Conceptual Physics: Chapter 13: Liquids
A 12-kg piece of metal displaces 2 L of water when submerged. Show that its density is 6000 kg/m3. How does this compare with the density of water?
(Since there are 1000 liters in 1 cubic meter, density may be expressed in units kg/m3). That's six times the density of water.
The sketch shows a wooden reservoir reinforced with metal hoops that supplies water to a farm. (a) Why is it elevated? (b) Why are the hoops closer together near the bottom part of the tank?
(a) The reservoir is elevated so as to produce suitable water pressure in the faucets that is serves. (b) The hoops are closer together at the bottom because water pressure is greater at the bottom. Closer to the top, the water pressure is not so great, so less reinforcement is needed there.
What is the mass of 1 L of water? Its weight in newtons?
1 L of water has a mass of 1 kg, and a weight of 10 N (more precisely, 9.8 N).
If the pressure in a hydraulic press is increased by an additional 10 N/cm2, how much extra load will the output piston support if its cross-sectional area is 50cm2?
500 N will be supported by the output piston (10 N/cm2 x 50 cm2 = 500 N).
A block of aluminum with a weight of 10 N is placed in a beaker of water filled to the brim. Water overflows. The same is done with another beaker with a 10-N block of lead. Does the lead displace more, less, or the same amount of water? (Why do your answers differ from the answers to the preceding two questions?)
A 10-N block of aluminum is larger than a 10-N block of lead. The aluminum therefore displaces more water. Only in Q 60 were the block volumes equal. In this and the preceding exercise, the aluminum block is larger.
Suppose that you balance a 5-kg ball on the tip of your finger, which has an area of 1cm2. Show that the pressure on your finger is 50 N/cm2, which is 500 kPa.
A 5-kg ball weighs 50 n, so the pressure is 50 N/cm2 = 500 kPa.
In answering the question of why bodies float higher in saltwater than in fresh water, your friend replies that the reason is that saltwater is denser than fresh water. Does your friend often recite only factual statements that relate to the answers and not provide any concrete reasons? How would you answer the same question?
A body floats higher in denser fluid because it does not have to sink as far to displace a weight of fluid equal to its own weight. A smaller volume of the displaced denser fluid is able to match the weight of the floating body.
A small, dry paper clip can rest on the surface of still water. Why can't a heavier paper clip do the same without sinking?
A heavier clip would push deeper into the water surface, overcoming the small force of surface tension whereupon it sinks.
Why is a high mountain composed mostly of lead an impossibility on Earth?
A mostly-lead mountain would be more dense than the mantle and would sink in it. Guess where most of iron in the world is. In Earth's center!
A block of aluminum with a mass of 1 kg is placed in a beaker of water filled to the brim. Water overflows. The same is done in another beaker with a 1-kg block of lead. Does the lead displace more, less, or the same amount of water?
A one-kilogram block of aluminum is larger than a one-kilogram block of lead. The aluminum therefore displaces more water.
If you release a ping pong ball beneath the surface of water, it will rise to the surface. Would it do the same if it were inside a big blob of water floating weightless in an orbiting spacecraft?
A ping-pong ball in water in a zero-g environment would experience no buoyant force. This is because buoyancy depends on a pressure difference on different sides of a submerged body. In this weightless state, no pressure difference would exist because no water pressure exists.
You know that a sharp knife cuts better than a dull knife. Do you know why that is so? Defend your answer.
A sharp knife cuts better than a dull knife because it has a thinner cutting area which results in more cutting pressure for a given force.
What geometric shape has the smallest surface area for a given volume?
A sphere has the least surface area for a given volume.
Distinguish between an immersed and a submerged body.
A submerged body is completely immersed, completely beneath the surface.
If you've ever wondered about the flushing of toilets on the upper floors of city skyscrapers, how do you suppose the pluming is designed so that there is not an enormous impact of sewage arriving at the basement level?
A typical plumbing design involves short sections of pipe bent at 45-degree angles between vertical sections two-stories long. The sewage therefore undergoes a succession of two-story falls which results in a moderate momentum upon reaching the basement level.
