F.1 Science

Question

1.Peter performs a magic to Mary.He puts a piece of paper on the top of a glass which is full of water and presses it against the opening of the glass.Then,he turns the glass upside down.When he withdraws his hands from the paper,the paper and the water do not fal.Can you explain why the water and the paper do not fall?


2.Can Peter use a glass full of sand instead of water to perform this magic?Why?
（第二題唔識答唔緊要）

Answer 1

1. It is because of air pressure.
The atmosphere exerts a pressure at the bottom of the paper, pushing it upwards. This atmospheric pressure is much greater than the weight of the water and paper, and thus they do not fall down.

2. No. It is because there is plenty of air space between sand particles. Thus there is air pressure exerted on both the top and bottom of the paper and this cancels one another out. The weight of the sand will make it to fall down.