The discussion revolves around checking a free body diagram related to a mobile crane, focusing on the forces acting on the system and their directions. Participants are exploring concepts related to force representation and equilibrium in the context of mechanics.
Some participants have provided guidance on the implications of force direction and the importance of consistency in applying chosen directions. There is an ongoing exploration of how forces interact, particularly concerning the stability of the crane and the role of the wheels.
Participants are considering the absence of outriggers or hydraulic stabilizers in the crane setup, which raises questions about the forces acting on the wheels and the overall stability of the system.
As I wrote, it doesn't really matter. As long as you apply the chosen directions consistently in the equations it will come out fine. Remember that there will be situations where it is not obvious which way the force will act, so it's rather important that it is not necessary to guess it correctly. (An exception to this is friction. You may sometimes need to consider two possibilities separately.)cracktheegg said:haruspex, can you tell me which are the force going the opposite way? I need to learn from my mistake. so please guide me thanks
Where should the cg be?haruspex said:As I wrote, it doesn't really matter. As long as you apply the chosen directions consistently in the equations it will come out fine. Remember that there will be situations where it is not obvious which way the force will act, so it's rather important that it is not necessary to guess it correctly. (An exception to this is friction. You may sometimes need to consider two possibilities separately.)
In the diagram you posted, it's not always clear how a force is being defined. E.g. consider R2y. Is that a vertical force exerted by the girder on the base, or an (equal and opposite) force exerted by the base on the girder? Whichever it is, I assume R3y is defined similarly. Now compare R2y and R3y. You show both pointing down. They must be in fact be in opposite directions to counter the moment from the load.
cracktheegg said:Where should the cg be?
haruspex said:CG of what? Your W1, W2, W3 and W4 look right.
There are forces from the ground surface on both the front and rear sets of wheels. One has to be certain that the vertical force from the ground on the rear wheels is upward, or else the rear wheels would lose contact with the ground and the crane would become unstable and start to tip over.cracktheegg said:Okay Ty, now I just worry about wether should I include the wheel. Because my classmate said the wheel should be in contact with ground, there are no outrigger or hydraulic, it is just stabilizer