The tension is known.  I'm just going to assume the rope is horizontal, otherwise the answer is different (obviously, you can lift the block if its weight is 3500N, but I'm sure the question didn't intend for anything but a horizontal rope fighting friction).

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QUESTION: So if the weight of the block is not equal to the tension in the rope, then how do I find the weight of the block? Because I thought that the force pulling on the rope (3500N) is directed along the rope, therefore the block would also be pulled with a force of 3500N. Everywhere I see, it says Fg = FT.

The mass of the block was the question, obviously they don't just give it to you.  You're mixing left-right forces (tension in the rope and friction) with up-down forces (normal force and mg).  You can't just set them equal, that's what the coefficient of friction is for in the first place.


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Dr. Stephen O. Nelson


I can answer most basic physics questions, physics questions about science fiction and everyday observations of physics, etc. I'm also usually good for science fair advice (I'm the regional science fair director). I do not answer homework problems. I will occasionally point out where a homework solution went wrong, though. I'm usually good at explaining odd observations that seem counterintuitive, energy science, nuclear physics, nuclear astrophysics, and alternative theories of physics are my specialties.


I was a physics professor at the University of Texas of the Permian Basin, research in nuclear technology and nuclear astrophysics. My travelling science show saw over 20,000 students of all ages. I taught physics, nuclear chemistry, radiation safety, vacuum technology, and answer tons of questions as I tour schools encouraging students to consider careers in science. I moved on to a non-academic job with more research just recently.

Ph. D. from Duke University in physics, research in nuclear astrophysics reactions, gamma-ray astronomy technology, and advanced nuclear reactors.

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