Levers and Cranes TBR

[greenseeking]

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Hello,

I was wondering if anybody could explain the concept of mechanical advantage with levers and cranes.

There is a fulcrum that looks like this: (please see attached picture)

Question: Given that Mechanical Advantage is dependent on FL/Fa=Xa/Xl,
what is the MA of the lever system in the picture?

Answer: The answer is between 1 and 2.

I said it's 2 because Fl/Fa=2X/X.
The answer explanation said that At first glance, it would appear that the mechanical advantage is equal to two because force is applied at exactly twice the distance from the fulcrum that the weight is positioned. this would lead to MA of two. But the center of mass of the bar is to the right of the fulcrum and the weight of that portion must be considered.

Could someone please explain to me what this is saying? I just followed what the passage gave me and figured out the MA with the formula. Why do you have to take the center of mass into account and how do you figure it out?

 

[chiddler]

We need to know if the bar has mass or is massless. This will tell us what the mechanical advantage is. If the bar has mass, which it apparently does in this question, then you add its mass to the free body diagram you draw when figuring this all out.

pretty much this. if there is a mass on the bar, then mechanical advantage is not exactly two because it takes more than half the weight of the mass to lift up the bar.

 

[greenseeking]

Thanks chiddler! It always makes more sense after somebody else explains it. The passage didn't say whether the bar had mass or not but I guess you should think about it in terms of center of mass. since the fulcrom isn't exactly at its center of mass it should be less than what you expect since it's out of balance. Thanks again!