Conceptual Physics Ch 13

surface tension

Tendency of the surface of a liquid to contract in area and thus behave like a stretched elastic membrane.

When a fish expands its air bladder, the density of the fish A) decreases. B) increases. C) remains the same. D) None of the above.

A) decreases.

When a submerged submarine expels water from its ballast tanks, its density A) decreases. B) increases. C) remains the same. D) None of the above.

A) decreases.

If a fish makes itself denser, it will sink; if it makes itself less dense, it will rise. In terms of buoyant force, why is this so?

When the fish decreases its volume, it displaces less water, so the buoyant force decreases. When the fish expands its volume, the buoyant force increases.

Suppose that when standing on a bathroom scale you raise one foot. Does the pressure you exert on the scale change? Is there a difference in the scale reading?

When you shift your weight by standing on one foot, pressure on the scale doubles. But the scale doesn't measure pressure- it measures weight. Except for some jiggling as you shift your weight the scale reading stays the same.

flotation

principle of flotation - A floating object displaces a weight of fluid equal to its own weight.

Weight density

weight / volume

total pressure of a liquid

weight density x depth + the pressure of the atmosphere

An object with a particular height is completely submerged in water. As the height of the object increases, the net buoyant force A) increases. B) remains the same. C) decreases.

A) increases.

Pascal's principle

Changes in pressure at any point in an enclosed fluid at rest are transmitted undiminished to all points in the fluid and act in all directions.

A river barge loaded with gravel approaches a low bridge that it cannot quite pass under. Should gravel be removed from or added to the barge?

? gravel should be added ?

The mass of a cubic meter of water is A) 1000 kg. B) 1 kg C) 9800 N. D) 10 kg. E) 100 kg.

A) 1000 kg.

Water pressure is greatest against the A) bottom of a submerged object. B) is the same against all surfaces C) sides of a submerged object. D) top of a submerged object.

A) bottom of a submerged object.

The net force due to water pressure, exerted on a flat surface of an object submerged in the water, A) is always directed perpendicular to the surface. B) Can be directed in a direction that is neither perpendicular nor parallel to the surface. C) is always directed parallel to the surface.

A) is always directed perpendicular to the surface.

The buoyant force on a submerged object A) is equal to the weight of the fluid displaced by the object. B) depends on the depth of the submerged object. C) is equal to the weight of the object.

A) is equal to the weight of the fluid displaced by the object.

Water pressure provided by a water tower is greater if the tower A) is taller. B) holds more water. C) Both A and B. D) None of the above.

A) is taller. Only depth, not amount of water, contributes to pressure.

Compared to an empty ship, the same ship loaded with Styrofoam will float A) lower in the water. B) higher in the water. C) at the same level in the water.

A) lower in the water.

All ice floats. This tells you that A) the solid form of water is less dense than the liquid form. B) ice must always be filled with air bubbles. C) When ice melts in a glass, the water level of the glass must change. D) ice expands as it sinks into water. E) ice is not really colder than water; it just feels colder.

A) the solid form of water is less dense than the liquid form.

A completely submerged object always displaces its own A) volume of fluid. B) weight of fluid C) density of fluid. D) all of these E) none of these

A) volume of fluid.

Pa

Abbreviation for the SI unit pascal.

buoyancy

Apparent loss of weight of an object immersed or submerged in a fluid.

On which of these blocks submerged in water is the buoyant force greatest? A) 1 kg of lead B) 1 kg of aluminum C) 1 kg of uranium D) All the same.

B) 1 kg of aluminum

What is the approximate mass of a 1-square-centimeter column of air that extends from sea level to the top of the atmosphere? A) 10 kilograms B) 1 kilogram C) 1 gram D) 100 kilograms

B) 1 kilogram

On a boat ride, the skipper gives you a life preserver filled with lead pellets. When he sees the skeptical look on your face, he says that you'll experience a greater buoyant force if you fall overboard than your friends who wear Styrofoam-filled preservers. A) He apparently doesn't know his physics. B) He is correct.

