A lever scale is balanced with a partially filled glass of water on one side. Will the balance be upset (and if so how?) if you put your finger carefully into the water without touching the glass? What happens if the beaker is filled with air instead of water?

Answer 1

The water exerts a force up on your hand equal to the weight of water displaced. Therefore an equal and opposite force is down on the scale. Ultimately that force comes from your arm pushing the finger down into the water.

In the air there will be no effect because you are not displacing any weight of air. The air you displace simply leaves the glass unlike the water that rises around your finger, thereby raising the pressure on the bottom of the glass.

Answer 2

Hello Damon i have asked my teacher and he said that the air part is not what you said. He gave me one more chance to tell him what is the right answer.