Which do you suppose exerts more pressure on the ground: a 5000-kg elephant or a 50-kg lady standing on spike heels? (Which will be more likely to make dents in a linoleum floor?) Approximate a rough calculation for each.
A woman with spike heels exerts considerably more pressure on the ground than an elephant! A 500-N lady with 1-cm2 spike heels puts half her weight on each foot, distributed (let's say) half on her heel and half on her sole. So the pressure exerted by each heel will be (125/N/1cm2) = 125 N/cm2. A 50,000-N elephant with 1000 cm2 feet exerting 1/4 its weight on each foot produces (12,500N/1000cm2) = 12/5N/cm2; about 10 times less pressure. So a woman with spike heels will make greater dents in a new linoleum floor than an elephant will.
Think about what happens to the volume of an air-filled balloon on top of water and beneath the water. Then rank the buoyant forces on a weighted balloon in water, from greatest to least, when it is a. barely floating with its top at the surface b. pushed 1 m beneath the surface c. pushed 2 m beneath the surface
A, B, C
Distinguish between adhesive and cohesive forces.
Adhesion is the attraction between unlike substances; cohesion is the attraction between like substances.
Place an egg in a pan of tap water. Then dissolve salt in the water until the egg floats. How does the density of an egg compare with that of tap water? With that of saltwater?
An egg is denser than fresh water, but less dense than salted water. Therefore an egg will float in salt water, but sink in freshwater.
We say that the shape of a liquid is that of its container. But, with no container and no gravity, what is the natural shape of a blob of water? Why?
Because of surface tension, which tends to minimize the surface of a blob of water, its shape without gravity and other distorting forces will be a sphere - the shape with the least surface area for a given volume.
How does water pressure 1 m beneath the surface of a lake compare with water pressure 1 m beneath the surface of a swimming pool?
Both are the same, for pressure depends on depth.
A block of aluminum with a volume of 10 cm3 is placed in a beaker of water filled to the brim. Water overflows. The same is done in another beaker with a 10-cm3 block of lead. Does the lead displace more, less, or the same amount of water?
Both blocks have the same volume and therefore displace the same amount of water.
How does the volume of a completely submerged object compare with the volume of water displaced?
Both volumes are the same.
What would you experience when swimming in water in an orbiting space habitate where the simulated gravity is g? Would you float in the water as you do on Earth?
Both you and the water would have the same weight density as on Earth, and you would float with the same proportion of your body above the water as on Earth.
Why does buoyant force act upward on an object submerged in water?
Buoyant force acts upward because there is more force beneath an object due to more pressure at greater depth.
Does the buoyant force on a submerged object depend on the volume of the object or on the weight of the object?
Buoyant force depends on the volume of the submerged object.
Is the buoyant force on a submerged object equal to the weight of the object itself or equal to the weight of the fluid displaced by the object?
Buoyant force equals the weight of fluid displaced.
How does the buoyant force on a submerged object compare with the weight of the water displaced?
Buoyant force equals the weight of the water displaced.
if liquid pressure were the same at all depths, would there be a buoyant force on an object submerged in the liquid? Explain.
Buoyant force is the result of differences in pressure; if there are no pressure differences, there is no buoyant force.
Will a swimmer gain or lose buoyant force as she swims deeper in the water? Or will her buoyant force remain the same at greater depths? Defend your answer, and contrast it with your answer to the previous question.
Buoyant force on a sinking swimmer will decrease as she sinks. This is because her body, unlike the rock in the previous exercise, will be compressed by the greater pressure of greater depths.
Will a rock gain or lose buoyant force as it sinks deeper in water? Or will the buoyant force remain the same at greater depths? Defend your answer.
Buoyant force will remain unchanged on the sinking rock because it displaces the same volume and weight of water at any depth.
