BUOYANCY

Buoyancy

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 the buoyant force

Bouyant 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 object more dense 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 in which it is immersed neither sinks nor floats.

Formula

Fb= buoyant force (N)

ρfluid= density of the fluid (kg/m3) v= volume (m3) g= gravity (9.81 m/s2)

Examples

1. Find the bouyant force on a body with a volume of 0.3 m3, whe 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. Its volume 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 volume of wood is under water?

Problems

1.- 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 a 0.0330 kg ice cube into her

glass of soda water. The ice cube has dimensions of 3cm x 3cm x 4cm. What is the volume of the ice out of water? What is the force needed to sink completely the ice cube?

3.- Eileen is floating on her back in the 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 volume of Eileen under water?