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I am having trouble using the right hand rule properly and often find myself putting my hand in awkward orientations. I know you point your hand in the direction of $r$ and then point your fingers in the direction of $F$ but that doesn’t really help me. Can anyone explain how to use it correctly and without awkward hand positions and apply it to the image below for F1, F2, and F3? I don't get the orientation of the thumb and how that dictates direction. In the first example for for F1 my thumb is pointed towards me to the right, F2 my thumb is pointed away from me, and F3 it is pointed towards me to the left. I dont get how this Shows me how F1 is clockwise, F2 counter, and F3 counter

Link to example 1

Also in this example I would have thought all of the forces are counterclockwise except D but they aren't, Help! :?

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After three years of physics at a university level and many more during school before, I still end up with awkward hand positions while trying to figure out the direction of the Lorentz force, for example. I fear there is no general way to avoid these. Maybe you could rephrase your question to concentrate on the application? –  Claudius Dec 2 '12 at 23:25
    
Clarified post thanks for the help Claudius! –  woody Dec 2 '12 at 23:37

2 Answers 2

up vote 3 down vote accepted

There are multiple ways to think about the right hand rule, the most popular are outlined here.

The one I find myself using the most is the "right hand grip rule". In your case, this would mean pointing your hand in the direction of r and then curling your fingers towards F, causing your thumb to point in the direction of the resulting vector (also the direction of the cross product of r and F).

Using the right hand grip rule for the torque due to F1, you can see that your fingers are curling in a clockwise direction and thus the torque about the center is in the direction of your thumb (into the page for F1, out of the page for F2 and F3).

You could also go the other way and give the direction of rotation based on a torque vector using the same rule... thumb points in the direction of the torque vector and your curled fingers will show the direction of rotation.

Hope this helps, and keep those wrists limber!!

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my left wrist is not nearly as limber as my right wrist... –  dinkelk Dec 4 '12 at 0:15
    
that's the mark of a true... physicist. –  Mik Cox Dec 4 '12 at 0:17
    
thanks Mik Cox! –  woody Dec 4 '12 at 21:29
    
Sure thing. Glad you figured it out! –  Mik Cox Dec 4 '12 at 22:31

The rule: To check the direction of $A\times B$ (where $A,B$ are vectors), you point your pointer-finger along $A$, then your middle-finger along $B$ (which may require you to twist your hand); your middle-finger should be orthogonal to your pointer-finger and so your palm will face the same direction as the middle-finger. Then you just put your thumb straight out (orthogonal to the other two fingers), and that's the desired direction.

You will see that for $F1$ your thumb points into the page (which is the direction for clockwise motion).

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Is the back of your hand supposed to face you or the palm? And then if your thumb points towards you its CCW? –  woody Dec 3 '12 at 3:24
    
@woody doesn't matter, the back of your hand faces whichever way it needs to so that your fingers point in the given directions. –  David Z Dec 3 '12 at 5:43

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