PRESSURE What is pressure Pressure is the force

PRESSURE

What is pressure? • Pressure is the force per unit area that is applied on the surface of an object • Pressure occurs at the points of contact between the two objects • The amount of pressure depends upon the strength of the force and on the size of the surface area over which the force acts

Area and Pressure • Area and pressure have an inverse relationship (indirect relationship) • This means these 2 variables change opposite to each other (one goes up, the other goes down, and vice versa)!

Small Area, Large Pressure area pressure EXAMPLE: nail tip (small contact area, so pressure is large enough to push apart the wood fibers)

Small Area, Large Pressure area pressure EXAMPLE #2: axe splits wood (small contact area, so pressure is large enough to push apart the wood fibers)

Small Area, Large Pressure area pressure EXAMPLE #3: break board with hand (small contact area, so pressure is large enough to break boards)

…and vice-versa

Large Area, Small Pressure area pressure EXAMPLE 1: bed of nails (larger area in contact with nail tips means pressure from any one nail is LOW)

Large Area, Small Pressure area pressure EXAMPLE 2: snow shoes (larger area prevents person from falling too deeply in the snow)

Force and Pressure • Force and pressure have a direct relationship • This means these 2 variables change together (one goes up, the other goes up, and vice versa)!

Large Force, Large Pressure EXAMPLE 1: blood pressure cuff force pressure

Large Force, Large Pressure EXAMPLE 2: squeezing water bottle with hole force pressure

Large Force, Large Pressure force pressure EXAMPLE 3: Hydraulic System; Syringes

…and vice-versa

Small Force, Small Pressure force pressure EXAMPLE 1: stop squeezing water bottle and stream slows down

Small Force, Small Pressure force pressure EXAMPLE 2: Accordian player stops compressing instrument and the music stops

Small Force, Small Pressure force pressure EXAMPLE 3: Nurse stops applying force to pump and pressure reading goes down

Calculating Pressure SI Unit for pressure is the pascal (Pa) which is equal to 1 N per 1 m 2

Practice Calculations Take out a calculator!

Example 1 P=F/A A water glass sitting on a table weighs 4 N. The bottom of the water glass has a surface of 0. 003 m 2. Calculate the pressure the water glass exerts on the table. P= ? f= 4 N P = 4 / 0. 003 A = 0. 003 m 2 P = 1, 333 Pa

Example 2 P=F/A A box that weighs 250 N is at rest on the floor. If the pressure exerted by the box on the floor is 25, 000 Pa, over what area is the box in contact with the floor? P = 25, 000 Pa 25, 000 = 250 / A f = 250 N A= ? 25, 000 A= 250 A = 250 / 25, 000 A = 0. 01 m 2

Fluids A fluid is any substance that has no definite shape and has the ability to flow. Both liquids AND gases are fluids (ex. Air can flow and has no definite shape)

Pressure in a Fluid What force is pulling down on this glass? How did the force change when the glass was filled with water? How did the pressure on the table change?

Pressure in a Fluid The water is exerting pressure on what two things? How would you calculate the pressure from the water glass onto the table? What happens to the force and pressure when more water is added?

Pressure and Fluid Height Where is the pressure from the water the greatest? The greater the height of fluid above an object, the greater the pressure on that object.

Pressure Increases with Depth When you swim underwater, what do you notice about how your ears feel? depth pressure As height of fluid above you increases, the weight of fluid above you also increases. Thus, the pressure increases with depth.

Pressure in ALL Directions * Pressure is on all sides of an object no matter what its shape. * Pressure is perpendicular (90⁰ angle) to the surface of the object.

Pascal’s Principle * Pressure applied to an enclosed fluid is transmitted undiminished to every point in the fluid and to the walls of the container * In other words, pressure is constant throughout the container

Atmospheric Pressure * The large thickness of the atmosphere exerts pressure on ALL objects at Earth’s surface. * Your body is filled with fluids such as blood that also exert pressure. * These inward and outward pressures balance each other

Atmospheric Pressure altitude air above person * As you go higher in the atmosphere, atmospheric pressure decreases because the amount of air above you also decreases. pressure

Underwater Pressure depth water & air above person * Water pressure increases as you travel downward * Water pressure is greatest at the ocean floor pressure

Barometer A barometer is a tool that measures atmospheric pressure How it works: the height of the liquid in the tube increases as the atmospheric pressure increases.

WRITE ANSWERS ONLY FOR THE FOLLOWING QUESTIONS Title: “Pressure Exit Slip” …and include a complete and proper header

Question 1 Pressure in a liquid _______ with increased height above sea level. A. decreases B. increases C. remains the same D. vanishes

Question 2 Is air a fluid? Why or why not?

Question 3 Describe the relationship between pressure and area and provide a specific example.