Right Hand Rule: Direction of Angular Momentum Vector

[scar123]

Homework Statement

Clock hands move slowly but they do have a small angular momentum. According to the right hand rule, in which direction, with respect to the clock face, does the angular momentum vector point?

Homework Equations

right hand rule

The Attempt at a Solution

Pointing right?

 

[Doc Al]

Do you know what the right hand rule is?

 

[scar123]

Do you know what the right hand rule is?

Yes... you have to curl your fingers, and then the thumb points in the direction of the vector?
I just don't understand this. Could you give me a hint?

 

[Doc Al]

Yes... you have to curl your fingers, and then the thumb points in the direction of the vector?

That's right. Make a half-closed fist with your right hand but with your thumb sticking straight out. Align your hand until your fingers curl in the direction that the hands are turning (which is clockwise, of course). The direction of your thumb is the direction of the angular momentum vector.

 

[scar123]

But your thumb can go in many directions... that doesn't make any sense.

 

[Battlecruiser]

But your thumb can go in many directions... that doesn't make any sense.

If you assume your four fingers (excluding the thumb), are sweeping an area in the plane made by the X and Y axis, the thumb will be point in the Z axis. So if there was a clock on your computer screen, the Z axis would be coming out of the screen and into the screen. Now, you have to determine whether the angular momentum is coming perpendicularly out of the plane of the clock and towards you, or in the opposite direction, away from you by seeing which way your four fingers are pointing (They should be moving in a clockwise direction).

 

[Doc Al]

But your thumb can go in many directions... that doesn't make any sense.

See a doctor... quick! (Just kidding.) Just make a fist with your thumb pointing up. Like you're making a "thumb's up" gesture.

Here's a picture (look at the right-most hand diagram in the middle): http://www.physics.brocku.ca/faculty/sternin/120/slides/rh-rule.html"

 

[scar123]

Ok I made a fist haha. But then it doesn't make sense of how i should allign it with the clock hands lol... Is it just going to point upward? hhaa

 

[Doc Al]

Ok I made a fist haha. But then it doesn't make sense of how i should allign it with the clock hands lol... Is it just going to point upward? hhaa

Describe your final answer with respect to the clock. You should know that when you use the right hand rule, your thumb direction is perpendicular to the plane of your curled fingers. The clock hands move in the plane of the clock, so the angular momentum vector must be perpendicular to the clock face. You have only two choices. Which must it be?

 

[scar123]

north east and north west are my two choices?

 

[Doc Al]

north east and north west are my two choices?

Specify the direction with respect to the clock. (We don't know which way the clock is pointing, so north and east mean nothing here.)

If the clock is on the wall, then the plane in which the clock hands move is the plane of the wall. The angular momentum vector must be perpendicular to the wall. So what are your choices? (There are only two ways something can point perpendicular to the wall--or the clock face.)

Read Battlecruiser's post--that might help you visualize what's going on.