Ch13.1 - Fluids!

Take a dive and discover what

pressure is all about

Like Freddie Mercury once said…

Pressure is something that is all around us, even in

Space. It comes from the force of different objects

interacting with one another. The atoms that make

up our atmosphere whiz around at close to 500m/s.

Due to interactions at the atomic level, these

molecules rebound in totally elastic collisions, so no

Kinetic Energy is lost.

Wiggle your toes a little to test the waters

Pressure is measure in an SI unit called the pascal (Pa).

P = F/AAt the surface of the Earth, the atmosphere exerts about

10N of force per cm2. This is caused by the weight of air above it (30 km high!)

1Pa = 10N/10-4m2

This tells us that for every square meter, the pressure is 100 kPa = 1x105 N/m2 = 1 atm

It’s too much Captain! She cannot take it!

The build up of pressure can cause mercury to rise in a

barometer, a whistle to sound in a kettle and can even

make a water heater shoot off like a rocket!

Don’t believe me? Check out what Jamie and Adam

did…

http://www.youtube.com/watch?v=JmJoyuUJj2Q

A hint… 350 lb/in2 = 2.4x103 kPa

Blaise Pascal (1623 – 1662)

Pascal was a child protégé.

He was first and foremost a

mathematician, helping to

found the areas of projective

geometry and probability theory.

As a physicist, Pascal was interested hydrostatics.

He discovered that pressure is transmitted

undiminished throughout a fluid.

The waters just fine, time for a swim

Soon after this discovery, Pascal built the first hydraulic

press. He found that by changing the area of two

surfaces, force can be multiplied.

P1 = F1/A1 P2 = F2/A2

F1/A1 = F2/A2

F1 = F2A1/A2 This works because

pressure in a fluid is

transmitted (passed on)

equally throughout it.

A hydraulic pump is filled with a fluid. A force of 50N

is applied to an area of 2m2. The area of the other

side of the pump is 10m2. What is the force exerted

on the second end?

F1 = 50N F1/A1 = F2/A2

A1 = 2m2 (50N)/(2m2) = F2/(10m2)

A2 =10m2 (50N)(10m2)/(2m2) = F2

F2 = ? F2 = 250N

Xzibit, Where art thou?

Hydraulics are found in many machines. Drills, pumps

and even aboard the Space Station.

And in this guy’s car…

http://www.metacafe.com/watch/1040017/hydraulic_suspension_car/

Getting to the bottom of it all

If you have ever swam to the bottom of the deep end of

your local swimming pool, you can feel a greater

pressure on your body. The deeper you swim, the

greater the pressure.

P = Fg/A

P = ρAhg/A = ρhg

The pressure at 5m below a pool is 49 kPa

Fg =ma , density,ρ=m/V, and V =

area x height, so Fg = ρVg

You sunk my Battleship!

The differences in pressure at one height versus another

can be described as Buoyancy. By comparing the

buoyant force on an object with its weight, we will

know if the object will sink or float.

FBuoyant = ρVg

Eureka! woot! 1337!

The Buoyant force is proportional to the volume of an

object. An equal volume of water is displaced by the

object, therefore the buoyant force , ρVg, is equal to

the weight of the displaced fluid. This is known as

Archimedes’ principle. Buoyant force does not depend

on the weight of the object, but on the weight of the

displaced fluid.

Don’t worry, you wont sink

So how do we figure out if something will float or sink?

We need to compare the Buoyant Force with the force

of gravity. (AKA the weight of the object)

If the buoyant force is greater, the object will float.

If the force of gravity is greater, the object will sink.

We all live in a yellow submarine…

A block of ice has a volume of 100 cm3 and a mass of 89g. The

weight of the ice cube is Fg= (.089kg)(9.8m/s2) = 0.88N.

We know for real world experience that ice floats, but does the

math support it?

Fg = 0.88N

FBuoyant = ρwaterVg

= (1x103 kg/m3)(1x10-4m3)(9.8m/s2)

= 0.98N

FBuoyancy = .98N

Fg = 0.88N

The density of the dead sea is 1.24x103 kg/m3. Mr. Gross has a volume of 0.08m3

FB= ρVg = (1.24x103)(.08m3)(9.8m/s2) = 972N

Fg = mg = (90kg)(9.8m/s2) = 882N

Not exactly buried treasure…

Homework:

Pg 303 Prac probs # 1 , 3

Pg 323 Review # 3,4

Pg 324 # 30, 31, 33

“What is man in Nature? A nothing in comparison with the infinite, and all compared with the nothing

– a mean between nothing and everything.”

- Blaise Pascal