Support Reactions: Identifying Moments & Forces

[princejan7]

I have trouble identifying what moments and forces are due to a support

http://postimg.org/image/a2xux5qjx/

Can someone explain the moments and forces (and their directions) due to the support in this picture?
http://postimg.org/image/kto0b0ft3/

Why isn't there a MAx and Ax due to the bearing in this problem?http://postimg.org/image/b5va9kq45/
Or for this one, why doesn't the support at B prevent rotation about any of the axii's?

 

[Simon Bridge]

I. Start by locating the pivot, then use your intuition to guide you about which way things will try to move if not supported. i.e. if the cord DE did not exist, the plate would rotate about pivot B.

It helps to sketch the plate with nothing else, then draw arrows for all the linear forces and their points of action.
Resolve the forces into components through the center of mass and perpendicular to that line.

II. If they were there - what would their magnitudes be?

III. Support B is a hinge - it is designed to allow rotation.

 

[princejan7]

It helps to sketch the plate with nothing else, then draw arrows for all the linear forces and their points of action.
Resolve the forces into components through the center of mass and perpendicular to that line.

So, for example, when considering the reactions due to the thrust bearing, should I imagine the plate without the rope and the friction-less bar AC?

I. Start by locating the pivot, then use your intuition to guide you about which way things will try to move if not supported. i.e. if the cord DE did not exist, the plate would rotate about pivot B.

I'm still not sure about the F2,M2,Fx and M1x reactions
Could you explain the reasoning behind them?

 

[Simon Bridge]

1. yes - isolate the component you are analyzing - replace any constraining objects, like bearings, pivots, and so on, by their corresponding linear forces.

I'm getting a "gateway time out" error o those images right now.

 

[Nickie]

I would like to first clarify that moments and forces are two different physical quantities. Moments are a measure of the rotational effect of a force, while forces are a measure of the push or pull on an object.

In the first image, it appears that there are two supports, A and B, holding up a beam that is being loaded with a weight at its center. The forces acting on the beam are the weight of the object, the upward reaction forces from the supports (RA and RB), and the downward force due to gravity. Moments, on the other hand, are the rotational effects of these forces. In this case, the moment due to the weight of the object is causing the beam to rotate clockwise, while the moments due to the reaction forces from supports A and B are countering this rotation and keeping the beam in equilibrium.

In the second image, it appears that there is only one support, A, holding up a beam that is loaded with a weight at its center. In this case, the force acting on the beam is the weight of the object, and the moment due to this force is causing the beam to rotate counterclockwise. However, since there is only one support, there is no opposing moment to keep the beam in equilibrium. This is why there is no MAx or Ax shown in this problem.

In the third image, there are two supports, A and B, holding up a beam that is loaded with a weight at its center. However, since the beam is symmetrical, the reaction forces from both supports are equal and opposite, resulting in a net moment of zero. This means that the support at B does not prevent rotation about any of the axis because its reaction force is balanced by the reaction force at A.

In summary, identifying moments and forces due to a support involves understanding the forces acting on an object and their rotational effects. It is important to consider the direction and magnitude of these forces to accurately determine the moments and forces due to a support.