MSTC Physics 2

1
MSTC Physics 2

• Chapter 9
• Section 3

2
Fluid

• A substance that can flow
• ex liquids, gases
• Liquids have definite volume but no definite
  shape
• Gases dont have definite volume or shape

3
Study of Fluids

• Statics concentrates on properties of
• fluids at rest
• Dynamics focuses on properties related
• to motion of fluids

4
Density

• Ratio of a bodys mass to its volume
• ? m/v ? rho ? kg/m3
• Characteristics property of a substance
• ex Al of any mass or volume will have the same
  ?

5
Common Densities
6
Common Densities

• Table p 304
• Gases have small ? because molecules are far
  apart so a volume has a relatively large amount
  of empty space
• ? of gases are particularly sensitive to changes
  in temperature and pressure

7
Sample Problem

• The body of a man whose weight is about 690 N
  typically contains about 5.2 x 10-3 m3 (5.5 qt)
  of blood. Calculate the weight of the blood and
  express this weight as a percentage of the body
  weight. The density of blood is 1.05 x 103 kg/m3.

8
Buoyant Force

• Sum of the fluid forces acting on all sides of an
  object
• Net upward force

9
Archimedes Principle

• Any fluid applies a buoyant force to an object
  that is partially or completely immersed in it
  the magnitude of the buoyant force equals the
  weight of the fluid the object displaces
• B ?gV ? density of liquid
• V volume of fluid
• displaced
• B Newtons, N

10
Buoyant Force contd.

• Effect of buoyant force depends on how strong it
  is compared with the other forces acting on it
• If B W the object will stay in a fixed position
  in the fluid

11
Sample Problem

• A solid, square, pinewood raft measures 4 m on a
  side and is 0.3 m thick. Determine whether the
  raft floats in water, and if so, how much of the
  raft is beneath the surface. The density of
  pinewood is 550 kg/m3.

12
Buoyant Force contd.

• Any object that is solid throughout, no matter
  its shape, will float in a liquid if and only if
  its density is less than or equal to the density
  of the liquid
• Even though steel is more dense than water it can
  float in water if its the right shape

13
Barge Building Competition

• You will be given a 4 x 4 piece of aluminum
  foil and approximately 5 minutes to construct a
  barge that will float the most pennies without
  sinking.
• When it is time to compete, you will place your
  barge on the surface of the water and place
  pennies one-by-one in the barge until it sinks.

14
Practice Problem

• A hollow log in the shape of a half cylinder of
  length 3 m and radius 0.35 m is used as a canoe.
  If the canoe weighs 1000 N, what is the maximum
  weight it can hold without sinking?

15
Practice Problem

• An object is suspended from a scale, first with
  the object in air and then with it submerged in
  water. Suppose that the object is a crown and
  that the scale read 9.8 N in air and 9 N in
  water. Is the crown solid gold?