Right handed rule for torque?

[ravupadh]

How do you apply the right handed rule for torque in order to see which direction the torque will go? And how can it be applied if there's more than one force being looke dat? I understand that the Force has to be perpendicular to the lever arm but I'm not sure how the direction of the torque is produced from this and what makes a torque negative or positive. Thanks.

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[90210]

How do you apply the right handed rule for torque in order to see which direction the torque will go? And how can it be applied if there's more than one force being looke dat? I understand that the Force has to be perpendicular to the lever arm but I'm not sure how the direction of the torque is produced from this and what makes a torque negative or positive. Thanks.

It seems like you don't completely understand what torque is. You're right that Torque = lever arm x Force (Perpendicular). The direction in which force points should tell you which way torque is moving:

If it's pointing down: it's clockwise
If it's pointing up: it's counterclockwise

Whether you call down positive or up negative is arbitrary. However, most conventions take the clockwise direction to be negative.

If there are more than one torque's acting in different directions, then the net force will dictate which direction torque will move. It's a fairly straightforward concept.

 

[DrShazam]

OP, torque used to be confuse me as well.

Best way to learn torque is to practice with some simple examples. First get the concept down, and then tackle the numbers. The right hand rule for torque seems pretty useless in my opinion because torque is something you can actually visualize.

I hope this exercise helps:
Grab a pencil and pen. Designate the left end of the pencil as the pivot, and hold it rigidly with your finger.
The pen is the force.
· If the pen is an upward force, you will be applying the force below the pencil, but directed upwards.
· If the pen is a downward force, you will be applying the force above the pencil, but directed downwards.
For each of these scenarios, notice which direction the pencil is moving in: clockwise or counterclockwise?

1. Apply an upward force through the center of the pencil (CCW)
2. Apply an upward force through the pivot (No movement)
3. Apply an upward force through the edge farthest from the pivot (CCW)

4. Apply a downward force through the center of the pencil (CW)
5. Apply a downward force through the pivot (No movement)
6. Apply a downward force through the edge farthest from the pivot (CCW)

By convention, CCW is positive torque and CW is negative torque. The direction of rotation depends not only on the direction of the force, but also what point is held as the pivot.

 

[ravupadh]

OP, torque used to be confuse me as well.

Best way to learn torque is to practice with some simple examples. First get the concept down, and then tackle the numbers. The right hand rule for torque seems pretty useless in my opinion because torque is something you can actually visualize.

I hope this exercise helps:
Grab a pencil and pen. Designate the left end of the pencil as the pivot, and hold it rigidly with your finger.
The pen is the force.
· If the pen is an upward force, you will be applying the force below the pencil, but directed upwards.
· If the pen is a downward force, you will be applying the force above the pencil, but directed downwards.
For each of these scenarios, notice which direction the pencil is moving in: clockwise or counterclockwise?

1. Apply an upward force through the center of the pencil (CCW)
2. Apply an upward force through the pivot (No movement)
3. Apply an upward force through the edge farthest from the pivot (CCW)

4. Apply a downward force through the center of the pencil (CW)
5. Apply a downward force through the pivot (No movement)
6. Apply a downward force through the edge farthest from the pivot (CCW)

By convention, CCW is positive torque and CW is negative torque. The direction of rotation depends not only on the direction of the force, but also what point is held as the pivot.

I see.. that actually makes a lot of sense. However I'm still wondering about the downward forces having a clockwise torque and upward forces having a counterclockwise torque rule that the second poster stated. For example, as shown in the 1st picture on this website: http://www.mrfizzix.com/review/torquerev/torquerev.htm

There are two downward forces but if we set the pivot point in the middle where that blue triangle is, wouldn't one force be clockwise and wouldn't another be counterclockwise even though they're both downward?

And the reason why I was asking about the right handed rule was because of the torque associted with a dipole in an electric field as shown in TBR physics chapter 8. They say the torque generated by the dipole charges is directed into the page which I can't really visualize without the right handed grip rule.

 

[90210]

I see.. that actually makes a lot of sense. However I'm still wondering about the downward forces having a clockwise torque and upward forces having a counterclockwise torque rule that the second poster stated. For example, as shown in the 1st picture on this website: http://www.mrfizzix.com/review/torquerev/torquerev.htm

There are two downward forces but if we set the pivot point in the middle where that blue triangle is, wouldn't one force be clockwise and wouldn't another be counterclockwise even though they're both downward?

And the reason why I was asking about the right handed rule was because of the torque associted with a dipole in an electric field as shown in TBR physics chapter 8. They say the torque generated by the dipole charges is directed into the page which I can't really visualize without the right handed grip rule.

