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

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!

jedishrfu
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.

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.

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?

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

enigma
Staff Emeritus