On Sun, 04 Sep 2016 22:58:12 -0400, FromTheRafters wrote:

Bill laid this down on his screen :
Bob F wrote:
On 9/4/2016 1:32 PM, 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

At the same time, you will weigh less by the same amount if you are on a
scale. If you put your whole body in a barrel of water, what would you
expect would happen? The barrel will weigh more, and you will weigh the
same amount less.

Surely your weight doesn't change just because you are in the water....
(geeze....) ; )

You weigh less above the water than you do in the water.

What are you talking about? Your weight doesn't change.

I must assume the cup is not full because there was no mention of overflow from the cup. The insertion of your
finger(s) will displace only air, which is lighter than water or your body. Therefore the weight registered on the
scale will increase.

Now if the cup was full and the overflow from the insertion of your fingers was made to magically disappear the weight
registered on the scale should remain relatively constant due to the similarity of body weight to water of the same
volume.

By the way, I'm not a physicist, but I saw one on TV once.