Torque and forces parallel vs perpendicular to the axis of rotation....

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SUMMARY

The discussion clarifies that forces acting parallel to the axis of rotation do not generate torque, as torque is defined as the cross product of the distance vector and the force vector. Specifically, the force components Fa (axial) and Fr (radial) do not produce torque about the A-A axis, while Ft (tangential) does. The reasoning is based on the angle between the force and the lever arm, where a zero angle results in zero torque. Additionally, Fa can cause rotation, but around an axis perpendicular to A-A, not along it.

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rashida564
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why force parallel to the axis of rotation do not make any torque.
 
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What is the usual definition of a torque ?
 
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is the tendency of force to rotate object around an axis .
 
or it's the cross product between the force and the distance vector .
 
rashida564 said:
why force parallel to the axis of rotation do not make any torque.

rashida564 said:
is the tendency of force to rotate object around an axis .

Can you answer your own question now ? How can a force acting parallel to an axis tend to cause rotation about that axis ?
 
it can't
 
but way .
 
Torque demo.jpg


In the diagram :

A-A is the axis
B-C is a lever arm at right angles to the axis
Fa , Ft and Fr are axial , tangential and radial force components acting on the end of the lever arm at point C .

Which of the force components could cause rotation of the lever arm about axis A-A and which could not ?
 
Last edited:
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i think Fr
 
  • #10
can anyone help me please
 
  • #11
rashida564 said:
i think Fr
No. Look at the diagram carefully.
 
  • #12
Fr don't make torque because it is perpendicular to axis of rotation.
Fa i think
 
  • #13
can anyone help me
 
  • #14
Ft wiil cause rotation about AA axis...
 
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  • #15
Correct . Can you explain in words now why Ft does produce rotation about A-A axis and why Fa and Fr do not ?
 
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  • #16
Look t=r×f...the anlge btween r and fr is sin0=0...so torque is zero...no question for rotation...
However fa will produce rotaion but the axis will be different...(an axis which is perpendicular to AA...BUT NOT ALONG AA)
 
  • #17
So fa will rotate the object
 
  • #18
Yes obviously fa will...but as i said corresponding rotation axis will be different
 

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