Buoyancy and Archimedes Principle - PHY 231 Fall 2004, Exams of Physics

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Prof. S. Billinge

Class 24

Archimedes Principle

Fluid flow

Prof. S. Billinge

Announcements

• Exam results.

Average grade

after extra credit corrections12.

Prof. S. Billinge

Problem Solving

Overview

Prof. S. Billinge

Buoyancy: Archimedes

principle

Forces we already know

about:

1. Easy to see (something

pushing or pulling): 1. Rope/cable pulling 2. Normal force due to a

surface

2. Harder to see:

1. Weight (force due to gravity) 2. Friction

Prof. S. Billinge

Buoyancy

• When is it relevant?:

• When an object is immersed

in a fluid

• Archimedes principle:

Bouyant force is ALWAYSequal to the weight of thefluid displaced

Prof. S. Billinge

Two objects are placed in a beaker of water.

One floats, the other sinks. Whichstatement is true?

The floating object becomes weightless inthe fluid

The floating object is held up by thebuoyant force but it doesn’t act on thesinking object

The buoyant force acts upwards on bothobjects

Prof. S. Billinge

Two blocks of identical size are placed in a

beaker of water and sink. Block A is made oflead and block B is made of Aluminum.Which statement is true about the buoyantforces on A and B: 1.

B A

B

B

B A

B

B

B A

B

B

Not enough information

Prof. S. Billinge

•^

ALWAYS true: The (magnitudeof the) buoyant force is given bythe weight of the fluid displaced

-^

Floating objects:–

The buoyant force equals theweight of the object– Volume of fluid displaced, V

<f

volume of object, V

obj

-^

Density of object < density of fluid

-^

Sunken objects:–

The buoyant force < weight of theobject– V

= Vf

obj

-^

Density of object > density of fluid

Prof. S. Billinge

Two identical glasses are filled to the samelevel with water. One of the two glasses hasice cubes floating in it.Which weighs more?1. The glass without ice cubes.2. The glass with ice cubes.3. The two weigh the same.

Prof. S. Billinge

Two identical glasses are filled to the samelevel with water. One of the two glasses hasice cubes floating in it.When the ice cubesmelt, in which glass is the level of the waterhigher? 1. The glass without ice cubes.2. The glass with ice cubes.3. It is the same in both.

Prof. S. Billinge

A lead weight is fastened on top of a largesolid piece of Styrofoam that floats in a containerof water. Because of the weight of thelead, the water line is flush with the top surfaceof the Styrofoam. If the piece of Styrofoamis turned upside down so that theweight is now suspended underneath it,1. the arrangement sinks.2. the water line is below the top surface ofthe Styrofoam.3. the water line is still flush with the topsurface of the Styrofoam.

Prof. S. Billinge

A lead weight is fastened to a large solidpiece of Styrofoam that floats in a containerof water. Because of the weight of thelead, the water line is flush with the top surfaceof the Styrofoam. If the piece of Styrofoamis turned upside down, so that theweight is now suspended underneath it, thewater level in the container1. rises.2. drops.3. remains the same.

Prof. S. Billinge

Consider an object that floats in water butsinks in oil.When the object floats in water,half of it is submerged. If we slowly pour oilon top of the water so it completely coversthe object, the object1. moves up.2. stays in the same place.3. moves down.

Prof. S. Billinge

Consider an object floating in a container ofwater. If the container is placed in an elevatorthat accelerates upward,1. more of the object is below water.2. less of the object is below water.3. there is no difference.