PRESSURE

Presented by:

Catherine G.TumaliuanNovaliches High SchoolQuezon City,Philippines

April 12,2010

Objectives

• Define the term pressure in terms of force and area• Apply the relationship of pressure

due to a liquid, P = hpg, to solve related problems• Cite at least two applications of

transmission pressure

Recall

• Force – a push or a pull• In Newton’s Second Law states that force, F is

equal to mass, m of a moving object times acceleration, a

• In equation: F = ma where: F – force m – mass a - acceleration due to gravity 10 m/s-2

http://www.google.com.sg/search?sourceid=navclient&ie=UTF-8&rlz=1T4RNTN_enSG377SG377&q=pressure-ppt

Weight is a Force

• Weight is an ever-present force exerted on every object due to gravity

• Therefore: W =mg (since weight (W) is a force due to gravity

Pressure - is defined as the force acting per unit areaPressure = force/area

In symbols, P = F / A where: p = pressuref = force (in Newton)a = area (in m2)

• The SI unit of pressure is Newton per square meter (N/m2) or Pascal (Pa)

How is pressure related to force?

• Force acting over a smaller area exerts more pressure

• Example: using a high heel shoes- force pushing

downward is due to her weight. The heel sink easily because her weight is acting on a smaller area

Sample Problem

• Calculate the pressure under a girl’s feet if her mass is 50 kg and the area of her shoes in contact with the ground is 2.00 cm2 (high heels ) and 200 cm 2 ( flat shoes ).

(Take gravitational field strength g = 10 N/kg)

Solution• Given: mass = 50 kg Weight of girl = mg = 50 kg x 10 N/kg = 500 NArea of high heels=2.00cm2 = 2.00x10-4 m2

A

FP

Paxmx

N 622

1050.21000.2

500

Area of flat soles = 200 cm 2 = 200x10-4 m2

Weight of girl = 500 N

A

FP

Pax

N410200

500

Pax 41050.2

Pressure in liquids• Pressure due to liquid column = height of

column x density of the liquid x gravitational field of strength or P = hpg

http://images.google.com.sg/imglanding?q=pressure%20in%20liquids&imgurl

P = hpgwhere:

h = height of liquid column (m) p=density of liquid ( kg m-3 )

g = acceleration due to gravity( m/s-2 )

Sample problem• A rectangular block with base area ( A )of

25 cm2 is submerge in water of density 1000 kg m-3 as shown in figure. .05 m

.07 m P1

P2

GCE’O’ Level. Physics Matters. Dr C. Chew et.al .pages 127-128

• Find the a. pressure p1 due to the water acting on the top surface of the block Solution:

Pressure exerted by water on top surface P = hpg = .0 5 m x 1000x kg m-3 x 10 N/kg = 5.0x10 2 Pa

b. pressure p2 due to the water acting on the bottom surface of the block (Take gravitational field strength g = 10 N/ kg)

Solution: P = hpg = .07 m x 1000 kg m-3 x 10 N/kg = 7.0 x 10 2 Pa

. Calculate the

a. force exerted by the water on the top surface of the block

given base area A = 25 cm2 =2.5x10-3 m2 Solution: P = F/A rearranging: F = p A F = p A = 5.0x102 x 2.5x10-3

= 1.25 N

b. force exerted by the water on the bottom surface of the block given base area A = 25 cm2 =2.5x10-3 m2 P = F/A rearranging: F = p A F = p A = 5.0x102 x 2.5x10-3

= 1.25 N

Transmission of Pressure in Liquids

• Hydraulic system make use of the transmission of pressure in liquids to do useful work , e.g. the

hydraulic press

http://imagaes.google.sg/images?um=1&hl=en&tbs=1sch:l&=hydraulic+jack&sa=N&satart=18&ndsp=18

The hydraulic brake system

http://images.google.com.sg/images?um=1&hl=en&rlz=1R2RNTN_enSG377&tbs=isch:1&q=hydraulic+brake&sa=N&start=36&ndsp=18

When the driver steps on the brake pedal, the pressure on the brake pedal is transmitted to the large

pistons on each side of a large disc on the wheel axle. This pressure causes the pistons to come into

contact with the disc to slow down the car by friction. The exact same

pressure is also applied to the other brakes in the car

• PASCAL’S PRINCIPLE –pressure applied to an enclosed fluid is transmitted unchanged to every part of the fluid, as well as the walls of the container

http://.google.com.sg/images?hl=en&q=pascal%20principle&um=1&ie=UTF-8&source=og&sa=N&tab=wi

Some daily applications of transmission of pressure

• Hydraulic brakes • Hydraulic car jack or Hydraulic garage lift

http:/www.google.com.sg/imglanding?q=hydraulic%20brake%system&imgurl

Evaluation

• Calculate the pressure produced by a force of 800 N acting on an area of 2.0 m2

answer

• Given : Force = 800 N Solution: Area = 2.0 m2 P = P = ?

= N

m2

= 400 Pa

A

F

0.2

800

• A glass is filled to the brim with water and has an ice cube floating in it. When the ice cube melts, what happens ? a. water spills out of the glass b. the water level in the glass drops c. the water level in the glass does not change

Assignment 1 .Are both direction and

magnitude needed to specify a pressure ?

2.Why is the sharp edge of a knife used for cutting made very thin?