# buoyancy of fluids

## buoyant force

Buoyancy is the ability of a fluid to exert an upward force on any object placed in the fluid. This upward force is called buoyant force.

## pressure and buoyant force

What explains buoyant force? Recall from the earlier lesson "Pressure of Fluids" that a fluid exerts pressure in all directions but the pressure is greater at greater depth. Therefore, the fluid below an object exerts greater force on the object than the fluid above the object. This is illustrated in Figure 15.12. Buoyant force explains why objects may float in water. No doubt youve noticed, however, that some objects do not float in water. If buoyant force applies to all objects in fluids, why do some objects sink instead of float? The answer has to do with their weight.

## weight and buoyant force

Weight is a measure of the force of gravity pulling down on an object. Buoyant force pushes up on an object. Weight and buoyant force together determine whether an object sinks or floats. This is illustrated in Figure 15.13. If an objects weight is the same as the buoyant force acting on the object, then the object floats. This is the example on the left in Figure 15.13. If an objects weight is greater than the buoyant force acting on the object, then the object sinks. This is the example on the right in Figure 15.13. Because of buoyant force, objects seem lighter in water. You may have noticed this when you went swimming and could easily pick up a friend or sibling under the water. Some of the persons weight was countered by the buoyant force of the water.

## density and buoyant force

Density, or the amount of mass in a given volume, is also related to buoyancy. Thats because density affects weight. A given volume of a denser substance is heavier than the same volume of a less dense substance. For example, ice is less dense than liquid water. This explains why ice cubes float in a glass of water. This and other examples of density and buoyant force are illustrated in Figure 15.14 and in the video at this URL: MEDIA Click image to the left or use the URL below. URL:

## archimedes law

Did you ever notice that when you get into a bathtub of water the level of the water rises? More than 2200 years ago, a Greek mathematician named Archimedes noticed the same thing. He observed that both a body and the water in a tub cant occupy the same space at the same time. As a result, some of the water is displaced, or moved out of the way. How much water is displaced? Archimedes determined that the volume of displaced water equals the volume of the submerged object. So more water is displaced by a bigger body than a smaller one. What does displacement have to do with buoyant force? Everything! Archimedes discovered that the buoyant force acting on an object in a fluid equals the weight of the fluid displaced by the object. This is known as Archimedes law (or Archimedes Principle). Archimedes law explains why some objects float in fluids even though they are very heavy. Remember the oil tanker that opened this chapter? It is extremely heavy, yet it stays afloat. If a steel ball with the same weight as the ship were put into water, it would sink to the bottom (see Figure 15.15). Thats because the volume of water displaced by the steel ball weighs less than the ball. As a result, the buoyant force is not as great as the force of gravity acting on the ball. The design of the ships hull, on the other hand, causes it to displace much more water than the ball. In fact, the weight of the displaced water is greater than the weight of the ship, so the buoyant force is greater than the force of gravity acting on the ship. As a result, the ship floats. You can check your understanding of Archimedes law by doing the brainteaser at this URL: . For an entertaining video presentation of Archimedes law, go to this URL: http://videos.howstuffworks.com/disc

## instructional diagrams

No diagram descriptions associated with this lesson

## questions

Only liquids have buoyancy.

``````a. true

-->  b. false
``````

Which statement is true about weight and buoyant force?

``````a. They work in opposite directions.

b. They are always equal in strength.

c. They determine whether an object floats.

-->  d. two of the above
``````

Any object sinks if its weight is greater than its volume

``````a. true

-->  b. false
``````

Some objects float in water because the objects

``````-->  a. weigh less than the weight of the water they displace.

b. have less mass when they are placed in water.

c. have greater density than water.

d. have the property of buoyancy.
``````

A denser object weighs more than a less dense object of the same size.

``````-->  a. true

b. false
``````

The amount of water that is displaced when you submerge yourself in a swimming pool is equal to your bodys

``````a. surface are

-->  b. volume.

c. weight.

d. mass.
``````