Rank the pressures from greatest to least for the following: a. Bottom of a 20-cm-tall container of saltwater b. Bottom of a 20-cm-tall container of fresh water c. Bottom of a 5-cm-tall container of mercury
C, A, B
Rank the following from greatest to least for the percentage of its volume above the water line: a. Basketball floating in fresh water b. Basketball floating in saltwater c. Basketball floating in mercury
C, B, A
How is the density of a fish controlled? How is the density of a submarine controlled?
Density is controlled in a fish by expansion or contraction of an air sac; in a submarine by the weight of water blown in or out of ballast tanks.
If you punch a hole in a container filled with water, in what direction does the water initially flow outward from the container?
Direction of water flow is at right angles to the container surface.
Calculate the average force per nail when Sara, who weighs 120 pounds, lies on a bed of nails and is supported by 600 nails.
Force per nail is 120 pounds/600 nails = 0.2 pounds per nail, which is quite tolerable.
Why isn't there a horizontal buoyant force on a submerged object?
Forces on opposite sides are equal and opposite and cancel.
If you swim beneath the surface in saltwater, will the pressure be greater than in fresh water at the same depth?
Greater pressure in salt water due to its greater density.
Why is it inaccurate to say that heavy objects sink and light objects float? Give exaggerated examples to support your answer.
Heavy objects may or may not sink, depending on their densities (a heavy log floats while a small rock sinks, or an ocean liner floats while a paper clip sinks, for example). Dense objects sink.
How does the height to which water is lifted in a capillary tube relate to adhesion and the weight of the water lifted?
Height of water occurs when adhesive forces balance the weight of water lifted.
Your friend of mass 100 kg can just barely float in fresh water. Calculate her approximate volume.
Human density is about water's, 1000kg/m3. From density = m/V, V= m/density = (100 kg)/(1000 kg/m3) = 0.1 m3
The relative densities of water, ice and alcohol are 1.0, 0.9, and 0.8, respectively. Do ice cubes float higher or lower in a mixed alcoholic drink? Comment on ice cubes submerged at the bottom of a cocktail.
Ice cubes will float lower in a mixed drink because the mixture of alcohol and water is less dense than water. In a less dense liquid a greater volume of liquid must be displaced to equal the weight of the floating ice. In pure alcohol, the volume of alcohol equal to that of the ice cibes weighs less than the ice cubes, and buoyancy is less than weight and ice cubes will sink. Submerged ice cunes in a cocktail indicate that it contains a high percentage of alcohol.
What happens to the pressure in all parts of a confined fluid if the pressure in one part is increased?
If pressure in one part is increased, the same increase in pressure is transmitted to all parts.
A small aquarium half-filled with water is on a spring scale. Will the reading of the scale increase or remain the same if a fish is placed in the aquarium? (Will your answer be different if the aquarium is initially filled to the brim?)
If water doesn't overflow, the reading on the scale will increase by the ordinary weight of the fish. However, if the aquarium is brim filled so a volume of water equal to the volume of fish overflows, then the reading will not change. We correctly assume that the fish and water have the same density.
When you are bathing at a stony beach, why do the stones at the bottom hurt your feet less when you're standing in deep water?
In deep water, you are buoyed up by the water displaced, and as a result, you don't exert as much pressure against the stones on the bottom. When you are up to your neck in water, you hardly feel the bottom at all.
What is the relationship between liquid pressure and the depth of a liquid? Between liquid pressure and weight density?
Liquid pressure is proportional to depth, and to weight density.
Why will a block of iron float in mercury but sink in water?
Mercury is more dense (13.6 g/cm3) than iron. A block of iron will displace its weight and still be partially above the mercury surface. Hence it floats in mercury. In water it sinks because it cannot displace its weight.
if water faucets upstairs and downstairs are turned fully on, will more water per second flow out of the upstairs faucets or the downstairs faucets?
More water will flow from open faucets downstairs because of the greater pressure. Since pressure depends on depth, a downstairs faucet is effectively "deeper" than an upstairs faucet. The pressure downstairs is greater by an amount = weight density x depth, where the depth is the vertical distance between faucets.
