# Homework Help: Can anyone help me check my free body diagram for mobile crane

1. Jan 5, 2014

### cracktheegg

Last edited: Jan 5, 2014
2. Jan 5, 2014

reposted

3. Jan 5, 2014

### haruspex

Your diagram looks ok. Some forces are shown going the opposite way to what will actually happen, but that won't matter - they'll just come out negative. you won't be needing Rpx.

4. Jan 5, 2014

### cracktheegg

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

Last edited: Jan 5, 2014
5. Jan 5, 2014

### haruspex

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.

6. Jan 5, 2014

### cracktheegg

Where should the cg be?

7. Jan 5, 2014

### haruspex

CG of what? Your W1, W2, W3 and W4 look right.

8. Jan 5, 2014

### cracktheegg

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

9. Jan 6, 2014

### PhanthomJay

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.
You have to be careful in drawing free body diagrams to isolate the members or joints you are interested in. For example, when looking at the entire crane system, the only forces acting are the weight forces and the reaction forces from the wheels. All other forces are internal to the system. It is not good practice to show all forces internal and external on one diagram without breaking up the system into its parts.

10. Jan 6, 2014

### cracktheegg

Thanks alot guys, i learn alot