# B Vanishing mass

1. Mar 11, 2016

### martix

First of all I apologize if silly speculative discussions like this don't belong here. With that disclaimer out of the way, here's a fun thought experiment:

Let's say an object, a cube that weighs 1kg rests on a rigid, level surface. And all of a sudden 99% of its mass disappears.
What happens?
Well, my thinking is, the cube is in equilibrium, so an equal amount of up and down force acts on it. Then 99% of the down force disappears in an instant and the cube is shot up, as if it had just bounced off.
Is that anywhere near reasonable?
What would the upwards force be? 0.99*9.8 newtons? How far up would it go?

2. Mar 11, 2016

### Staff: Mentor

"What do the laws of physics predict if we violate the laws?"
Why would the force upwards not disappear in the same moment?

To find some physical situation: if you split the block horizontally in two parts and rapidly remove the upper part, the lower block expands a bit (because pressure from above reduces), which can in theory let it jump up a bit like a spring. Won't lead to an actual jump with most materials, but with rubber and similar things it should be possible. The achievable height depends on material parameters and the way the block is split.

3. Mar 11, 2016

### martix

:) That's why I said silly.
Oh... that does indeed seem similar mechanically.
Sadly my physical intuition fails me here, that's why I was asking.

Am I correct in interpreting your point as, if the object(and ground) are non-deformable(i.e. not springy), nothing would happen to the lower piece?

4. Mar 11, 2016

### HallsofIvy

The only reason a surface pushes up on an object resting on it is that the object, through gravitation force, presses down on the object. If part or all of the mass disappears, the downward force and, therefore the upward force, also disappears. It would be exactly the same as if the smaller portion that is left, alone, were sitting on the surface.