How to determine direction of forces in free body diagrams?

[Yoriko]

For instance, in the below diagram:

[PLAIN]http://img101.imageshack.us/img101/3205/59914396.png

How do you know if the reaction force on the beam is in Direction A or B? How do you determine the correction direction?

 

[Redbelly98]

Welcome to Physics Forums.

Don't worry about getting the correct direction. Break up the reaction force into x and y components, and solve for the forces that way. If the x-component turns out to be positive (towards the right), then the correct direction is B. If the x-component is negative, then it's A.

 

[issacnewton]

yes, I agree with redbelly, don't bother about the direction now. since the movement of the beam is being prevented in x and y direction at the hinge, just show forces in x and y direction. then when you use stability conditions, you will get correct magnitude and the direction anyway

 

[Yoriko]

Welcome to Physics Forums.

Don't worry about getting the correct direction. Break up the reaction force into x and y components, and solve for the forces that way. If the x-component turns out to be positive (towards the right), then the correct direction is B. If the x-component is negative, then it's A.

Ok thanks!

 

[rwisz]

In order to determine the direction of forces in free body diagrams, it is important to understand the principles of equilibrium and Newton's laws of motion. In the given example, the reaction force on the beam can be determined by first identifying all the forces acting on the beam. These forces include the weight of the beam, the applied load, and the reaction force at the support.

Next, we can use the principle of equilibrium, which states that the sum of all forces acting on a body must be equal to zero in order for the body to remain in a state of rest or uniform motion. This means that the reaction force must be equal and opposite to the combined forces of the weight and the applied load.

To determine the correct direction of the reaction force, we can apply Newton's third law of motion, which states that for every action, there is an equal and opposite reaction. This means that the reaction force will act in the opposite direction of the applied load.

In the given diagram, Direction A represents the direction of the applied load, so the reaction force will act in the opposite direction, which is Direction B. This can also be confirmed by the principle of equilibrium, as the sum of forces in the vertical direction must be equal to zero.

In summary, the direction of forces in free body diagrams can be determined by considering the principles of equilibrium and Newton's laws of motion. By analyzing all the forces acting on a body and applying these principles, we can accurately determine the direction of reaction forces.