Reading Quiz
A cube made of solid wood floats when
put into water. This means that
•the cube weighs less than an equal
volume of water.
•the cube weighs the same as an equal
volume of water.
•the cube weighs more than an equal
volume of water.
Previous Unit: The Newtonian Revolution
What have we learned….
Force, F=ma
constant acceleration
Third Law
torque
Gravitational force law
g= 9.8 m/s/s
Work, energy, power
Conservation of energy
Conservation of momentum Conservation of angular momentum
The New Unit: Fluids and Heat
Questions:
What are fluids, liquids, gases?
What forces do liquids apply to objects?
Is the atmosphere a fluid?
How do fluids flow?
A woman’s high heels sink into the soft ground,
but the larger shoes of the much bigger man do not.
Pressure = force/area
The pressure exerted on the piston extends uniformly
throughout the fluid, causing it to push outward with equal
force per unit area on the walls and bottom of the cylinder.
Pascal’s Principle:
Any change in the pressure of a fluid is
transmitted uniformly in all directions
throughout the fluid.
A small force F1 applied to a piston with a small
area produces a much larger force F2 on the larger
piston. This allows a hydraulic jack to lift heavy objects.
Torricelli filled a tube with mercury and
inverted it into an open container of
mercury. Air pressure acting on the
mercury in the dish can support a column
of mercury 76 cm in height.
How much does the
atmosphere weigh? The
same as 76 cm of
mercury.
76cm mercury=1.01x105 pascals
Standard atmospheric pressure
32 feet of water or 14.7Lbs/in2
A balloon that was partially inflated near sea level
expanded as the experimenters climbed the mountain.
Atmospheric
pressure changes
with altitude.
P1V1 = P2V2
Temperature Constant
Boyle’s Law
The density of a column of air decreases as altitude
increases because air expands as pressure decreases.
P1V1 = P2V2
Boyle’s Law
The atoms in a liquid are closely packed while those
in a gas are separated by much larger distances.
Gas have a density ~ 1/1000 x liguid density
In Boyle’s experiment, adding mercury to the open
side of the bent tube caused a decrease in the
volume of the trapped air in the closed side.
P1V1 = P2V2
Boyle’s Law
Archimedes’ Principle:
The buoyant force acting on an
object fully or partially submerged
in a fluid is equal to the weight of
the fluid displaced by the object.
The weight of a column of water is proportional to
the volume of the column. The volume V is equal
to the area A times the height h.
Equilibrium…
The pressure acting on the bottom of the suspended
metal block is greater than that acting on the top
due to the increase of pressure with depth.
Lecture Quiz - Question 1:
A large metal tank is filled with high pressure of
Helium, used to fill Helium balloons. Consider how
much the tank weighs when it is full, and when
empty.
A. The weight is the same full or empty.
B. The empty tank weighs less than the full tank.
C. The tank weighs less when it is full, because
Helium is lighter than air.
Lecture - Question 2:
One cubic centimeter of Aluminum weighs 2.7 gms
(rAl=2.7 gms/cm3). The Al cube is placed in a
beaker of water.
A. The upward buoyant force on the cube equals the
downward force (weight).
B. The weight of the displaced water equals the weight
of the cube.
C. The weight of the cube is reduced in water do to the
buoyant force.
D. The cube floats, because Aluminum is lighter than
water.
Lecture - Question 3:
A block of wood is added to a beaker of water. It floats.
The beaker rests on a scale that reads its weight.
A. There is no change in weight, because the buoyant
force equals the weight of the water.
B. The weight increases by the weight of the block.
C. The weight increases by an amount less than the weight
of the block.