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#1
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I can't seem to get the correct answer when working out the following question:

A block of 60kg on a ramp inclined at an angle (x) to the horizontal where tan (x) = 3/4. A rope is attached to the block and held parallel to the line of greatest slope of ramp.

The coefficient of friction between ramp and block = 2/3. Modelling the block as a particle, find the largest tension in rope that can be applied before the block moves up the ramp.

The proper answer is 744.8N, but I get 666.4N, by taking the force from the mass down the plane and the friction from the tension. What am I doing wrong here?
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Eru Iluvatar
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Ermm... im not sure really. Could you leave a step by step, so its clear what you have done, and easier to spot mistakes. It sounds right though, but this is the sort of thing a usually struggled on as well.
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OK, this is what I've done.

Just before the tension pulls the block up the ramp, it must be the same as the forces acting against it i.e. 60gsin(x), which is the force of the weight when resolved along the plane, and the friction F, which is 2/3 * 60gcos(x). By taking these two forces away from T and adding it to the RHS (where the RHS is 0 since I'm assuming the block is not moving), I would get T= 666.4N.
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Eru Iluvatar
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As i said, im not very good at this really. What is the RHS?
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Right hand side of the equation. I've done about ten similar questions and despite following the method, most of the answers are wrong. Sniff.
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Ive tried all the methods i know of doing this, and ive got answers between 200 and 900, but none near the answer they said was right (744.8N).
I'll have a think about it, but i've got an exam at about half 1, so i better get ready for that soon.
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OK, good luck.
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Eru Iluvatar
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Yeah, thanks. I should get the answer though. I did my mechanics exam easily, and got those sorts of Q's right all through my revision.
Maybe im just not concentrating.
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I have found one way of satisfying the answer, but it defies logic completely. By adding F and uR to make 666.4 (like I had before), then taking uR from F (to make 39.2), multiplying the answer and adding it to the 666.4, 744.8N is made. Of course, I would never think of doing that in the exam.
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Yeah. There probably is some logic to it, but ive got an exam soon so my brain isnt turned on.
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What exam?
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Eru Iluvatar
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Business
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I've taken business as well, although my exams are bunced near the end (middle of June).
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Eru Iluvatar
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Well ive got 3, 1 hour exams in a row starting about half past 1.
And the rest of my exams are all this week.
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Are they all business or different subjects?
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All my business ones are today, but the rest of the week is all my other exams.
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I'm dreading the maths exams I have next Friday - I didn't do too well last year in As, so basically this year I have to take the whole A ans AS levels again - it does help that the stuff in AS compliments the A level content though.
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Well i have problems in pure and mechanics, but ive already taken mechanics, and i did really well. Ive got pure tomorrow, and stats on thursday.
Its fairly easy though in my opinion, as its mostly logical.
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Originally posted by Unregistered
I can't seem to get the correct answer when working out the following question:

A block of 60kg on a ramp inclined at an angle (x) to the horizontal where tan (x) = 3/4. A rope is attached to the block and held parallel to the line of greatest slope of ramp.

The coefficient of friction between ramp and block = 2/3. Modelling the block as a particle, find the largest tension in rope that can be applied before the block moves up the ramp.

The proper answer is 744.8N, but I get 666.4N, by taking the force from the mass down the plane and the friction from the tension. What am I doing wrong here?
The answer 744.8 N is wrong. 666.4 N is correct. You should have more trust in yourself :-).

Regards,
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