m1 - pulleys question Watch

wizz_kid
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Hi,
Please excuse my diagram. Any ways, here is the question :





One end of a light string is attached to a block of mass 40 kg which is at rest on a horizontal shelf. The string passes over a fixed smooth pulley and a block of mass 10 kg is atttached to the other end of the string. The parts of the string between the blocks and the pulley arevertical. Find the magnitude of the force exerted by the shelf on the 40 kg block.


Attempt :

I found the tesion in the string and i assumed it to be the right answer but its not so...i dont know what iam looking for!
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lordcrusade9
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erm the tension the string does not equal to the weight!!!!!!!!!!!! the net force on the 40kg mass is 0.

the tension is acting upward

weight is acting downward

their discrepency is the reaction of the table/shelf
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wizz_kid
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(Original post by lordcrusade9)
erm the tension the string does not equal to the weight!!!!!!!!!!!! the net force on the 40kg mass is 0.

the tension is acting upward

weight is acting downward

their discrepency is the reaction of the table/shelf
so...what are we actually looking for?
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lordcrusade9
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what is your tension?
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spook2575
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by any chance is the answer 40g-T
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lordcrusade9
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that is what i implied, but it is not very helpful to tell someone the answer without explaining. it is better to guide someone toward the solution than tell them
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spook2575
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(Original post by lordcrusade9)
erm the tension the string does not equal to the weight!!!!!!!!!!!! the net force on the 40kg mass is 0.

the tension is acting upward

weight is acting downward

their discrepency is the reaction of the table/shelf
lordcrusade9 wrote the explanation in a clear manner.
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wizz_kid
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hmmm....
the tension which i got is 98 N. The real answer is 294 N
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wizz_kid
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(Original post by spook2575)
by any chance is the answer 40g-T
yea! can u explain plz
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lordcrusade9
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tension is 10g or 98 N

for the 40kg mass, the force upward is the tension which is 98 newton.
the force downard is 40g but the net force is 0N because it is not acclerating.

so the Reaction force must be 40g-10g = 30g

they didnt ask for the tension in the question, they asked for reaction force
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spook2575
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newton's 3rd law 'For every action, there is an equal and opposite reaction.'

so as the 40kg particle rests on the shelf, its weight is 40g. Therefore the reaction (R) exerted by the shelf is 40g as well, otherwise the particle would fall through the shelf. However since the particle is attached to a string, the whole weight of the particle is not felt by the shelf. It is the weight- tension (since tension acts upwards) which is 40g-T.

Edit: too slow lol.
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wizz_kid
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(Original post by lordcrusade9)
tension is 10g or 98 N

for the 40kg mass, the force upward is the tension which is 98 newton.
the force downard is 40g but the net force is 0N because it is not acclerating.

so the Reaction force must be 40g-10g = 30g

they didnt ask for the tension in the question, they asked for reaction force
oh yea!

thnkx
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