In a lecture demonstration, an empty can is used to demonstrate the force exerted by air pressure (Fig. 9.35). A small quantity of water is poured into the can, and the water is brought to a boil. Then the can is sealed with a rubber stopper. As you watch, the can is slowly crushed with sounds of metal bending. (Why is a rubber stopper used as a safety precaution?)

(a) This is because of (1) thermal expansion and contraction, (2) a higher steam pres-sure inside the can, (3) a lower pressure inside the can as steam condenses. Why?
(b) Assuming the dimensions of the can are 0.24 m x 0.16 m x 0.10 m and the inside of the can is in a perfect vacuum, what is the total force exerted on the can by the air pressure?