Moment is calculated using the following formula:
Key points to note:
-
D is the perpendicular distance. You find that you need less strength to balance the grocery bag moment as you un-tilt your hand to the vertical position,
even though your arm length remains the same. This is because the perpendicular distance decreases.
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Because moment is the product of force (N) and distance (M), its unit is Nm.
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Moment also has a rotational direction, being either clockwise (CW) or counter-clockwise (CCW) depending on the force direction.
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The sign convention is +ve for CCW and –ve for CW.
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Moment about an axis is also known as torque. We’ll be covering more on torque in Solid Mechanics I.
Couple
When two equal but opposite forces act on a body at an offset, the forces cancel out but the offset causes a net moment, which we call a couple. A typical example is the torque wrench:
We see that the forces cancels each other out, but the net effect is a couple (moment) about the centre of the torque wrench, which is what untightens the screw.
Principle of Transmissibility
The concept of couple can be used to “transmit” the force that is acting at an offset on a body. That is, we move the offset force to another point on the body, but at the same time we add the equivalent moment magnitude that is caused by the offset force, as such:
This might sound a little complicated, and the best way to learn is by looking at an example.
Moment is calculated using the following formula:
Key points to note:
-
D is the perpendicular distance. You find that you need less strength to balance the grocery bag moment as you un-tilt your hand to the vertical position,
even though your arm length remains the same. This is because the perpendicular distance decreases.
-
Because moment is the product of force (N) and distance (M), its unit is Nm.
-
Moment also has a rotational direction, being either clockwise (CW) or counter-clockwise (CCW) depending on the force direction.
-
The sign convention is +ve for CCW and –ve for CW.
-
Moment about an axis is also known as torque. We’ll be covering more on torque in Solid Mechanics I.
Couple
When two equal but opposite forces act on a body at an offset, the forces cancel out but the offset causes a net moment, which we call a couple. A typical example is the torque wrench:
We see that the forces cancels each other out, but the net effect is a couple (moment) about the centre of the torque wrench, which is what untightens the screw.
Principle of Transmissibility
The concept of couple can be used to “transmit” the force that is acting at an offset on a body. That is, we move the offset force to another point on the body, but at the same time we add the equivalent moment magnitude that is caused by the offset force, as such:
This might sound a little complicated, and the best way to learn is by looking at an example.