The mountains of the Himalayas are slightly less dense than the mantle material upon which they "float". Do you suppose that, like floating icebergs, they are deeper than they are high?
Mountain ranges are very similar to icebergs. Both float in a denser medium and extend farther down into that medium than they extend above it. Mountains, like icebergs, are bigger than they appear to be. The concept of floating mountains is Isostacy - Archimedes' principle for rocks.
When you are standing, blood pressure in your legs is greater than in your upper body. Would this be true for an astronaut in orbit? Defend your answer.
No, in orbit where support is absent there are no pressure differences due to gravity.
The weight of the human brain is about 15 N. The buoyant force supplied by fluid around the brain is about 14.5 N. Does this mean that the weight of fluid surrounding the brain is at least 14.5 N? Defend your answer.
No, there does not have to actually be 14.5 N of fluid in the skull to supply a buoyant force of 14.5 N on the brain. To say that the buoyant force is 14.5 N is to say that the brain is taking up the space that 14.5 N of fluid would occupy if fluid instead of the brain were there. The amount of fluid in excess of the fluid that immediately surrounds the brain does not contribute to the buoyancy on the brain.
It was emphasized earlier that the buoyant force does not equal an object's weight but does equal the weight of displaced water. Now we say buoyant force equals the object's weight. Isn't this a grand contradiction? Explain.
Not a coincidence because in the case of floating the buoyant force equals both weight of the object as well as the weight of water displaced.
So you're having a run of bad luck, and you slip quietly into a small, calm pool as hungry crocodiles lurking at the bottom are relying on Pascal's principle to help them to detect a tender morsel. What does Pascal's principle have to do with their delight at your arrival?
Part of whatever pressure you add to the water is transmitted to the hungry crocodiles, via Pascal's principle. If the water were confined, that is, not open to the atmosphere, the crocs would receive every bit of pressure you exert. But even if you were able to slip into the pool to quietly to float without giving pressure via swimming strokes, your displacement of water raises the water level in the pool, is an ever-so-welcome signal to the hungry crocodiles.
Calculate the pressure a 10-N block exerts on the table it rests on if its area of contact is 50 cm2.
Pressure = 10 N/50 cm2 = 0.2 N/cm2
A 1-m tall barrel is closed on top except for a thin pipe extending 5 m up from the top. When the barrel is filled with water up to the base of the pipe (1 meter deep), the water pressure on on the bottom of the barrel is 10 kPa. What is the pressure on the bottom when water is added to fill the pipe to its top?
Pressure = weight density x depth = 10,000 N/m3 x (5+1)m = 10,000 N/m3 x 6 m = 60,000 N/m2 = 60 kPa.
A 1-m-tall barrel is filled with water (with a weight density of 10,000 N/m3). Show that the water pressure on the bottom of the barrel is 10,000 N/m2, or equivalently, 10 kPa.
Pressure = weight density x depth = 10,000 N/m3 x 1m = 10,000 N/m2 = 10 kPa
The top floor of a building is 20 m above the basement. Show that the water pressure in the basement is nearly 200 kPa greater than the water pressure on the top floor.
Pressure = weight density x depth = 10,000 N/m3 x 20 m = 200,000 N/m2 = 200 kPa
The depth of water behind the Hoover Dam is 220m. Show that the water pressure at the base of this dam is 2200 kPa.
Pressure = weight density x depth = 10,000 N/m3 x 220 m = 2,200,000 N/m2 = 2,200 kPa
Show that the water pressure at the bottom of the 50-m-high water tower in the chapter-opening photo is 500,000 N/m2, which is approximately 500 kPa.
Pressure = weight density x depth = 10,000 N/m3 x 50 m = 500,000 N/m2 = 500,000 Pa = 500 kPa
How does pressure at the bottom of a body of water relate to the wright of water above each square meter of the bottom surface?
Pressure is due to the wright of water above (and total pressure, plus the weight of the atmosphere).
How does pressure relate to force?
Pressure is force per area.
Which is more likely to hurt; being stepped on by a 200-lb man wearing loafers or being stepped on by a 100-lb woman wearing high heels?
