- #36

jrmichler

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Look carefully at the dimensions in the FBD. The portion of the vertical rod that is above the COM is not shown because it has no mass, and therefore no effect on the solution. The wheels are massless, so the FBD has exactly two masses, both of which are shown as point masses:Devin-M said:Correct me if I’m wrong

1) The COM of the vertical rod.

and

2) The added mass out in front.

Since this problem is defined as translation only, with zero rotation, all masses are shown at their COM. If you were working on a different problem involving angular acceleration, you would need to show distributed masses as distributed masses. The vertical rod would be one such mass.

The secret to solving these types of problems is to put most of your effort into getting the FBD correct, and without any extraneous detail. The best FBD has only the information needed to solve the problem.

If you want to better understand FBD's, solve this one several different ways:

1) By sum of moments about point P, the point of application of the force from the car.

and

2) By sum of moments about the COM of the vertical rod.

and

3) By sum of moments about the mass M.

and

4) By sum of moments about the center of the wheels.

Looking for an equation online will, all too often, lead you to a wrong answer because there is no guarantee that the online equation is correct for your FBD. If the FBD is correct, then the correct equation is easily found.Devin-M said:According to this page,