Download Properties of Fluids

Properties of
Fluids
What is a Fluid?
• Fluids include two states of
matter
–Liquids
–Gases
Sink or Float?
• We have said that density is what determines
whether an object will sink or float in a fluid.
• This is only part of the story!
• Another factor affecting this is called the
buoyant force.
Buoyant Force
• Buoyant force (measured in Newtons) is the
upward force exerted by a fluid on an
immersed object.
– Buoyant force > weight
• Balloon rises
– Buoyant force < weight
• Balloon sinks
– Buoyant force = weight
• Balloon floats
Buoyant Force
Archimedes’ Principle
• The buoyant force on an object in a fluid
equals the weight of the fluid displaced by the
object.
Buoyant Force
• Determine whether each object will float or sink.
A.
53 N
20 N
11 N
B.
200 N
302.6 N
D.
C.
53 N
1200 N
9N
E.
302.4 N
1350 N
Calculating Buoyant Force of Water
1. How much water does the object displace?
(In other words, what is the volume of the
object?)
2. One cubic foot of water has a buoyant force
of about 62.43 pounds.
3. Multiply the volume of the object (in ft3) by
62.43 to find the buoyant force.
4. If buoyant force is greater than the weight of
the object, the object will float! If buoyant
force is less than weight, the object will sink.
Example
• A cube weighing 500 lbs. and measuring 3x6x2
ft is dropped into water. Will it float or sink?
• First, find the volume (l x w x h).
3 x 6 x 2 = 36 ft3
• Multiply by 62.43 lbs to find the buoyant force.
Buoyant force = 2,247.5 lbs
• Compare buoyant force to the weight of the
object.
Buoyant force > weight
Object will float
Viscosity
• Liquid’s resistance to flow
• Depends on the attraction between particles
– Stronger attraction = more viscous (does not flow
easily)
• Can be affected by temperature
Pascal’s Principle
• Named for French
scientist Blaise
Pascal
• Explains how
pressure is
transmitted through
fluids
– Pressure = force
exerted on a specific
area
Pascal’s Principle
• Pressure applied to a fluid is transmitted
throughout the fluid
–Example: squeezing a tube of
toothpaste
Applying Pascal’s Principle
• Hydraulics machines are based on Pascal’s
Principle.
– Transmit pressure through a fluid to
multiply force
– Save us some work
• Examples: hydraulic lifts, dentist chairs,
hospital beds, etc.
Hydraulics
• You input a small amount of force over a small
area
• The output is a large force over a large area
Hydraulics
Pascal’s Principle
• We can use Pascal’s Principle to calculate how
much force will result from a hydraulic device
• Pressure = Force/Area
– Measured in pascals (Pa)
Pascal’s Principle
• A car weighing 1000 N sits on a 250 m2 platform. What
force is needed on the 10 m2 plunger to keep the car
from sinking?
GIVEN:
WORK:
Platform:
F = 1000 N
A = 250 m2
Plunger:
F=?
A = 10 m2
1000 N = F2
250 m2
F1 F2

A1 A2
10 m2
(1000N)(10m2)=(250m2)F2
F2 = 40 N
Pascal’s Principle
• A disgruntled cow that weighs 1000 N sits on a 5 m²
piston. What force would need to be applied to a 2
m² piston?
GIVEN:
WORK:
Platform:
F = 1000 N
A = 5 m2
Plunger:
F=?
A = 2 m2
1000 N = F2
5 m2
F1 F2

A1 A2
2 m2
(1000N)(2m2)=(5m2)F2
F2 = 400 N
Pascal’s Principle
• You apply 20 N of force to a 3 m2 piston on a
hydraulic car lift. What would the area of the
platform have to be to lift a 2000 N car?
GIVEN:
Platform:
F = 2000 N
A=?
Plunger:
F = 20 N
A = 3 m2
WORK:
2000 N = 20 N
A1
3 m2
(2000 N)(3 m2) = (20 N)(A1)
A1 = 300 m2
Exit Ticket
1. An object weighing 56 N displaces 89 N of
water. Will it sink or float?
2. A 50 N object displaces 200 mL of water
weighing 1.96 N. Draw a diagram illustrating the
opposing forces on the object. Will it sink or
float?
3. A mechanic exerts 10 N of force on a 0.8 m2
piston. How much force can the large piston
(area = 23 m2) support?
4. You exert 17 N of force on the 2 m2 input piston
of a hydraulic device. What must be the area of
the large piston to lift 70 N?
Quiz Review
• Matching:
– Kinetic Theory of Matter
– Law of Conservation of Mass
– Law of Conservation of Energy
– Buoyant force
– Archimedes’ Principle
– Viscosity
– Pascal’s Principle
Quiz Review
• Archimedes’ Principle problems:
– Buoyant force vs. Weight
– Will objects sink or float?
– An object weighing 33 N displaces 72 N of water. Will it
sink or float?
– A 50 N object displaces 200 mL of water weighing 1.96 N.
Draw a diagram illustrating the opposing forces on the
object. Will it sink or float?
– An object weighing 23 N is dropped into an overflow can.
The displaced water is collected in a 40 N beaker. If the
weight of the beaker full of water is 61 N, did this object
sink or float?
Quiz Review
• Pascal’s Principle Problems
– A hydraulic hospital bed requires 10 N of force to
be exerted on a 0.5 m2 piston. If the area of the
bed is 5 m2, what is the maximum weight for a
patient it can lift?
– A mechanic needs to lift a 5000 N truck. If he
exerts a 30 N force on a 1 m2 piston, what must be
the area of the platform the truck sits on?
Pascal’s Practice
• A mechanic exerts 10 N of force on a 0.8 m2
piston. How much force can the large piston (area
= 23 m2) support?
• You exert 17 N of force on the 2 m2 input piston
of a hydraulic device. What must be the area of
the large piston to lift 70 N?
• A hydraulic lift has a 3000 N car on the 40 m2
large piston. How much force needs to be exerted
on the small piston (area= 5 m2)?
Bernoulli’s Principle
As the velocity (speed) of a fluid
increases, the pressure of the
fluid decreases.
Bernoulli’s Principle
Airplane Lift
• Airplane wings are
designed so air (a fluid!)
flows more quickly over
the top of the wing.
• The higher speed causes
lower pressure above the
wing.
• Underneath the wing, air
pressure is higher… this
helps lift the airplane up.