Pressure would be appreciably greater by the woman because of the relatively small area of contact at the heal, which would hurt you more.
How does the water pressure 1 m below the surface of a small pond compare with the water pressure 1 m below the surface of a huge lake?
Pressures will be the same at the same depth.
Why is it easier to float in saltwater than in fresh water?
Saltwater is denser than freshwater, which means you don't "sink" as far when displacing your weight. You'd float even higher in mercury and sink completely in alcohol.
Soap greatly weakens the cohesive forces between water molecules. You can see this by adding some oil to a bottle of water and shaking it so that the oil and water mix. Notice that the oil and water quickly separate as soon as you stop shaking the bottle. Now add some liquid soap to the mixture. Shake the bottle again and you will see that the soap makes a fine film around each little oil bead and that a longer time is required for the oil to coalesce after you stop shaking the bottle. Soap breaks the surface tension around each particle of dirt so that the water can reach the particles and surround them.
Share this info about why soap cleans well with your friends.
Fill in the blanks: An object denser than water will _______ in water. An object less dense than water will _______ in water. An object that has the same density as water will ________ in water.
Sink; float; neither sink nor float.
A chunk of steel will sink in water. But a steel razor blade, carefully placed on the surface of water, will not sink. what is your explanation?
Surface tension accounts for the "floating" of the razor blade. The weight of the blade is less than the restoring forces of the water surface that tends to resist stretching.
What is the cause of surface tension?
Surface tension is caused by molecular attractions.
A ship sailing from the ocean into a freshwater harbor sinks slightly deeper into the water. Does the buoyant force on the ship change? If so, does it increase or decrease?
The buoyant force does not change. The buoyant force on a floating object is always equal to that object's weight no matter what the fluid.
Why does an inflated beach ball pushed beneath the surface of water swiftly shoot above the water surface when released?
The buoyant force on the ball beneath the surface is much greater than the force of gravity on the ball, producing a large net upward force and large acceleration.
A can of diet soda floats in water, whereas a can of regular soda sinks. Explain this phenomenon first in terms of density and then in terms of weight versus buoyant force.
The can of diet drink is less dense than water, wehreas the can of regular drink is denser than water. Water with disolved sugar is denser than pure water. Also, the weight of the can of diet drink is less than the buoyant force that would act on it if totally submerged. So it floats, where buoyant force equals the weight of the can.
The photo shows physics instructor Marshall Ellenstein walking barefoot on broken glass bottles in his class. Discuss the physics concept that Marshall is demonstrating. Whys is he careful to be sure that the broken pieces are small and numerous?
The concept of pressure is being demonstrated. Marshall is careful that the pieces are small and numerous so that his weight is applied over a large area of contact. Then the sharp glass provides insufficient pressure to cut the feet.
A merchant in Katmandu sells you a solid gold 1-kg statue for a very reasonable price. When you get home, you wonder whether or not you got a bargain, so you lower the statue into a container of water and measure the volume of displaced water. Show that, for pure gold, the volume of water displaced will be 51.8 cm2.
The density of gold is 19.3 g/cm3. Your gold has a mass of 1000 grams, so 1000g/V = 19.2 g/cm3. Solving for V, 1000g/19.3 g/cm3 = 51.8 cm3
How much force is needed to hold a nearly weightless but rigid 1-L carton beneath the surface of water?
The force needed will be the weight of 1 L of water, which is 9.8 N. If the weight of the carton is not negligible, then the force needed would be 9.8 N minus the carton's weight, for then the carton would be helping to push itself down.
Consider both a 50-ton boat and a 100-ton boat floating side by side in the gondola of the Falkirk Wheel, while the opposite gondola carries no boats at all. Why do the gondolas nevertheless weight the same?
The gondolas weigh the same because the floating boats have displaced a weight of water equal to their own weights, equaling the weight of the brim filled gondola with no boat.
One gondola in the Falkirk Wheel carries a 50-ton boat, while the other carries a 100-ton boat. Why do the gondolas nevertheless weigh the same?
