Jeff Wisnia wrote:

I'm always interested in learning new things to try and spotted this
one in a book of puzzles SWIMBO just gave me. I just had to try it.

Put a paper or Styrofoam cup with water in it on a kitchen or postal
scale and note its weight. Leave it on the scale. (A glass of water
might be too heavy for the scale.)

Now stick a finger or two into the water without touching the side or
bottom of the cup and watch what happens.

The scale reading increases.

Did you expect that? I didn't.

I'm sure any physicists here will explain why.

Jeff

I'm not a physicist, but it sounds like the bottom of your finger had
been exerting more vertical force (in the downward direction) on the
"surface" of the water, than the air had. There is also Newton's 3rd
Law (which I just looked up), €œFor every action, there is an equal and
opposite reaction.€� OTOH, I wouldn't have expected to see the
phenomenon you described either! ; )

Bill