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![[Post New]](/templates/default/images/icon_minipost_new.gif) 3 Jul 2007 20:36:29 IST
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A ball of mass m tied to the end of a massless string is revolved in a vertical circle of radius R. The net force at the lowest and highest points of the circle directed vertically downwards are ? (T1, T2 denote the tension and V1, V1 denote the speed of the stone in the lowest and highest point respectively. )
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3 Jul 2007 20:47:11 IST
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T1=m(V1)^2/R
T2 = m(V2)^2/R
..so net force at bottommost point = mg-T1
net force at top most point = mg+T2
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3 Jul 2007 20:52:00 IST
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Your answer is right. But I have a few doubts. I always get confused with this centripetal force.
Centripetal force is directed towards the center right? If the net force at the bottom is mg-T, shouldn't the string break, as the mg is greater than tension?
Similarly, at the top point, mg+T is directed towards the center, that is downwards, and centripetal force is directed towards the center too, so again, the string should collapse right? Clear my doubt please!
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3 Jul 2007 20:55:03 IST
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ur right if the string was stationary..but since it is in circular motion...centrifugal force prevents dis...
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3 Jul 2007 21:03:55 IST
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I still don't get it dude. Can you explain more elaborately?
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3 Jul 2007 21:11:19 IST
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ya string will break...fall on ur head ....and u'll go 2 hosiptal....u'll soon b discharged and come back 2 goiit and ask the same ques again ok????
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3 Jul 2007 21:46:14 IST
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What a lame joke... lamer. Go get a life. And learn how to spell hospital. I bet you didn't understand head or tail of the question, so just shut up.
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3 Jul 2007 21:50:24 IST
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who said that mg is greater than tension at bottommost point!!
net force will be negative if we have the exact values...
the string rotating because of the fact that T>mg at bottommost point and T<mg at topmost point!
any force acting towards the centre provides the centripetal acceleration...
centrifugal force is just an imaginary force...u may think that it is acting outwards frm the circle...and then balance forces
they will bring out the same result..
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3 Jul 2007 21:51:31 IST
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Net force in the radial direction in circular motion is always equal to centrifugal force that is mv2/r
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4 Jul 2007 00:57:56 IST
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Understand this first: The external forces on the particle are the tension and the weight. So we have to find the vector sum of these force at the top and bottom points.
At the lowest point
T1 - mg = mv12/R, which is the resultant force, directed upwards.
And at the highest point,
mg + T2 = mv22/R, which is the resultant force.
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4 Jul 2007 02:46:01 IST
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Now I get it. I was under a wrong notion that centripetal force is directed outward. Now I understood clearly.
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