EK 1001 Phys #245

[member232]

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245. Two masses hang across a massless, frictionless pulley. One mass is m, while the other mass is 2m. As the masses become infinitely large, the tension in the rope becomes:

A. 0 N
B. 500 N
C. 1000 N
D. infinite

The back of the book has a typo for the answer, so I just wanted to confirm my answer. I'm getting D, the tension will be infinite. Any thoughts? Thanks in advance

 

[jHustle]

I picked D as well

My reasoning:

T - mg = ma, so in this case T = mg + ma which equates to T = m(g+a), so with a rising mass, the T rises as well

Alternatively, they didn't list any specific masses, so the choices with specific numbers should be eliminated off the bat right? Making D the correct answer?

Hopefully this is right. Where is that milksi guy? lol

 

[Arctangent]

I would say infinite. B and C can be ruled out, given that they are finite non-zero numbers in a question that really can't prompt such an answer. A should be ruled out as in any system of 2 masses across a pulley, given non-zero masses there will be non-zero tension.

 

[milski]

Yes, it's infinity, the tension is mg. For m->inf, T=mg->inf. Of course, that's ignoring all the relativistic effects that would come up with such a mass but I don't think the question is about that.

While correct in this case, keep in mind that expressions can approach arbitrary numbers when some of the variables in them approach infinity.

 

[member232]

Thanks everyone! 🙂