The gondolas weigh the same because they're brim full, and whatever the weight of a floating boat, the same weight of water was displaced when the boat entered the gondola.
In the hydraulic arrangement shown in figure 13.22, the multiplication of force is equal to the ratio of the areas of the large and small pistons. Some people are surprised to learn that the area of the liquid surface in the reservoir of the arrangement shown is immaterial. What is your explanation to resolve this confusion?
The increased pressure in the reservoir is a result of the applied force distributed over the input piston area. This increase in pressure is transmitted to the output piston. In figure 13.22 however, the pressure increased is supplied by the mechanical pump, which has nothing to do with the area of fluid interface between the compressed air and the liquid. Many hydraulic devices have a single piston upon which pressure is exerted.
Sprinkle some black pepper on the surface of some pure water in a saucer. The pepper floats. Add a drop of liquid dish soap to the surface and the pepper grains repel from the soap droplet. Stir gently once or twice and watch the pepper sink.
The pepper grains float due to surface tension. When the surface tension is diminished by the addition of soap, the grains sink.
In the hydraulic pistons shown in the sketch, the small piston has a diameter of 2 cm. The large piston has a diameter of 6 cm. How much more force can the larger piston exert compared with the force applied to the smaller piston?
The relative areas are as the squares of the diameters; 6^2/2^2 = 36/4 = 9. The large piston can lift 9 times the input force to the smaller piston.
In 1960, the U.S. Navy's bathyscaphe Trieste (a submersible) descended to a depth of nearly 11 km in the Marianas Trench near the Philippines in the Pacific Ocean. Instead of a large viewing window, it had a small circular window 15 cm in diameter. What is your explanation for so small a window?
The smaller the window area, the smaller the crushing force of water on it.
In the hydraulic arrangement shown, the larger piston has an area that is 50 times that of the smaller piston. The strong man hopes to exert enough force on the large piston to raise the 10 kg that rest on the small piston. Do you think he will be successful? Defend your answer.
The strong man will be unsuccessful. He will have to push with 50 times the weight of the 10 kilograms. The hydraulic arrangement is arranged to his disadvantage. Ordinarily, the input force is applied against the smaller piston and the output force is exerted by the large piston - this arrangement is just the opposite.
When the wooden block is placed in the beaker, what happens to the scale reading? Answer the same question for an iron block.
The total weight on the scale is the same either way, so the scale reading will be the same whether or not the wooden block is outside or floating in the beaker. Likewise for an iron block, where the scale reading shows the total weight of the system.
Discuss which teapot holds more liquid, and why?
The water can be no deeper than the spouts, which are at the same height, so both teapots hold the same amount of liquid.
Float a water-soaked Ping-Pong ball in a can of water held more than a meter above a rigid floor. Then drop the can. Careful inspection will show that the ball was pulled beneath the surface as both the ball and the can drop. What does that say about surface tension? More dramatically, when the can makes impact with the floor, what happens to the ball, and why?
The wetted ball is pulled by surface tension beneath the surface when the system is weightless (dropping). When the can makes impact the submerged ball, much lighter than the water it displaces, is popped with great force out of the water/
Why are persons who are confined to bed less likely to develop bedsores on their bodies if they rest on a waterbed rather than an ordinary mattress?
There is less pressure with a waterbed due to the greater contact area.
Why do the gondolas of the Falkirk Wheel have the same weight whether or not they carry boats?
They have the same weight because when carrying a boat, the weight of the boat is the same as the weight of water that overflows.
If a 1-L container is immersed halfway into water, what is the volume of the water displaced? What is the buoyant force on the container?
Volume of displaced water will be 1/2 L. Buoyant force will be 5 N.
what common liquid covers more than two-thirds of our planet, makes up 60% of our bodies, and sustains our lives and lifestyles in countless ways?
Water covers most of Earth and is essential to human life.
Why does water "seek its own level"?
