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Topic 6: Buoyancy

Topic 6: Buoyancy. Buoyancy – the tendency for materials to rise or float in a fluid - the ability of a fluid to support an object floating in or on the fluid. Floating – occurs when an object does not fall in air or sink in water.

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Topic 6: Buoyancy

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  1. Topic 6: Buoyancy

  2. Buoyancy – the tendency for materials to rise or float in a fluid - the ability of a fluid to support an object floating in or on the fluid

  3. Floating – occurs when an object does not fall in air or sink in water

  4. Buoyant force – the upward force exerted on objects in fluids. Gravity pulls down. Buoyant force pushes up.

  5. Forces are measured in Newtons (N).

  6. Average Density – the total mass of all substance divided by the total volume.

  7. Objects float in water if they have an average density that is less than that of water. The density of water is 1.00 g/mL. A ship floats because its large hollow hull ensures that the average density of the ship is less than this, as low as 9.0 g/cm3. (1cm3 = 1 mL) Iron has a density of 7.87 g/cm3 so a nail will not float.

  8. A fish floats in water because of a swim bladder. This fish regulates the amount of air in the bladder to increase or decrease its average density so it can float to the surface or dive down. A submarine uses ballast tanks in much the same way. If the submarine wants to sink it fills the tanks with water, when it is to move up it pushes air into the ballast tanks forcing the water out.

  9. A dirigible, can float in air because it is more buoyant, has a lower average density than that of the air surrounding it.

  10. Archimedes

  11. Archimedes was a famous mathematician and scientist that lived 212 BC. He was given a problem by his king, Hiero II, the ruler of Syracuse (Greece). The king had a crown made and suspected that is was not made of pure gold.. Archimedes was to figure out how to prove if the crown was of pure gold.

  12. He didn’t know how to do this until getting into his bathtub. When he got in the water gushed over the sides – Eureka!!! A solid object added to a container displaces the fluid.

  13. To determine density we use volume and mass. Now he could determine the density of the crown. He found that the crown was a mixture of gold and silver and that the goldsmith had tried to cheat the king. The crown and the goldnugget have equal weight The crown displaces morewater than the gold nugget

  14. Archimedes did other experiments with buoyancy and fluids. In object immersed in a fluid does not rise or sink if the amount of force pulling down (gravity) equals the amount of force pushing up (buoyancy).

  15. Neutral buoyancy – when gravity equals buoyancy. Archimedes’ principle – the buoyant force acting on an object equals the weight (force of gravity) of the fluid displaced by the object.

  16. How Buoyancy and Density are Related Buoyant forces depend on density. The more dense a fluid the more easily an object will float in it. If fluids have a different density the less dense one will float on the denser one.

  17. Which is more dense, water or oil? Water

  18. Hydrometer – used to measure the density of a liquid

  19. Where would a hydrometer be used? -         determine if the antifreeze in a car is of the right mixture -         determine the sugar content of canned fruit syrup content of alcohol in wine

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