PHYSICS
Frequently Asked Questions:
Density & 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 4WD 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.
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Quiz:
Density
1. The measure of how close and how heavy are the particles in an object is called its: |
2. The rule to calculate density is: |
3. The density of water in g/mL is: |
4. One millilitre has a volume of: |
5. A substance whose density is greater than 1 g/mL will: |
6. The density of a feather in g/mL would be about: |
7. The density of ice in g/mL is: |
8. The density of petrol in g/mL is about: |
9. An object is suspended in each of two full containers, one containing water and the other petrol. In which container will the object be higher in the liquid? |
10. If a sinker made of lead and another identical sinker made of aluminium were dropped into saltwater, why would the lead sinker reach the bottom first?: |
11. A density of 2 g/mL is equal to a density of: |
12. An object has a mass of 10 grams and a volume of 5 ml. What will happen when the object is placed in water, and why? |
13. A stone has a mass of 200 grams. When it is immersed in a measuring cylinder of water, the water rises 100 millilitres. What is the density of the stone? |
14. The least dense gas is: |
15. The density of carbon dioxide gas is greater than that of air. In a container of air, carbon dioxide gas will be found: |
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Quiz:
Pressure
1. The concentration of a force is called: |
2. The rule for pressure is: |
3. The unit for pressure is: |
4. Using the same muscular force, a karate chop with the side of the hand will hurt more than a hit with the open hand because: |
5. Four-wheel drive owners let air out of their tyres before driving on sand. This is because: |
6. A thumb tack has a large flat surface and a pointed end. With the same applied pressure, which part will have the greatest pressure? |
7. What happens to the water as one goes deeper into the ocean? |
8. Ships are made from iron. The reason why they do not sink is because: |
9. Air pressure is less than that of liquids. This is because: |
10. Atmospheric pressure at sea level is greater than on top of mountains. This is because: |
11. Water boils on Mount Everest at a temperature less than 100 degrees Celsius. This is because: |
12. The force of an object is 100 Newtons. The area on which the force acts is 2 square metres. What is the pressure in Newtons per square metre? |
13. Two identical blocks of wood are placed side by side on a table. One lies on its larger side, and the other stands on its smaller end. Which is exerting the greatest pressure on the table? |
14. A 12 Newton force acting on an area of 3 square centimetres exerts a pressure of: |
15. A person whose mass is 50 kg (weight about 500 N) has a surface area on one foot of 0.01 square metres. What pressure does the person exert on the ground if standing on both legs? |
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