The shape of Earth

[downbeatplumb]

Depends on how solid the rubber, but if it is less dense than the medium it is surrounded by then it will float. 

Using the displacement of liquid to measure the volume of a stone? Not sure how that proves Newton's theories about perpetual motion, you might want to explain that a little more.

Same goes for using fire to suck up liquid into a vacuum.

Yes, precisely. If it is less dense than the surrounding medium, it will float. Why? because at that point the force of buoyancy is larger than that of gravity.

Now, take a ball that is more dense than water. it will sink. But how much will it weigh in the water? The answer is the weight outside of the water minus the weight of the water that would occupy that volume.

The same happens in air: the weight of anything (rock, balloon, etc) will be the weight outside of the air minus the weight of the air for that volume.

In the case of the rock, the weight outside of the air is much larger than the weight of the air, so it sinks. For a balloon, it is less, so it floats.

Now, why is there a weight outside of the air?

https://www.youtube.com/watch?v=E43-CfukEgs