Water seeking its own level is a consequences of pressure depending on depth. In a bent U-tube full of water, for example, the water in one side of the tube tends to push water up the other side until the pressures at the same depth in each tube are equal. if the water levels were not the same, there would be more pressure at a given level in the fuller tube, which would move the water until the levels were equal
What is the condition in which the buoyant force on an object does equal the weight of the object?
When an object floats, buoyant force equals its weight.
Why will a volleyball held beneath the surface of water have more buoyant force than if it is floating?
When the ball is held beneath the surface, it displaces a greater weight of water.
If you punch a couple of holes in the bottom of a water-filled container, water will spurt out because of water pressure. Now drop the container, and as it freely falls, not that the water no longer spurts out! If your friends don't understand this, could you figure it out and explain it to them?
When the can is held still, pressure due to the weight of water in the can accounts for the spurt. But when dropped, the weight of water and the water pressure are nil, and no spurt occurs.
When an ice cube in a glass of water melts, does the water level in the glass rise, fall, or remain unchanged? Does your answer change if the ice cube has many air bubbles in it? How about if the ice cube contains many grains of heavy sand?
When the ice cube melts the water level at the side of the glass is unchanged (neglecting temperature effects). To see this, suppose the ice cube is a 5-gram ice cube; then while floating it will displace 5 grams of water. But when melted, it becomes the same 5 grams of water. Hense the water level is unchanged. The same occurs when the ice cube that contains air bubbles melts. Hence the water level is unchanged. The same occurs when the ice cube that contains air bubbles melts. Whether the ice cube is hollow or solid, it displaces as much water floating as when it melted. If the ice cube contains grains of heavy sand, however, upon melting, the water level at the edge of the glass will drop.
Why will hot water flow more readily than cold water through small leaks in a car radiator?
When water is hot, the molecules are moving more rapidly and do not cling to one another as well as when they are slower moving, so the surface tension is less. The lesser surface tension of hot water allows it to pass more readily through small openings.
Why is the buoyant force on a submerged submarine appreciably greater than the buoyant force on it while it is floating?
While floating, BF equals the weight of the submarine. When submerged, BF equals the submarine's weight plus the weight of water taken into its ballast tanks. Looked at another way, the submerged submarine displaces a greater weight of water than the same submarine floating.
A dike in Holland springs a leak through a hole of area 1 cm2 at a depth of 2 m below the water surface. How much force must a boy apply to the hole with his thumb to stop the leak? Could he do it?
Yes. First find the pressure. It is weight density x depth = (10,000 N/m3) (2 m) = 20,000 N/m2, or 20,000 Pa. Force is pressure x area, and 1 cm2 = 10^-4 m2) = 2N. It would be easy for the boy to exert this force. It is about the weight of a notebook or small box of cereal. (Note: Air pressure is not figured into this calculation because its effect in pushing down on the water from above is cancelled by its effect in pushing from outside the hole against the leaking water).
The density of a rock doesn't change when it is submerged in water, but your density changes when you are submerged. Explain.
You are compressible, whereas a rock is not, so when you are submerged, the water pressure tends to squeeze in on you and reduce your volume. This increases your density.
Why does your body get more rest when you're lying down than when you're sitting? Is blood pressure in your legs greater?
Your body gets more rest when lying than when sitting or standing because when lying, the heart does not have to pump blood to the heights that correspond to standing or sitting.
Why is blood pressure measured in the upper arm, at the elevation of your heart?
Your upper arm is at the same level as your heart, so the blood pressure in your upper arms will be the same as the blood pressure in your heart.
In lab you find that a 1-kg rock suspended above water weighs 10 N . When the rock is suspended beneath the surface of the water, the scale reads 8 N . a. What is the buoyant force on the rock? b. If the container of water weighs 10 N on the weighing scale, what is the scale reading when the rock is suspended beneath the surface of the water? c. What is the scale reading when the rock is released and rests at the bottom of the container?
a. BF is 10 N - 8 N = 2 N b. The gain in scale reading is 2 N; total weight = 12 N. c. Weight of the rock is 10 N, so total weight is 20 N.