Oh I see. Well with regard to the pivot point (treat it as though it's the center of a circle) so that the downward force to the left of the pivot turns counter clockwise, while the downward force to the right of the pivot turns clockwise.

The pivot point is the axis of rotation. If the pivot point was at the left end, then you would see the downward arrows to the left of the pivot (in the picture) turn clockwise. Remember the rotation is with respect to the pivot point.

 

[DrShazam]

I see.. that actually makes a lot of sense. However I'm still wondering about the downward forces having a clockwise torque and upward forces having a counterclockwise torque rule that the second poster stated. For example, as shown in the 1st picture on this website: http://www.mrfizzix.com/review/torquerev/torquerev.htm

There are two downward forces but if we set the pivot point in the middle where that blue triangle is, wouldn't one force be clockwise and wouldn't another be counterclockwise even though they're both downward?

You're absolutely correct. The force on the left produces a CCW torque and the force on the right produces a CW torque. Your intuition is spot on. That's why when looking at torque questions, you have to consider BOTH the pivot point and the direction of the force.

If, on the other hand, the 30 N force was pointing upward, then both the 30 N and 35 N forces would be working synergistically to produce a CW motion.

And the reason why I was asking about the right handed rule was because of the torque associated with a dipole in an electric field as shown in TBR physics chapter 8. They say the torque generated by the dipole charges is directed into the page which I can't really visualize without the right handed grip rule.

This is the best explanation I found of the right hand grip rule:
If you take your right hand and wrap it around a rotational axis with your fingers pointing in the direction of the force, then the direction of torque will be in the same direction as your thumb.

I doubt you will be asked to determine whether the torque is into or out of the plane of the page, but if you were, instead of using the complicated right-hand rule, just determine the direction of rotation (which is far more intuitive) and then use this(one of the few times I encourage memorization):
Clockwise rotations are tied to torques pointing into the page, and counter-clockwise rotations are tied to torques pointing out of the page. Just think of how a common screw works. If you are looking at its head from above with it pointing away from you, then if it spins clockwise, or 'righty', it will move downward, or into the page. If it spins counter-clockwise, or 'lefty', it will move upward, or out of the page.

If you placed the dipole in Fig 8-7 into the field shown in Fig 8-6 (which is what I think they were referring to on pg. 122), the dipole would rotate CW, meaning the torque acting on the dipole is into the plane of the page.

Example 8-6a is another good example. We know the dipole will rotate CCW because the negative charge of the dipole wants to align with the positive charges in the field and the positive charge on the dipole wants to align with the negative charges in the field. You know which side has positive and negative charges because field lines always go from the positive to the negative charge (in this case, that means negative charges on the left and positive charges on the right). Since CCW rotation is produced, it means the torque produced by the field on this dipole is out of the plane of the page.

 

[ravupadh]

You're absolutely correct. The force on the left produces a CCW torque and the force on the right produces a CW torque. Your intuition is spot on. That's why when looking at torque questions, you have to consider BOTH the pivot point and the direction of the force.

If, on the other hand, the 30 N force was pointing upward, then both the 30 N and 35 N forces would be working synergistically to produce a CW motion.



This is the best explanation I found of the right hand grip rule:
If you take your right hand and wrap it around a rotational axis with your fingers pointing in the direction of the force, then the direction of torque will be in the same direction as your thumb.

I doubt you will be asked to determine whether the torque is into or out of the plane of the page, but if you were, instead of using the complicated right-hand rule, just determine the direction of rotation (which is far more intuitive) and then use this(one of the few times I encourage memorization):
Clockwise rotations are tied to torques pointing into the page, and counter-clockwise rotations are tied to torques pointing out of the page. Just think of how a common screw works. If you are looking at its head from above with it pointing away from you, then if it spins clockwise, or 'righty', it will move downward, or into the page. If it spins counter-clockwise, or 'lefty', it will move upward, or out of the page.

If you placed the dipole in Fig 8-7 into the field shown in Fig 8-6 (which is what I think they were referring to on pg. 122), the dipole would rotate CW, meaning the torque acting on the dipole is into the plane of the page.

Example 8-6a is another good example. We know the dipole will rotate CCW because the negative charge of the dipole wants to align with the positive charges in the field and the positive charge on the dipole wants to align with the negative charges in the field. You know which side has positive and negative charges because field lines always go from the positive to the negative charge (in this case, that means negative charges on the left and positive charges on the right). Since CCW rotation is produced, it means the torque produced by the field on this dipole is out of the plane of the page.

Ahh I see thanks it makes a lot of sense now! I guess the right handed rule isn't really needed for torques at all if you just do some memorization.