C HAPTER N INE : F ORCES AND F LUIDS -- B UOYANCY Forces influence the motion and properties of fluids.

9.1 F ORCES AND B UOYANCY A force is anything that causes a change in the motion of an object. A force can be like a push or a pull. Forces sometimes work together, or against each other.

Balanced forces are equal in strength and opposite to each other in direction.

Unbalanced forces happen when one force acting on an object is greater than another, causing the object to move in a certain direction and perhaps speed up or slow down.

MASS VS. WEIGHT Mass is the amount of matter in an object. Mass remains constant everywhere in the universe. Weight is a measurement of the pull, or force, of gravity acting on an object. Weight is measured in Newtons. The weight of an object can change because the force of gravity is not the same everywhere in the universe.

B UOYANCY : T HE A NTI - GRAVITY F ORCE Buoyancy, or buoyant force, is the upward force exerted on an object when it is submerged or floating in water or air. A buoyant force pushes away from the centre of the Earth.

A RCHIMEDES ’ P RINCIPLE Archimedes was a Greek scientist who made some discoveries about the connection between the weight of an object and buoyancy. He found that the buoyant force acting on an object equals the weight (force of gravity) of the fluid displaced by the object. This is known as Archimedes’ principle.

P OSITIVE, N EGATIVE AND N EUTRAL B UOYANCY

Positive and Negative buoyancy are the result of unbalanced forces. Positive Buoyancy : the buoyant force is greater than the force of gravity, therefore the object floats. Negative Buoyancy : the force of gravity is greater than the buoyant force, therefore the object sinks. Neutral buoyancy results from balanced forces. Neutral Buoyancy : the buoyant force and the force of gravity are equal, or balanced, and the object is suspended in the fluid.

A VERAGE D ENSITY The average density of an object is the total mass of all the substances that make up the object divided by the total volume. Average density is very useful because it means that objects that would otherwise sink, like large ships, are able to float.

Fish also use average density to control how deep in the water they can swim. They have a large organ in their body called a swim bladder. This sac is filled with a mixture of water and air. When there is less air in the sac, the fish sinks lower. When there is more air in the sac, the fish rises closer to the surface.

The swim bladder of a fish has been copied by engineers to help control the depth of a submarine.