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Thread: Why is torque relevant?

  1. #1 Why is torque relevant? 
    墨子 DaBOB's Avatar
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    In a car engine why is the force of torque, which is perpendicular to the rotation of the crank shaft, important? Would there be a significant difference, for example, to the functioning of an engine if the whole system was mirrored so that the torque's direction, or vector (?), was reversed?

    Oh, by the way. I'm using the car engine as an example but I'm really just curious about torque which is why I'm putting this in the physics section.


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    Another torque question. Is torque refer to two fores? The for on the lever and the perpendicular force at the fulcrum. Would these forces then be the same since the length of the lever is not changing?


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    If you reverse the torque of the engine, then your car will go backwards, which is kind of obvious so that you must mean something else by your question.

    Consider two people on a seesaw. If they are not accelerating, then the sum of the torques must be zero, and the sum of the forces must be zero. The torque is the weight of the person multiplied by the length of the lever arm, with one person applying a torque in the opposite direction to the other. The forces involved are the weights of the person, which are applied in a downward direction, balanced by an upward force at the fulcrum.
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    But isn't there a another force going perpendicular to the forces on the lever arms? I read it noted as an "F" with and upside-down "T" after it. My question about the car was, if there is this perpendicular force how is it relevant? This is confusing me.

    Also, I remember way back, when I actually was taking physics courses, we did a little experiment where a person sits on a swivel chair with a bicycle wheel in their hands they spin the wheel and the chair. Then they flip the wheel the other way around and the chair slows and starts spinning the other way. How does this happen?
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    Dabob, can we really call torque a force? One equation for torque is T= R x F, where F is force and R is the distance from the axis of rotation (or lever length). Could this equation be telling us that torque is not a force in itself but a result of a force? I have heard torque described as a spinning force. Giving the equation above it would seem that torque refers to a single force rather then two. Though torque can be understood as a spinning force, I'm still looking for what the torque vector actually means, though, I am unsure if it is an actual force...

    I too remember the swivel chair experiment. Its interesting that once the wheel is flipped around, the swivel chair rotates in the direction opposite to the wheel. This is key, the chair swiveling the opposite direction has something do to with the conservation of angular momentum. Newtons third law also has a part in describing why the chair spins counter to the wheel: Since your hands are holding the spinning wheel, and apply a force (remember the resistance you feel when you turn the wheel upside down?) your hands will feel an equal and opposite force in the other direction, since your sitting on a swival chair you start to spin in that other direction!
    Last edited by Aconic; July 26th, 2011 at 03:16 PM.
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    Quote Originally Posted by DaBOB View Post
    But isn't there a another force going perpendicular to the forces on the lever arms? I read it noted as an "F" with and upside-down "T" after it. My question about the car was, if there is this perpendicular force how is it relevant? This is confusing me.

    Also, I remember way back, when I actually was taking physics courses, we did a little experiment where a person sits on a swivel chair with a bicycle wheel in their hands they spin the wheel and the chair. Then they flip the wheel the other way around and the chair slows and starts spinning the other way. How does this happen?
    There isn't a force going perpendicular to the lever arm. The direction of the torque vector is taken as along the axis of rotation, using the right hand rule. This doesn't mean there is any motion or force along the axis. It is just a mathematical convention.

    What you have to remember about torque and rotational inertia is that each particle behaves according to Newton's laws. There is nothing magic, it just seems that way. In the case of the bicycle wheel and swivel chair consider this. Let's say you are holding the wheel in front of you and it's not spinning. Then you turn the axle counterclockwise as if you are turning a car's steering wheel to the left. The bicycle wheel just tilts to the left, as expected.

    Now what happens if the wheel is spinning forward. Consider a particle of mass at the top of the wheel at a certain instant. It is traveling away from you. A particle at the bottom of the wheel is traveling toward you. When you turn the wheel to the left, you are tending to make the particle at the top go left, but since it has some momentum, it tends to be deflected left but still maintains some speed forward. Likewise the particle at the bottom of the wheel wants to go to the right and rear. The only way this can occur is for your chair to swivel left.
    Last edited by Harold14370; July 27th, 2011 at 11:51 AM.
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    Quote Originally Posted by Harold14370 View Post
    There isn't a force going perpendicular to the lever arm. The direction of the torque vector is taken as along the axis of rotation, using the right hand rule. This doesn't mean there is any motion or force along the axis. It is just a mathematical convention.
    So, the only reason torque is given a vector is because it is adopting it from Force (or originally acceleration)? I still don't understand what the point is. The vector is the direction of the Force, correct? If, in the case of torque, this vector is merely a mathematical convention than where is the direction of the Force? Am I just thinking about this too hard? I remember having trouble with this the first time I studied it.

    Ooooh... I think I just hit on something I wasn't considering before and that is the vector product. However, the term itself isn't really helping me.

    This is from Wikipedia: Cross product - Wikipedia, the free encyclopedia

    "The direction of the vector n is given by the right-hand rule, where one simply points the forefinger of the right hand in the direction of a and the middle finger in the direction of b. Then, the vector n is coming out of the thumb (see the picture on the right). Using this rule implies that the cross-product is anti-commutative, i.e., b a = -(a b). By pointing the forefinger toward b first, and then pointing the middle finger toward a, the thumb will be forced in the opposite direction, reversing the sign of the product vector."

    Maybe I shouldn't be relying on Wikipedia. How does the vector become perpendicular and why is it important? Sorry if this has already been explained but I'm still not grasping it.

    So, if torque is rF (t=rF) than it could written as kg(m/s)^2?

    Force is kg(m/s^2) and that is multiplied by the length of the lever arm in meters. Correct?
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    I like the description using point particles in the spinning wheel, it helps me try to imagine whats going on. I am also still having difficulty imagining the torque vector but perhaps this cant be done; Harold says its just a mathematical convention. I wonder if this convention has been adopted so that we can analyze situations with multiple torques, a positive and negative label would remind us to add torques spinning in the same direction and to subtract torques spinning in opposite directions. If the torque vector really is used for denoting spin direction then we instantly have the answer to your question of why it is important to consider. If we didn't have the torque vector we might get confused and do stupid things like get red in the face trying to turn a bolt from opposing directions and then complaining that the bolt wont budge.

    Anyhow, I think your right about the units for torque, we could equivalently write it in newton meters (NM), because t=RxF is the multiplication of a length (m) by a force (N, or kg(m/s^2).
    Last edited by Aconic; July 28th, 2011 at 04:09 PM.
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