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▪ The apparent loss of weight of objects when submerged in a liquid.
▪ The liquid exerts an upward force that is opposite in direction to gravity. This upward force is called thebuoyant force

Archimide’s principle

▪ An immersed object is buoyed up by a force equal to the weight of the fluid it displaces.

Does it sink, or does it float?

1. An objectdenser than the fluid in which it is immersed sinks.

2. An object less dense than the fluid in which it is immersed floats.

3. An object with density equal to the density of the fluid inwhich it is immersed neither sinks nor floats.



▪ Fb= buoyant force (N)

▪ ρfluid= density of the fluid (kg/m3)

▪ v= volume (m3)

▪ g= gravity (9.81 m/s2)Examples

1. Find the buoyant force on a body with a volume of 0.3 m3, when it is summerged in a liquid with a density of 1100 kg/m3.

2. A rock of 70 kg is in the bottom of a lake. Itsvolume is 0.03 m3. What is the force needed to raise the rock?

3. A 1200 kg piece of wood with a density of 0.6x103 kg/m3 floats in a river. What force is needed to sink the wood?

Problems1.- A 1.9 kg piece of wood from a sunken pirate ship has a volume of 2.16 x 10-3 m3. Will this piece of wood float to the surface of the water or remain submerged with the ship?

2.- Ursula drops a0.0330 kg ice cube into her glass of soda water. The ice cube has dimensions of 3cm x 3cm x 4cm. What is the force needed to sink completely the ice cube?

3.- Eileen is floating on her back inthe beautiful blue Caribbean during her spring vacation. If Eileen’s density is 980 kg/m3 and she has a volume of 0.060 m3. What is the buoyant force that supports her in the sea water? What is theforce needed to sink Eileen completely under water?

4.- A bar of soap has dimensions of 9cm x 6cm x 3cm, and a density of 975 kg/m3. The bar is floating on water, a) what is buoyant force acting...
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