Effect of Air Pressure • Each square inch exerts a force of 14.7 lbs
CRUSH • barrel 55 • classroom crush • TANKER
What is a Fluid? • FLUID: a substance, as a liquid or gas, that is capable of flowing and that changes its shape at a steady rate when acted upon by a force tending to change its shape • Water is a fluid and so is air. • All liquids and gases are fluids.
Water Pressure When a diver is underwater, gravity pulls on the water above him and the atmosphere presses down on the water. A diver is experiencing two types of pressure…. 1. water and 2. Atmospheric Styrofoam water pressure on body
Density Water is about 1000 times more dense than air. Water exerts a greater pressure than air.
Variation of pressure with depth in a liquid • Pressure increases with increasing depths • The increasing water pressure with depth limits how deep a submarine can go • dive suit crush depth 2200 ft
Liquid Pressure • The liquid in the first container is twice as deep, so the pressure on the bottom is twice that in the second container. • Two blocks exert twice as much pressure on the table.
WHAT WOULD HAPPEN IF A DIVERS PRESSURE IS NOT EQUALIZED? dumpster dive Dumpster dive 2 effect on fish how whales hold breath
Why does P increase with depth? the block on the bottom supports all the blocks above it this layer of fluid must support all the fluid above it
14.7 lbs/in3 h How much does P increase • At the surface of a body of water the pressure is 1 atm =14.7 lbs/in3 • As we go down into the water, pressure doubles, from 1 atm to 2 atm At 10 meters or about 33 feet stryofoam cups weight of water
Pressure increases with depth, so the speed of water leaking from the bottom hole is larger than that from the higher ones.
How much does 1 gallon of water weigh? • 8.3 pounds
19.1Liquid Pressure • The forces against a surface add up to a net force that is perpendicular to the surface. • Liquid escaping through a hole initially moves perpendicular to the surface.
Pressure depends only on depth Dam • The pressure at the bottom of the lake is higher than at the top • The dam must be thicker at its base • The pressure does not depend on how far back the lake extends
19.1Liquid Pressure The water pressure is greater at the bottom of the deeper lake. The dam holding back water twice as deep must withstand greater average water pressure, regardless of the total volume of water.
Blood Pressure • The blood pressure in your feet can be greater than the blood pressure in your head depending on whether a person is standing or reclining
Archimedes principle The buoyant force on an object in a fluid equals the weight of the fluid which it displaces. • this works for objects in water • helium balloons (density of He = 0.18 kg/m3) • hot air balloons the density of hot air is lower than the density of cool air so the weight of the cool air that is displaced is larger than the weight of the balloon
Floating objects heavier object lighter object
Floating in a cup of water Only a thin layer of water around the hull is needed for the ship to float!
2. NITROGEN NARCOSIS: Nitrogen narcosis is a condition that occurs in divers breathing compressed air. Below depths of approximately 100 ft, increase in the partial pressure of nitrogen produces an altered mental state similar to alcohol intoxication.
HOW DOES PRESSURE AFFECT A SCUBA DIVER? • Decompression Sickness or “ The Bend” caused by nitrogen creating bubbles ACCUMULATED IN body DURING DIVE Problem OCCURS during ascent MUST occur slowly called "Off-gassing"
Oil Tankers empty tanker full tanker
Clicker Question Consider holding a ping-pong ball and a golf ball of the same size under water. Then release them both. The ping-pong ball will rise to the surface whereas the golf-ball will fall. Why? The buoyant force is smaller on the golf ball than on the ping-pong ball. Both the buoyant force and the gravitational force are larger on the golf ball than on the ping-pong ball due to its greater mass. The buoyant force is the same on both balls but less than the gravitational force for the golf ball and larger than the gravitational force for the ping-pong ball. More than one above is true None of the above is true Answer: C
The density of the block of wood floating in water is Clicker Question 1. greater than the density of water.2. equal to the density of water.3. less than half that of water.4. more than half the density of water. 5. … not enough information is given.
