Statics: Find resultant force and couple moment

In summary, the conversation discusses the calculation of the resultant force's magnitude and the couple moment in a given situation involving a triangle and rectangle load. The resultant force is calculated to be 24.69kN, and the location of the loads are determined to be 1.233m and 5.550m. However, there is an error in the calculation of the moment due to W1, and the total moment will depend on the support at joint B.
  • #1
Chandasouk
165
0
http://imageshack.us/a/img842/2497/112639506.jpg

Calculate the resultant force’s magnitude and the couple moment.

Let a = 3.70 m
w1=5.35kN/m
w2=4.00kN/m

The resultant force of the triangle is 1/2(3.70)(5.35)=9.898 kN

The resultant force of the rectangle is (4)(3.70) = 14.8kN

So FR (The force's magnitude) is 24.69kN

I found the location of the triangle's load to be 1/3(3.7) = 1.233m

I found the location of the rectangles load to be 1/2(3.70)+3.7 = 5.550m

So I calculated the couple moment MRa (Taking counter clockwise to be POSITIVE) I get
(9.898)(1.233)+(14.8)(5.55) = 94.3kN*m

but when I put these answers down for MasteringEngineering, they are incorrect. Where did I go wrong?
 
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  • #2
W1 and W2 are acting in opposite directions.
Even if you correct that your FR calculation will still be wrong. You don't want the total vertical force. You only want the load at A. To get that, take moments about B.
When you take moments about A for the second part, W1 has a negative moment. (I assume it's only asking for the moment about A due to W1 and W2. If the joint at B is holding the beam then there will also be a moment from that, and the total moment will be zero.)
 
  • #3
W1 and W2 are acting in opposite directions.
Even if you correct that your FR calculation will still be wrong. You don't want the total vertical force. You only want the load at A. To get that, take moments about B.
When you take moments about A for the second part, W1 has a negative moment. (I assume it's only asking for the moment about A due to W1 and W2. If the joint at B is holding the beam then there will also be a moment from that, and the total moment will be zero.)
 

1. What is the difference between a resultant force and a couple moment?

A resultant force is a single force that represents the combined effect of all the individual forces acting on a body, while a couple moment is a pair of equal and opposite forces that act along parallel lines but in opposite directions. Resultant forces produce linear motion, while couple moments produce rotational motion.

2. How do you find the resultant force and couple moment of a system of forces?

To find the resultant force, you must first determine the horizontal and vertical components of each individual force. Then, add all the horizontal and vertical components separately to find the total horizontal and vertical forces. Finally, use the Pythagorean theorem to find the magnitude of the resultant force and trigonometry to find its direction. To find the couple moment, you must find the perpendicular distance between the line of action of the forces and the moment center, then multiply this distance by the magnitude of one of the forces.

3. Can the resultant force and couple moment be in equilibrium?

Yes, it is possible for the resultant force and couple moment to be in equilibrium. This means that the sum of all the forces and moments acting on a body is equal to zero, resulting in no linear or rotational motion.

4. What is the difference between a coplanar and non-coplanar system of forces?

A coplanar system of forces is one in which all the forces lie in the same plane, while a non-coplanar system of forces is one in which the forces do not lie in the same plane. Coplanar systems are easier to analyze because they can be represented graphically on a 2-dimensional plane, while non-coplanar systems require vector algebra and 3-dimensional analysis.

5. How does the direction of the forces affect the resultant force and couple moment?

The direction of the forces can greatly affect the resultant force and couple moment. If all the forces are parallel, they will produce a resultant force but no couple moment. If the forces are not parallel, they will produce both a resultant force and a couple moment. Additionally, the angle between the forces can affect the magnitude and direction of the resultant force and couple moment.

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