What is the physical meaning of moment about an axis?

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SUMMARY

The discussion centers on the physical meaning of the moment about an axis, particularly in the context of torque and rotational dynamics. It is established that the moment, calculated as the cross product of the position vector R and the force vector F (R x F), provides insight into how force is applied at different distances from an axis of rotation. Practical applications include scenarios like closing a door, where the distance from the hinge affects the force required to achieve the same moment. The conversation emphasizes that calculating moments about an axis is crucial for designing rotating parts and understanding the dynamics of mechanical systems.

PREREQUISITES
  • Understanding of vector mathematics, specifically cross products
  • Familiarity with the concepts of torque and moment
  • Knowledge of mechanical systems and dynamics
  • Basic principles of rotational motion
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Mechanical engineers, physics students, and anyone involved in the design and analysis of rotating systems or mechanical components will benefit from this discussion.

jehan60188
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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!
 
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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.
 
jedishrfu said:
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?
 
cross-product indicates an axis and gives a notion of rotational direction: clockwise vs counterclockwise.
 
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.
 
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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