BUOYANCY

What makes some objects float and some objects sink?

1.

An object placed in water experiences an upward force called BUOYANT FORCE [B f ].

Pressure acting at the sides are equal.

Upward force is greater because of greater pressure = Fluid pressure increases with depth.

2.

I feel I’ve lost a lot of weight!

Since BUOYANT FORCE acts opposite that of gravity, this force makes an object feel lighter.

3.

Archimedes, a Greek Mathematician, studied the force of buoyancy. He came up with this PRINCIPLE: “The BUOYANT FORCE acting on an object is equal to the WEIGHT of the fluid DISPLACED by the object.”

Archimedes supposedly ran out to the street naked!

ILLUSTRATION

The weight of fluid collected is equal to the B f acting on the object submerged.



The WEIGHT of the water the man displaces is equal to the SIZE of the B f acting on him.

4.

When does an object float?

When the B f is greater than the WEIGHT of the object, the object floats. The object is described as POSITIVELY BUOYANT.

When does an object sink?

When the WEIGHT of the object is greater than the B f acting on the object, the object sinks. The object is NEGATIVELY BUOYANT.

KINDS OF BUOYANCY

Positive Buoyancy: Neutral Buoyancy: Negative Buoyancy:

Does SINKING or FLOATING depend on the object’s weight?

NO. It depends on the DENSITIES of the object and the fluid where it is placed.

Recall…

DMV?

Density = Mass ÷ Volume

M D V