# Frequently Asked Questions: Density and Pressure

Q. If a person is swimming in the Dead Sea in Israel, why is it almost impossible to drown?
A. The water has so much salt that it is extremely dense and has a lot of buoyancy.

Q. From where does cork come?
A. Cork comes from the bark of the cork tree grown mainly in Portugal and Spain.

Q. Why does cream float on milk?
A. Cream contains a lot of fat which is less dense than milk.

Q. Why are bubbles spherical?
A. The sphere shape allows equal air pressure on all parts of the bubble.

Q. Why does a karate chop or a closed-fist punch hurt more than a flat-handed slap?
A. The force of the punch is concentrated into a smaller area, so it has greater pressure.

Q. Why is a sharp knife more effective than a blunt one?
A. The force of the knife cut is concentrated into a smaller area on the sharper knife, so it has greater pressure.

Q. Why do our ears 'pop' when we travel up a mountain in a car?
A. There is a tube called the Eustachian tube connecting our nose with the middle ear. Its purpose is to equalise the air pressure between the outside air and inside the middle ear. When we travel up a mountain, there is less air pressure outside. This causes the air inside the middle ear to quickly move against the eardrum causing a pop.

Q. Why do four-wheel drive vehicles deflate their tyres when driving on sand?
A. Letting down the tyres means that there is more surface area in contact with the sand. This has two effects - it reduces the pressure of the car on the sand so the car won't bog in so much, and it allows more friction or grip with the sand.

Q. Why are pressure-pak cans banned from aircraft?
A. Inside an aircraft in a slightly lower air pressure. Because aerosol cans have a higher gas pressure, they could blow apart and damage the aircraft while in flight.