Download Chapter 13 Forces in Fluids

Chapter 13 Forces in
Fluids
It’s a bit of a review…
Pressure


Pressure is force exerted over an
area
P = Force
---------
Area

Pressure is measured in kilopascals
(kPa), Remember?
Pressure


If a weightlifter exerts 30,000
Newtons of force on a barbell 3.5m2,
what is the resultant Pressure?
P=F/A
= 30,000N / 3.5m2
= 8571.43 kPa
Water Pressure


Water pressure
increases with depth
Pressure in a fluid at
any given depth is
constant, and is
exerted equally in all
directions
Air Pressure



Air is a mixture of gases that has
mass
The weight of the earth’s atmosphere
exerts a pressure of 101kpa at sea
level
You cannot feel this pressure
because your body is used to it, but
if you were on another planet, you’d
notice a change
Planetary Atmospheric Pressures
Air Pressure


Air pressure decreases with altitude: the
higher up you go, the less air there is, and
the less pressure it exerts
This is why mountain climbers need
oxygen – there is not enough air to breath
at that altitude
Air Pressure



The weight of the atmosphere above
you is exerting a force of 1000N at
all times
Your body has a similar pressure
which balances out the force of the
atmosphere
Ever have your ears pop?
Pascal’s Principle


The pressure applied to a fluid is
transmitted unchanged throughout
the fluid.
When you squeeze one end of the
tube, the pressure is transmitted
through the entire fluid, and the
paste will pushed out of all
openings.
Bernoulli’s Principle

States that as the velocity of a fluid
increases, the pressure exerted by
that fluid decreases.
Airplane wing
Bernoulli’s Principle
Buoyancy


Is the ability of a fluid (liquid or gas) to
exert an upward force on an object
immersed in it.
This upward force is called the buoyant force.
Force of the
object pushing down on the water.
Buoyant force of
water pushing up on
the object.
Buoyant Force


The amount of buoyant force determines
if an object will sink or float.
If the buoyant force is less than the
object’s weight, it will sink. If the
buoyant force is greater than the
weight, it will float.
Archimedes's Principle

States that the buoyant force on an
object in a fluid is equal to the
weight of the fluid displaced by the
object.
Same mass of
steel
Ship
hull
Since hull
displaces more
water, the
buoyant force is
greater. It floats.