Boy, Oh Buoyancy

• Does it Float?

• Does it Sink?

What is density?

• A measure of how much material is packed into a unit volume of the material

• The fewer particles packed into a given volume, the less dense the material

• Density depends on mass of the atoms that make up the

material distance between the particles

Things float if they are less dense than their surrounding.

• Helium is less dense than the surrounding air.

The density of the ‘surroundings’ makes a difference!

• The Dead Sea in Israel has very salty water. People can float very high with very little effort.

What is a force?

• Any push or pull

• If a force in one direction is equal to a force in the opposite direction, then there is no movement

• If a force in one direction is stronger than the force in the opposite direction, then there will be movement

Buoyant force

• the upward force on an object in a fluid exerted by a surrounding fluid

• Example: When you push a beach ball under water, the water exerts an upward force on the ball. When you push more of the ball under water, more buoyant force is exerted. When you let go, the ball pops out of the water.

Archimedes said….

• In order to float, the weight of the water that an object displaces must be equal to – or greater than – the weight of the object.

Buoyant Force and Gravity

•A fluid exerts pressure on any object that is in the fluid.

• Gravity is the downward force on the object due to its weight.

•Archimedes’ principle: the buoyant force on an object is an upward force equal to the weight of the fluid that the object takes the place of.

Weight Versus Buoyant Force

• Sinking An object in a fluid will sink if its weight is greater than the buoyant force.

• Floating An object will float only when the buoyant force on the object is equal to the object’s weight.

• Neutrally buoyant An object will be suspended in water when its weight is exactly equal to the buoyant force

Weight Versus Buoyant Force

• Will an object sink or float? That depends on the whether the buoyant force is less than or equal to the object’s weight.

Buoyant Force and Gravity

• There is more pressure at the bottom of an object because pressure increases with depth. This results in an upward buoyant force on the object.

But the ability to float is more than density…

Water is “displaced” as you enter a bathtub, but you do not float!

So….

• In a pool, you spread your arms, displacing more water. The water that is displaced weighs as much or more than you do and you float!

Changing Overall Density

•The secret of how a ship floats is in the shape of the ship.

•The overall density of the boat is made less than the density of water by making the volume of the boat larger.

• Submarines have ballast tanks that can be opened to allow sea water to flow in.

• As water is added, the submarine’s mass increases, but its volume stays the same.

CHANGING MASS

•Like a submarine, some fish adjust their overall density to stay at a certain depth in the water by using an organ called a swim bladder.

CHANGING VOLUME

• Ships float because they have air in their hulls.

• Due to density of the surrounding fluid, they float at different heights in different types of water.

• Salt water is more dense than fresh water so the oil tanker floats high in the water.)

How does the surrounding fluid affect the way a ship floats?

In Fresh water the same oil tanker floats lower in the water.

Temperature of the surrounding water causes different densities

• In cold water, ships float higher. In tropical waters, ships float lower in the water.

This Could be a Problem!!• If a ship is loaded to the max

in a salt water port • comes across the ocean to

unload in a fresh water port• There could be a serious

problem if the ship was loaded too heavily at the beginning.

• Remember, salt water has a higher density and a greater buoyant force than fresh water

Plimsoll Lines

Samuel Plimsoll

(1824-1898) a member of the British Parliament was concerned with the loss of ships and crews due to overloading.

He called them “coffin ships”.

Plimsoll Lines

• To save sailors’ lives, he persuaded Parliament to provide for the marking of a line on ships’ sides that would disappear below the water line if the ship was overloaded.

Plimsoll Lines

• T = Tropical• TF = Tropical Fresh• F = Fresh• S = Summer• W = Winter• WNA = Winter North

Atlantic

In summary,Archimedes principle:

• If an object sinks, its volume = volume of water displaced

• If an object floats, the weight of object = the weight of the fluid it displaces

• the denser the fluid (example salt water versus fresh water) the greater the buoyant force exerted by the fluid