Answer: 4 Principle of flotation: A floating object displaces a weight of fluid equal to its own weight. Weight = density x volume, so density-of-block = ¾ (density-of-water) A very-low-density object, like an inflated balloon, floats high on the water, and a denser object, like a piece of hardwood, floats lower into the water. An object half as dense as water floats halfway into the water (because it weighs as much as half its volume of water). Wood that floats 3/4 submerged, is 3/4 as dense as water—like the block in question—more than half the density of water. The density of the block compared to the density of water is the same as the fraction of the block below the water line.
Compared to an empty ship, will a ship loaded with a cargo of Styrofoam float lower in water or higher in water? 1. Lower in water 2. Higher in water
Answer: 1, Lower in water The ship loaded with Styrofoam will float lower in water. A ship will float highest when its weight is least—that is, when it is empty. Loading any cargo will increase its weight and make it float lower in the water. Whether the cargo is a ton of Styrofoam or a ton of iron, the water displacement will be the same.
Concept Check – Hydrostatic Pressure (2) While swimming near the bottom of a pool, you let out a small bubble of air. As the bubble rises toward the surface, what happens to its diameter? 1. bubble diameter decreases 2. bubble diameter stays the same 3. bubble diameter increases
Concept Check – Hydrostatic Pressure (2) While swimming near the bottom of a pool, you let out a small bubble of air. As the bubble rises toward the surface, what happens to its diameter? 1. bubble diameter decreases 2. bubble diameter stays the same 3. bubble diameter increases As the bubble rises, its depth decreases, so the water pressure surrounding the bubble also decreases. This allows the air in the bubble to expand (due to the decreased pressure outside) and so the bubble diameter will increase.
Concept Check – Hydrostatic Pressure (3) When you drink liquid through a straw, which of the items listed below is primarily responsible for this to work? 1. water pressure 2. gravity 3. inertia 4. atmospheric pressure 5. mass
Concept Check – Hydrostatic Pressure (3) When you drink liquid through a straw, which of the items listed below is primarily responsible for this to work? 1. water pressure 2. gravity 3. inertia 4. atmospheric pressure 5. mass When you suck on a straw, you expand your lungs, which reduces the air pressure inside your mouth to less than atmospheric pressure. Then the atmospheric pressure pushing on the liquid in the glass provides a net upward force on the liquid in the straw sufficient to push the liquid up the straw.
Eg. Hold a garden hose filled with water, and hold both ends at same height, water stays. Now if raise one end, water flows out lower end, even through an “uphill” path. Question: Which pot holds more tea? (They are identical except that the left one is taller) The both hold the same! The water cannot be no deeper than the spouts, which are at the same height. Relevant to the unnecessarily elaborate acqueducts the Romans made, very carefully ensuring water would flow downhill at all points – but actually water can flow upwards in between.
Ranking Task – Hydrostatic Pressure The figures below show side views of six hollow cylinders that have varying amounts of water in them. The widths of the cylinders and the heights to which they have been filled with water vary. The cylinders all have a hole cut in the side. All of the holes are the same size and they are all at the same height above the bases of the cylinders. There are corks in all of the holes.Rank these situations, from greatest to smallest, on the basis of the pressure on the cork by the water.
Ranking Task – Hydrostatic Pressure Greatest Smallest 1 ________ 2 ________ 3 ________ 4 ________ 5 ________ 6 ________ Or, the pressure is the same for all these cases. ________ Explain:
Ranking Task – Hydrostatic Pressure Greatest Smallest 1 ________ 2 ________ 3 ________ 4 ________ 5 ________ 6 ________ Or, the pressure is the same for all these cases. ________ Explain: Hydrostatic pressure is a function of density, depth, and g. In all cases, the density is the same, so the greatest pressure will be at the greatest depth. CF A BDE
Hydrometer • Instrument that measures density or specific gravity of fluids • Can you drown in quick sand?