# What is the physical meaning of moment about an axis?

1. Feb 16, 2012

### jehan60188

i.e.

"A F = <0,0,10> is exerted at the point (1,1,1)
what is the moment about the X axis?"

i know the answer is <sqrt(3),sqrt(3),sqrt(3),> DOT (<1,1,1>x<0,0,10>)
but the physical meaning of that escapes me.
in what applications would an axis be more useful than a point?
thanks!

2. Feb 16, 2012

### Staff: Mentor

moments a vector that is the cross product of R and some force ie R x F

The physical meaning is found in closing a door. If I use my finger to close a door pushing on the door knob then I need to use a some small amount of force. If I instead move close to the door hinge then I have to use a lot more force to close it. In either case, the moment for the door remains the same because as the radius is decreased the force must be increased. Moment is also known as torque a kind of angular cousin to force.

3. Feb 16, 2012

### jehan60188

why is that not calculated by the moment about the point on the hinge?
what is the physical meaning behind calculating it across the entire hinge-axis?

4. Feb 16, 2012

### Staff: Mentor

cross-product indicates an axis and gives a notion of rotational direction: clockwise vs counterclockwise.

5. Feb 16, 2012

### enigma

Staff Emeritus
Moment is basically a measurement of how the mass is distributed about a point or axis. The larger the values, the more torque is needed to spin it.

You would want to calculate about an axis if you're designing a rotating part.

Just as a for instance, if you have a nominal axis of rotation of a part, and then a worst case tolerance stackup, you might need to see what that does to the part's dynamics if it was machines to the worst case.

6. Feb 16, 2012

### jehan60188

so, the necessary force/torque to make a wrench work should be calculated by using a point, but the force necessary to raise a bridge should be calculated using a line (the line being the 'hinge' equal to the width of the bridge)?

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