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Thread: Torque

  1. #1 Torque 
    Forum Sophomore NimaRahnemoon's Avatar
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    So here's my problem:

    Can an object rotate if its net torque is 0?

    I had this on a test, my teacher said the answer is "Yes", but I said it is "No" and I still think it is "No"

    Here's my train of thought:

    You're in space and you throw an object up, while you are throwing it the object has some amount of torque. Once the object leaves your arm, the torque becomes 0, but the object is still rotating. So the net torque can never be 0, because there was some torque originally.

    Right? :?


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  3. #2 Re: Torque 
    Forum Professor wallaby's Avatar
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    Quote Originally Posted by Nima Rahnemoon
    So here's my problem:

    Can an object rotate if its net torque is 0?

    I had this on a test, my teacher said the answer is "Yes", but I said it is "No" and I still think it is "No"

    Here's my train of thought:

    You're in space and you throw an object up, while you are throwing it the object has some amount of torque. Once the object leaves your arm, the torque becomes 0, but the object is still rotating. So the net torque can never be 0, because there was some torque originally.

    Right? :?
    like with anything else i can think of involving forces if the net torque is 0 then the object is in a state of equilibrium and will either continue rotation at a constant rate or remain at rest.


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  4. #3  
    Forum Sophomore NimaRahnemoon's Avatar
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    Hmm, but for it to spin in the first place some torque was needed. Therefore the net torque can never be 0 and it can't spin in the first place without ever having any torque.
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  5. #4 Re: Torque 
    Forum Sophomore Absane's Avatar
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    Quote Originally Posted by Nima Rahnemoon
    Can an object rotate if its net torque is 0?
    I would say yes. Torque = force*radius. force = mass*acceleration.

    Think of spinning around on a marry-go-round with no resistance. It keeps spinning, yet you are applying no torque to it. If there is resistance, you need to keep the force up to keep it going.. hence you get a net torque != 0.
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  6. #5  
    Forum Professor wallaby's Avatar
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    Quote Originally Posted by Nima Rahnemoon
    Hmm, but for it to spin in the first place some torque was needed. Therefore the net torque can never be 0 and it can't spin in the first place without ever having any torque.
    the question was can an object rotate when the net torque is 0, thats not to say that it was always 0, but at any moment when the object is rotating at a constant rate there is no net force or torque

    the if the net torque were never 0 then the rate at which the object spins would accelerate.

    a F<sub>net</sub>>0 amount of torque was needed to get it in motion, but if the object is spining a a constant rate then the net torque is still 0.

    newtons first law.
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  7. #6 Torque 
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    Nima Rahnemoon asks Can an object rotate if the net torque is zero.
    I understand the question.But to put the question more accurately it should read as "Can an object rotate when the torque acting on the object externally is zero ?"
    An obect continues to be in the state of equlibrium or state of motion as long as no external force acts on it.That is the Newtons first law of motion.If you set an object ,say a round glass ball,to rotate by spinning
    it on a smooth surface ,the glass ball rotates for some time and finally comes to halt. This is because external forces acted on it and brought its motion to halt. The external forces are 1) frictional force between the glass ball bottom and the smooth surface on which it is rotating and 2)the frictional force between the surface of theball and the surrounding air.
    Thus when the torque applied initially is counteracted upon and is made zero the ball comes to a halt.
    If you are talking about space,where there is no subtance causing
    friction ,and where there is no possibility of the initially applied torque to become zero the object will continue to rotate.
    But if the torque becomes zero for some reason or other ( like internal liquids inside the body -even this is considered as external force as far as Newtons forst law is concerned)the rotation of the object stops.
    Science and method of science reached great heights in ancient India.Today blind faith overrules.What stopped the growth of science in India later.Any answer?
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