BUOYANCYBUOYANCY

Definition:Definition:

BUOYANCY:BUOYANCY: Is a measure of the upward Is a measure of the upward

force a fluid exerts on an object.force a fluid exerts on an object.

(The ability to float or rise in a (The ability to float or rise in a fluid)fluid)

(it is a property of liquids and (it is a property of liquids and gases)gases)

NewtonNewton

Isaac Newton came up with the Isaac Newton came up with the explanation for force.explanation for force.

Therefore the unit for force is a Therefore the unit for force is a NEWTONNEWTON..

Archimedes’ PrincipleArchimedes’ Principle

Archimedes is a Greek Archimedes is a Greek Mathematician that realized Mathematician that realized that the-- that the-- force exerted force exerted on an object in a liquid on an object in a liquid is equal to the weight of is equal to the weight of the fluid displaced by the fluid displaced by the object.the object.

ExampleExample

A piece of string attached to a rock A piece of string attached to a rock weighs 2.25 newtons. Next the rock weighs 2.25 newtons. Next the rock is immersed in water, but not is immersed in water, but not allowed to touch the bottom or sides allowed to touch the bottom or sides of the container. Now the rock’s of the container. Now the rock’s weight measures 1.8 newtons. The weight measures 1.8 newtons. The water has an exerted force of 0.45 water has an exerted force of 0.45 newtons.newtons.

Buoyancy explains why some Buoyancy explains why some objects sink and others float. If objects sink and others float. If the buoyant force is greater than the buoyant force is greater than its weight, the object floats. In its weight, the object floats. In the example before, we would the example before, we would need a buoyant force greater need a buoyant force greater than 2.25 newtons to make the than 2.25 newtons to make the rock float.rock float.

2 Principles to remember:2 Principles to remember:

1.1. If the buoyant force is If the buoyant force is lessless than its weight, then than its weight, then the object will the object will sinksink..

2. If the buoyant force is 2. If the buoyant force is greatergreater than its weight, than its weight, then the object will then the object will floatfloat..

Question???Question???

Why does a block Why does a block of steel sink, but of steel sink, but a steel boat will a steel boat will float?float?

Answer:Answer: 1 m1 m33 of steel has a weight of of steel has a weight of

76,400 newtons. When placed 76,400 newtons. When placed in the water, the block displaces in the water, the block displaces 1m1m33 of water. The water would of water. The water would have a weight of 9,800 newtons. have a weight of 9,800 newtons. The weight of the steel block is The weight of the steel block is much greater than the weight of much greater than the weight of the displaced water. Therefore, the displaced water. Therefore, the block sinks.the block sinks.

Answer cont.Answer cont.

But…if the same block of steel is But…if the same block of steel is flattened into a thin sheet, its sides flattened into a thin sheet, its sides bent up in the shape of a boat might bent up in the shape of a boat might occupy 10m3 of space instead of occupy 10m3 of space instead of 1m3. 10 m3 of displaced water has 1m3. 10 m3 of displaced water has a weight of 98,000 newtons. Now a weight of 98,000 newtons. Now the displaced water weighs more the displaced water weighs more than the steel which still weighs than the steel which still weighs 76,400 newtons.76,400 newtons.

Answer cont.Answer cont. When you put the steel ship in When you put the steel ship in

the water, it will settle in the the water, it will settle in the water until it reached a level water until it reached a level where it displaced 76,400 where it displaced 76,400 newtons of water. Then the newtons of water. Then the upward force exerted by the upward force exerted by the water would equal the downward water would equal the downward force exerted by the boat.force exerted by the boat.