Forces in Fluids

Pressure

Pressure Pressure = Force in N F(N) Area in m2 A N/m2 is called a Pascal (Pa) 1000 Pa = 1 kPa P (N/m2) A (m2)

Pressure What 2 things influence pressure? If force is increased, pressure increases If area is increased, pressure decreases When do you exert more pressure on the floor - when standing on one foot or two feet?

Our Atmosphere Think about it: You live near the bottom of an “ocean” of air We don’t talk about depth of air like depth of water Altitude – height above sea level As altitude increases what happens to air pressure? Air pressure decreases, because the column of air above your head weighs less. As you increase your altitude, your ears “pop” to equalize pressure.

Air Pressure Air is a fluid (mixture of gases) so it exerts a pressure on objects At sea level, 1 atmosphere is 14.7 psi or 101,300 Pa Why don’t we feel this? Pressure inside our bodies balances the pressure outside Therefore, the net force is zero and we don’t get crushed What happens when the pressures are not equal? http://www.youtube.com/watch?v=Uy-SN5j1ogk http://www.youtube.com/watch?v=E_hci9vrvfw

Fluid Pressure Fluid – substance that does not have a fixed shape. Liquids and gases are fluids. Fluids exert pressure on the substances within them.

Fluid Pressure Fluid pressure is often measured using units: Atmospheres (atm) Millimeters of mercury (mm Hg) Pounds per square inch (psi)

Going Deeper In Water Have you ever dove deep into a pool, lake, or ocean? How does it make your ears feel? Gases can be compressed, or squeezed into a smaller space. http://www.youtube.com/watch?v=FkhBPF4yfkI&feature=related

How does depth affect pressure? The greater the depth within a fluid the greater the pressure. The deeper you go, the greater the weight of the column of fluid above you and therefore a greater force over the same size area. The pressure in a fluid at any given depth is constant and exerted equally in all directions. The 2 things that determine pressure exerted by a fluid that is not moving are DEPTH and TYPE OF FLUID.

Check your understanding Where is the pressure greater: 3 m deep in the ocean or 8 m deep in the ocean? Where is the pressure greater: 25 cm deep in water in a bathtub or 25 cm deep in water in a lake? Where is pressure greater: 3 feet deep in water or 3 feet deep in air?

Rising to the Surface – The “Bends” Under high pressure, higher concentration of gases dissolved in blood. As you rise to the surface, pressure decreases If ascent is made too rapidly, dissolved gases form bubbles that can migrate to various parts of body, especially noted in the joints. This causes Decompression Sickness, or the “Bends” - pain and various other ailments