The weight of the displaced water in question 3 equals the

``````a. force of gravity acting on your body.

-->  b. buoyant force acting on your body.

d. two of the above
``````

Helium balloons float in air because helium is purer than air.

``````a. true

-->  b. false
``````

The buoyant force acting on an object in a fluid always equals the objects weight.

``````a. true

-->  b. false
``````

What happens if an object is placed in water and its density is greater than water?

``````a. The object always sinks.

b. The object always floats.

-->  c. The object may sink or float.

d. The objects density decreases.
``````

Archimedes determined that the mass of fluid displaced by an object equals the mass of the object.

``````a. true

-->  b. false
``````

The more fluid an object displaces, the greater the buoyant force acting on the object.

``````-->  a. true

b. false
``````

Buoyancy is a property of an object that can float in a fluid.

``````a. true

-->  b. false
``````

Fluids exert pressure only in an upward direction.

``````a. true

-->  b. false
``````

Buoyant force explains why some objects float in water.

``````-->  a. true

b. false
``````

Objects float because fluids exert only upward pressure.

``````a. true

-->  b. false
``````

amount of mass in a given volume

``````a. buoyant force

b. displacement

c. buoyancy

d. float

e. weight

f. gravity

-->  g. density
``````

Buoyancy is a property of some objects when placed in fluids.

``````a. true

-->  b. false
``````

force that causes an object to sink in a fluid

``````a. buoyant force

b. displacement

c. buoyancy

d. float

e. weight

-->  f. gravity

g. density
``````

to remain at or near the surface of a fluid

``````a. buoyant force

b. displacement

c. buoyancy

-->  d. float

e. weight

f. gravity

g. density
``````

If two objects have the same volume but differ in density, the denser object will weigh more.

``````-->  a. true

b. false
``````

force that causes an object to float on a fluid

``````-->  a. buoyant force

b. displacement

c. buoyancy

d. float

e. weight

f. gravity

g. density
``````

Archimedes law explains why heavy objects can float if they displace enough water.

``````-->  a. true

b. false
``````

The buoyant force acting on an object in a fluid depends on the total volume of the fluid.

``````a. true

-->  b. false
``````

act in which an object moves fluid out of its way

``````a. buoyant force

-->  b. displacement

c. buoyancy

d. float

e. weight

f. gravity

g. density
``````

ability of a fluid to exert upward force

``````a. buoyant force

b. displacement

-->  c. buoyancy

d. float

e. weight

f. gravity

g. density
``````

measure of the force of gravity pulling down on an object

``````a. buoyant force

b. displacement

c. buoyancy

d. float

-->  e. weight

f. gravity

g. density
``````

Which statement explains buoyant force?

``````a. Denser fluids exert less pressure.

-->  b. Fluid pressure is greater at greater depths.

c. An object weighs less in water.

d. all of the above
``````

What determines whether an object floats or sinks in water?

``````a. the buoyant force acting on the object

b. the force of gravity acting on the object

c. the objects weight

-->  d. all of the above
``````

Ice cubes float on water because ice is

``````-->  a. less dense than water.

b. colder than water.

c. heavier than water.

d. less stable than water.
``````

When you sit in a tub of bath water, the water rises because it

``````a. becomes less dense.

-->  b. is displaced.

c. gets warmer.

d. floats.
``````

Buoyancy is a property of

``````a. gases.

b. liquids.

c. solids.

-->  d. two of the above
``````

Where is water pressure greatest on an object in the water?

``````a. on top of the object

b. on the sides of the object

-->  c. on the bottom of the object

d. two of the above
``````

You feel lighter in the water than on land because

``````a. gravity is not as strong in the water.

-->  b. the buoyant force of the water counters some of your weight.

c. your mass is less in the water than on land.

d. you are trying to stay afloat.
``````

## diagram questions

No diagram questions associated with this lesson