In the system of parallel forces , all the forces are non-colinear. So how to they balance each other?.enter image description here

Here in this case a wooden plank is supported at its end by two supports. All the forces here are non-colinear. So how do the tension in that supports balance the weight of plank?

And if they balance each other , then why the equal and opposite forces in couple don't balance?

I don't understand principal of moments intuitively.

So , do explain

  • $\begingroup$ Colinear forces are the most trivial case to avoid avoid twisting. The example of the picture shows that it is not necessary. $\endgroup$ May 20 at 21:51

1 Answer 1


In order for the plank to be in equilibrium, the sum of the vertical forces must be zero and the sum of the moments about any point on the plank must be zero. There are no horizontal forces.

This means the sum of the upward reaction forces at A and B must equal the sum of the downward applied forces at D and C, and the clockwise moments about any point must equal the counterclockwise moments about the same point.

For example the clockwise moments about A due to downward forces C and D must equal the counterclockwise moment about A due to the upward reaction force at B. That will be the case if the distance between C and D is 1 meter (it isn't shown in the diagram)

I don't understand principal of moments intuitively.

Think about a moment or torque as being the turning effect about a point due to the application of a force. An example is using a wrench to turn a bolt. You apply a force perpendicular to the arm of the wrench. The moment or torque is the product of your applied force and the length of the arm of the wrench between the bolt and the point where you apply the force.

Hope this helps.

  • $\begingroup$ Why don't equal and opposite forces in couple balance each other? $\endgroup$ May 21 at 6:10
  • $\begingroup$ physics.stackexchange.com/questions/709468/… . Would you pls answer this question? $\endgroup$ May 21 at 6:13
  • $\begingroup$ Let’s do one question At a time. When you say “couple” are you referring to a “force couple”? $\endgroup$
    – Bob D
    May 21 at 7:52
  • $\begingroup$ Yes , two equal and opposite unlike parallel forces acting on two different point on a single object. $\endgroup$ May 21 at 8:31
  • $\begingroup$ What do you mean by “ unlike “? $\endgroup$
    – Bob D
    May 21 at 9:31

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