# How much of an object's mass behaves as if it is located at the center of gravity? [closed]

We recently finished up a physics lab where a 200g weight was balanced at the end of a ruler on a fulcrum (measuring torque).

In the analysis portion of the lab, we are asked: "How much of the mass of an object behaves as if it is located at the center of gravity?"

My initial answer is that all of an object's mass acts as if it is located at the center of gravity, but that doesn't make sense in terms of real life.

Any tips?

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## 2 Answers

It depends on what the object is doing.

If “behaves” means “exerts torque around a fulcrum”, then you can pretend that 100% of the mass is at the center of gravity.

But if “behaves” means “has angular momentum because of rotation around an axis”, then 0% of the mass acts as if it is at the center of gravity; you have to take into account how the mass is distributed.

My answer would have been all too. You can substitute the physical object with a point mass placed on center of mass of that object for mathematical purposes. Then it(point mass) behaves exactly like the real object. Because the mass of the point is equal to the mass of the object, all of the mass of the object acts like it's located in the center of mass. But only when calculating the torque.