Forces in fluids can be explained by several principles: 1) Pressure is exerted equally in all directions and depends on depth, with deeper depths experiencing greater pressure from the weight of fluid above. 2) Whether an object floats or sinks depends on if its density is greater or less than the density of the surrounding fluid based on Archimedes' principle. 3) Bernoulli's principle states that as the speed of a fluid increases, the pressure exerted decreases, which can create lift.

1. Forces in Fluids
Pressure, Floating/Sinking,
Pascal’s Principal
&
Bernoulli’s Principal

2. Pressure (1)
 Pressure relates to anything exerting
force on another object by pressing on
it.
Pressure = Force/Area
Units will be in Newton per squared
meter: Nm2

3. Pressure (2)
 Theamount of pressure you exert
depends on the area over which you
exert a force.
 Examples:
– Snow shoes vs. regular shoes
– Light pressure used with needles for shots
– High heels in grass vs. regular shoes

4. Fluid Pressure (1)
 Remember back to fluids…
 Fluids are a material that can easily
flow and change shape.
 If you understand the pressure in
fluids, you can predict whether an
object will float or sink.
**** Fluids are not just liquids!! They can
also be gasses- look at the definition!!!

5. Fluid Pressure (2)
 In a fluid, all of the forces exerted by
the individual particles combine to
make up the pressure exerted by the
fluid.
– The denser the fluid, the more pressure it
will exert (because there are more
particles to exert pressure).

6. Fluid Pressure (3)
 Air pressure is also considered to be a
type of fluid pressure.
 We are constantly surrounded by this
pressure of the air around us.
 Although it changes at different
elevations, our bodies are designed to
withstand it without it crushing us.
– Sea Level atmospheric pressure is a
whopping 101,300 Pa!!!!! (Pascal units)

7. Measuring Pressure
 Weuse Barometers to measure
atmospheric pressure.
 Peopleto make weather predictions
use Barometers to help understand
weather patterns and make forcasts.

8. Floating and Sinking (1)
 Remember Density? Mass per unit
volume???
 We can calculate density by dividing
mass by volume!
Density = Mass / volume

9. Floating and Sinking (2)
 If you know the densities of objects you
can predict whether they will sink or
float when placed in a fluid of a given
density.
 Remember if the density is larger than
the fluid it’s placed in it will sink, if it’s
smaller it will float. (let’s see what
you’ve learned…)

10. Floating and Sinking (3)
 IfI had water (1g/cm3) would the
following float or sink?
– Wood (.7 g/cm3 )
– Tar (1.02 g/cm3 )
– Corn Oil (.925 g/cm3 )
– Mercury (13.6 g/cm3 )
– Corn Syrup (1.38 g/cm3 )

11. Buoyancy (1)
 Buoyancy is the ability to float.
 Water and other fluids exert an upward
force on any object submerged into it.
 Buoyancy acts in the opposite direction
of gravity, so it makes an object feel
lighter.
– Ever notice how it’s much easier to lift or
carry a friend in a pool than it is outside
the water?

12. Buoyancy (2)
 Weight of submerged objects is a
downward force and the buoyancy of
an object is an upward force.
 If an objects weight is greater than the
buoyant force it will sink.
 If an objects weight is equal to the
buoyant force it will not sink.

13. Buoyancy (3)
 Pressure gets greater, the lower you
are in a fluid.
– You have the weight of all of the fluid on
top of you and the lower you get, the more
weight is on top of you.
– It’s like playing “dog pile”. You never want
to be on the bottom or you get crushed!

14. Archimedes Principal (1)
 Archimedes Principal states that the
buoyant force acting on a submerged
object is equal to the weight of the
volume of the fluid displaced by the
object.
 Meaning, if you hand displaces 50 ml
of water when I stick in a cup (stupid, I
know) then the buoyant force acting on
it would be equal to the weight of 50ml.

15. Archimedes Principal (2)
 So how does this apply to ships?
 If the buoyant force of an object is
equal to the weight of the displaced
fluid, then a large fluid that displaces a
lot of fluid should have a strong
buoyant force.
 Ifyou are confused, check out the
picture on page 429 in your text book.

16. Pascal’s Principal (1)
 Fluids exert pressure on any surface
that it touches.
 Pascal’s Principal states that when a
force is applied to a confined fluid, the
change in pressure is transmitted
equally to all parts of the fluid.
– An example of Pascal’s principal at work
is hydraulic pistons.

17. Pascal’s Principal (2)
 Hydraulic systems multiply forces by
applying the force to a small surface
area. The increase in pressure is then
transmitted to another part of the
confined fluid, which pushes on a
larger surface area.
– Hydraulic lifts
– Hydraulic breaks

18. Bernoulli’s Principal (1)
 Fluids (remember, not just liquids) flow
from areas of high pressure to low
pressure. This is an attempt to keep
things balanced.
 The pressure of a moving fluid is
different than the pressure of a fluid at
rest (not moving).

19. Bernoulli’s Principal (2)
 Bernoulli’s principal states that the
faster a fluid moves, the less pressure
the fluid exerts.
– Therefore, as the speed increases, the
pressure exerted by the fluid decreases.

20. Bernoulli’s Principal (3)
 This principal is used to explain the
flight of objects (kites, airplane, frisbee
etc).
 If the air moves faster above the
object, fluid pressure pushes the object
upward.
– The air moves faster due to the
shape/design of the object (typically a
curved shape with a slightly rounded top)