# A doubt regarding torque

by dreamz25
Tags: doubt, torque
 P: 26 If a body is rotating in a circular orbit then what is the moment of net force acting on it about the axis of rotation?
 Mentor P: 41,568 What do you think?
 P: 26 0 but how?
Mentor
P: 41,568
A doubt regarding torque

 Quote by dreamz25 0 but how?
Which way does the net force act? What's the definition of torque?
 P: 26 in gravitation i have read that... the force on a planet towards radius is given by GMm/r^2 and the force which acts radially outwards is Mv^2/r so dunno where the net force acts... !!!:{ and torque = F X R if the direction of net force is towards center then it makes and angle of 180 degrees which gives ex. torque = 0 as sin 180 = 0 .... but just tell me about the direction of net external force...!! (Thanks in advance)
Mentor
P: 41,568
 Quote by dreamz25 in gravitation i have read that... the force on a planet towards radius is given by GMm/r^2
OK.
 and the force which acts radially outwards is Mv^2/r
There is no outward force.
 so dunno where the net force acts... !!!:{
The only force acting is gravity.
 and torque = F X R if the direction of net force is towards center then it makes and angle of 180 degrees which gives ex. torque = 0 as sin 180 = 0 ....
Good.
 P: 26 How then do we derive the velocity of a plannet in circular orbit????? when we equate both of them... v = root[GM/R]...!!
Mentor
P: 41,568
 Quote by dreamz25 How then do we derive the velocity of a plannet in circular orbit?????
By applying Newton's 2nd law. The only force is gravity. Set that equal to mass X the centripetal acceleration.

No need for any mysterious outward force.
 P: 26 ok.. so u mean both of them acts towards the centre... Right? since the force on the particle by the center equals GMm/R^2 and also by Newton's second law of motion, F = ma so, F = m x centripetal acceleration (which is towards the center) = m x v^2/r and thus we get, v = root[GM/r] ...?
Emeritus
PF Gold
P: 5,196
 Quote by dreamz25 ok.. so u mean both of them acts towards the centre... Right? since the force on the particle by the center equals GMm/R^2 and also by Newton's second law of motion, F = ma so, F = m x centripetal acceleration (which is towards the center) = m x v^2/r and thus we get, v = root[GM/r] ...?
Your math is right, I think you just have a conceptual problem. When you say "both of them acts towards the centre", my question to you is, both of what?

We're not talking about two distinct forces here. Gravity IS the centripetal force in this situation. Centripetal force is always just a requirement for circular motion. It has to be provided by something real, like gravity, or tension in a string. Without something like this to provide (or act as) a centripetal force, there simply won't be any circular motion.
 P: 26 got ur point.... but i too meant the same... i m nt differentiating the two force i just meant the different expressions for a single force.. the force between them is GMm/r^2 which also equals mv^2/r (the centripetal force which acts towards the center to keep the body rotating in a circular path) and thus gets the formula derived.....
P: 26
 Quote by cepheid Centripetal force is always just a requirement for circular motion. It has to be provided by something real, like gravity, or tension in a string. Without something like this to provide (or act as) a centripetal force, there simply won't be any circular motion.
wonderful lines... thanks..!!

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