Calculating Couple Moment - Explaining Negative Values

In summary, the conversation discusses the calculation of couple moment and how to use negative values in the equation. The example provided shows the use of negative values in resolving the couple forces, with the convention that clockwise direction is negative and counter-clockwise is positive. The conversation also brings up a question about the use of '8ft' in the equation.
  • #1
unknown_2
29
0
Hey, I'm having some difficulty calculating couple moment. i understand that [tex]\sum[/tex]M = [tex]\sum[/tex]Fd. here's an example:
question.jpg

it asks to resolve the couple forces.

the answer is:
(4/5)(60lb) = 48lb
and
(40lb)(cos30)

[tex]\sum[/tex]M = -(48lb)(4ft) + (40lb)(cos30)(8ft)

Now, i don't understand why "-(48lb)(4ft)" is negative. I'm not sure when to use a negative value. Can someone explain this to me?

Cheers
 
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  • #2
unknown_2 said:
Hey, I'm having some difficulty calculating couple moment. i understand that [tex]\sum[/tex]M = [tex]\sum[/tex]Fd. here's an example:
question.jpg

it asks to resolve the couple forces.

the answer is:
(4/5)(60lb) = 48lb
and
(40lb)(cos30)

[tex]\sum[/tex]M = -(48lb)(4ft) + (40lb)(cos30)(8ft)

Now, i don't understand why "-(48lb)(4ft)" is negative. I'm not sure when to use a negative value. Can someone explain this to me?

Cheers

It's a matter of convention. Typically, the clockwise direction is negative and counter-clockwise is positive. So if the applied force causes rotation in the clockwise direction, the moment is negative and vice-versa.

CS
 
  • #3
I don't see where '8 ft' comes in. Shouldn't that be 'd ft'?
 

1. What is a couple moment?

A couple moment, also known as a torque, is a measure of the rotational force applied to an object. It is calculated by multiplying the force applied by the distance between the force and the point of rotation.

2. How is a couple moment calculated?

A couple moment is calculated by multiplying the force applied by the distance between the force and the point of rotation. The formula is M = F x d, where M is the moment, F is the force, and d is the distance.

3. What do negative values for a couple moment indicate?

Negative values for a couple moment indicate that the rotational force is acting in the opposite direction of the desired rotation. This means that the object will rotate in the opposite direction of the applied force.

4. Why do negative values occur when calculating a couple moment?

Negative values occur when the force and the point of rotation are in opposite directions. This can happen when the force is applied on the opposite side of the object from the point of rotation, or when the force is applied in a direction that causes the object to rotate in the opposite direction.

5. How do you interpret negative values for a couple moment?

Negative values for a couple moment indicate that the object will rotate in the opposite direction of the applied force. This is important to consider when designing and analyzing structures and machines, as it can impact their stability and functionality.

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