Notes Buoyancy

8/3/2019 Notes Buoyancy

1/4

Buoyancy is the reduction in weight on an object in liquid. The object weights less than

its actual weight when it is in water.

The ship is afloat because the weight of the ship is balanced by a buoyant force acting

upward. If more cargo is loaded onto the ship, it would sit lower in the water and

displace more water, thus receiving a higher buoyant force to balance the increasedweight.

Buoyant force is the upward force that causes a reduction in the weight of an object in

water. It is equal to the weight of the liquid displaced. This force is responsible for the

object weighing less than its normal weight when the object is fully or partially in a liquid

or a gas. It also responsible for keeping the ship afloat. The buoyant force does not

depend on what the object is made of.

The buoyant force which act upward on an object fully or partially in a fluid is related to

the weight of the fluid that is being displaced by the objects but not the weight of the

objects


2/4

Have you ever experienced lifting a heavy object in water? Yes, you would find that the

object is not as heavy as when it is not in water. In other words, an object is lighter in

water and easier to lift. Why this is so? When an object is in the water, there is always

an upward force or thrust that causes the object to weigh less that its actual weight. The

upward thrust is called the buoyant force.In this sub-section, you will be introduced to the buoyant force and the concept of

floatation. Archimedes' Principle will be introduced and the mathematical formula of this

principle will be derived. The daily applications of Archimedes' Principle will also be

discussed. They include the concept of the hot air balloon, the submarine, the ship and

the hydrometer.

y Buoyancy

A solid body immersed in a fluid will have upward buoyant force acting on it and this is

equal to the weight of the displaced fluid. This is due to the hydrostatic pressure in thefluid. The buoyant force causes the solid object to weigh less. The reduction in weight in

water is called buoyancy. The upward force that causes the reduction of the object in

the water is called the buoyant force.

An example of buoyancy

In the case of a container ship, for instance, its weight is balanced by a buoyant force

from the displaced water, allowing it to float. If more cargo is loaded onto the ship, it

would sit lower in the water, displacing more water and thus receiving a higher buoyant

force to balance the increased weight

The discovery of the principle of buoyancy is attributed to Archimedes (287-212 BC).

The buoyant force

In physics, buoyancy is the upward force on an object produced by the surrounding fluid

(i.e., a liquid or a gas) in which the object is fully or partially immersed. The buoyancy is

due to the pressure difference of the fluid between the top and the bottom of the object.

The net upward buoyant force is equal to the magnitude of the weight of the fluid

displaced by the object.

This force enables the object to float or become lighter when lifted in the water.

Buoyancy is important for many objects including boats, ships, balloons and ships. The

buoyant force acts against the force of gravity and so this makes objects seem lighter

with respect to gravity. The buoyant force does not depend on what the object is made

of.


3/4

Density and buoyancy

From Archimedes' Principle, when a body is partially or completely immersed in a liquid,

it experiences an upward buoyant force which is equal to the weight of the fluid

displaced by the immersed part of the body.

V is the volume of liquid displaced and p is the density of the fluid. Then the mass of the

fluid displaced is

From Archimedes' Principle which states that the upward buoyant force is equal to the

weight of the fluid displaced by the immersing part of the body, we obtain the following:

From equation (3), we can see that the buoyant force can be expressed using the

following equation:

where p is the density of the fluid, V is the volume of the object submerged and g is the

gravitational acceleration (10 ms-2). The negative sign must be used since the buoyancy

is opposite in direction to the acceleration due to gravity.

The weight mentioned is the weight of the fluid displaced by the immersing part of the

body. Thus, when we take the volume of the object, we only take the volume of the

part that immersed in the water.


4/4

y Archimedes' Principle

Archimedes' Principle, or the law of up thrust, states that,

"...Any body wholly orpartially immersed in a fluid experiences an upthrust equal

to, but opposite in sense to, the weight of the fluid displaced... " (Fluid means

liquid orgas).In other words, when a body is partially or completely immersed in a liquid, it

experiences an upward buoyant force which is equal to the weight of the fluid displaced

by the immersing part of the body. This is true only as long as one can neglect the

surface (capillarity) tension acting on the body.

Partially or totally immersed objects

There are two forces acting on an object immersed partially or totally in a fluid

Application of Archimedes' Principle

Archimedes's Principle has many applications in our everyday life. The applications of

this principle include the floating ship, the submarine, the hydrometer and the hot air

balloon.

How does a heavy ship remain floating without sinking in the water? Archimedes's

Principle makes this possible.

When a ship is partially immersed in the sea, the weight of the water displaced is equal

to the weight of the ship. And because the volume of the displaced water is large, the

buoyant force exerted upward is also large. This large buoyant force keeps the ship

afloat.

Documents

Notes Buoyancy