What is the physical meaning of moment about an axis?

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Discussion Overview

The discussion revolves around the physical meaning of the moment about an axis, particularly in the context of applications such as torque and rotational dynamics. Participants explore the implications of calculating moments about points versus axes, with examples drawn from everyday scenarios like closing a door and engineering applications.

Discussion Character

  • Exploratory
  • Technical explanation
  • Conceptual clarification
  • Debate/contested

Main Points Raised

  • One participant presents a specific example involving a force exerted at a point and questions the physical meaning of calculating the moment about the X axis.
  • Another participant explains that moments are calculated as the cross product of the position vector and the force vector, using the example of closing a door to illustrate how distance from the hinge affects the force required.
  • Some participants question why moments are not calculated about the hinge point itself and seek to understand the significance of calculating moments across an entire axis.
  • It is suggested that the moment reflects how mass is distributed about a point or axis, with implications for torque and the dynamics of rotating parts.
  • A participant proposes that different scenarios, such as using a point for a wrench and an axis for a bridge, may require different approaches to calculating necessary forces or torques.

Areas of Agreement / Disagreement

Participants express differing views on the significance of calculating moments about points versus axes, with no consensus reached on the best approach or the implications of each method.

Contextual Notes

Some assumptions about the definitions of moments and torque are not fully explored, and the discussion includes various applications without resolving the mathematical or conceptual complexities involved.

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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