Statics - Confusion about forces direction

In summary, the conversation discusses the concept of sign conventions and the use of equations to determine the direction and magnitude of forces in a problem involving rotation. The confusion arises when trying to solve for two unknowns in two equations, as the sign chosen for each force can affect the final result. It is suggested to assume all forces are positive in the positive X and Y directions, and to work through the problem with different assumptions to see how the equations and answers are affected.
  • #1
Femme_physics
Gold Member
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This is NOT a homework question. Just trying to understand a basic concept about signs convention...
[PLAIN]http://img638.imageshack.us/img638/9334/spring62009.jpg

Support A and B allow rotation, but nothing else.

Now, if I know W = 400 [N] for instance, using sum of all moments on B and sum of all forces on X I can find Bx = 920 and Ax = 920. It's easy for me to know which sign to use since there's only 1 force in each equation.

Here comes the kicker, though. I need to find Ay and By. I got two equations and two unknowns.
Sum of all moments on W, and sum of all forces on Y axis.
But, if I write Ay and By with the same signs (both plus or both minuses) they'd end up a different result if they had different signs (one plus the other a minus). My lecturer always told us that the sign of the result will let us know whether the original sign we picked was correct. But, when I have two equations and two unknowns, if I get a sign wrong it makes a different result!

So, am I suppose to intuitively know Ay and By are opposite signs because of some mechanically principle of rotation, or is there a way to mathmatically verify it through the equations?
 
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  • #2
It doesn't matter what direction you choose. If you "guess" the wrong direction, the magnitude of the force willl come out as a negative number.

In complicated problems it is often easiest just to say that all forces as positive in the positive X and positive Y directions, and let the algebra sort out the minus signs for you.

Work through the question making different assumptions and you will see how the equations change when you sum the forces and moments, and how the answers change when you solve the equations.
 
  • #3
Sorry it took me A WHILE to reply. I was meaning to get back to that. It's just that I'm still confused.

Yes, I do understand the idea that minus and plus are about direction. But reviewing my question, I made a mistake, and as it turns out, the terms cancel each other out and I get 0 = 0!

I want to rephrase the question so I'm making another topic in HW section, because I want to go over this specific question and this specific equations. Thanks though.

https://www.physicsforums.com/showthread.php?p=3249340#post3249340
 

FAQ: Statics - Confusion about forces direction

What is statics?

Statics is the branch of mechanics that deals with the behavior of objects at rest, or in a state of constant motion, under the action of forces. It is concerned with the analysis of forces acting on objects and the effects of these forces on the objects.

What is the difference between a force and a moment?

A force is a push or pull acting on an object, while a moment is a rotational force that causes an object to rotate around a fixed point. In other words, a force is a linear force while a moment is a rotational force.

How do you determine the direction of a force?

The direction of a force can be determined by using a free body diagram. This involves drawing a simplified diagram of the object and labeling all the forces acting on it. The direction of a force is indicated by an arrow pointing in the direction of the force.

What is the difference between internal and external forces?

Internal forces are forces that act within an object, while external forces are forces that act on an object from the outside. Internal forces do not change the motion of an object, while external forces can cause an object to accelerate or decelerate.

How do you deal with forces in opposite directions in statics?

In statics, forces acting in opposite directions are considered to be equal and opposite, and thus cancel each other out. This means that their effects on the object will be balanced, and the object will remain in a state of rest or constant motion.

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