B) He is correct. but what he doesn't tell you is you'll drown! Your life preserver will submerge and displace more water than those of your friends who float at the surface. Although the buoyant force on you will be greater, the net force downward is greater still!

What is required such that an object can float on water? A) The density of the material that the object is made out of is less than the density of water. B) The weight of the water displaced by the object, when completely submerged, is greater than the weight of the object.

B) The weight of the water displaced by the object, when completely submerged, is greater than the weight of the object.

How many forces act on a submerged body at rest in a fluid? A) One—buoyancy B) Two—buoyancy and the force due to gravity. C) None—in accord with the equilibrium rule. D) None of the above.

B) Two—buoyancy and the force due to gravity.

As you sit in class, is there a buoyant force acting on you? A) No, as evidenced by an absence of lift B) Yes, due to displacement of air

B) Yes, due to displacement of air

A cook who measures a specific amount of butter by placing it in a measuring cup with water in it is using the A) principle of buoyancy. B) displacement rule. C) concept of density. D) All of the above.

B) displacement rule.

An object with a particular height is completely submerged in water. As the object is lowered within the water (away from the surface), the net buoyant force A) decreases. B) does not change. C) increases.

B) does not change.

When a fish decreases the size of its air bladder, the density of the fish A) decreases. B) increases. C) remains the same. D) None of the above.

B) increases.

When a fish makes itself less dense, the buoyant force on it A) decreases. B) increases. C) remains the same. D) None of the above.

B) increases.

When a submarine takes water into its ballast tanks, its density A) decreases. B) increases. C) remains the same. D) None of the above.

B) increases.

About what percentage of the molecules that make up the atmosphere are below an aircraft that flies at an altitude of 6 kilometers? A) 40% B) more than 50% C) 30% D) 50% E) 20%

B) more than 50%

When you stand on one foot instead of two, the force you exert on the floor is A) less. B) the same. C) more. D) None of the above.

B) the same.

As a boulder thrown into a deep lake sinks deeper and deeper into the water. Does the buoyant force upon it increase or decrease?

Buoyant force remains unchanged as the boulder sinks. because the boulder displaces the same volume and same weight of water at any depth.

If a block of oak with 75% the density of water is placed in a bucket of water, what percentage of the wood will be above water? A) 75% B) 100%, because it will float C) 25% D) it is impossible to tell without knowing the density of water. E) 0% because oak is heavy enough to sink.

C) 25%

The reason a person finds it easier to float in saltwater compared with freshwater is that in saltwater A) the buoyant force is greater. B) a person feels less heavy. C) Neither of these. D) None of the above.

C) Neither of these. A person floats higher because a smaller volume of the denser saltwater is displaced.

Two solid blocks of identical size are submerged in water. One block is lead and the other is aluminum. Upon which is the buoyant force greater? A) On the lead block B) On the aluminum block C) Same on both blocks D) There is not enough information to decide.

C) Same on both blocks

You place an object in a container that is full to the brim with water on a scale. The object floats, but the water spills out. How does the weight of the object compare with the weight of the water displaced? A) Weight of object is greater than weight of water displaced. B) Weight of object is less than weight of water displaced. C) Weight of object is equal to weight of water displaced. D) There is not enough information to decide.

C) Weight of object is equal to weight of water displaced.

In drinking soda or water through a straw, we make use of A) capillary action B) surface tension. C) atmospheric pressure. D) Bernoulli's principle. E) none of these

C) atmospheric pressure.

A bubble of air released from the bottom of a lake A) rises to the top at constant volume. B) becomes smaller as it rises. C) becomes larger as it rises. D) becomes denser as it rises. E) none of these

C) becomes larger as it rises.

The pressure in a liquid depends on liquid A) density. B) depth. C) both of these D) neither of these

C) both of these

When you stand on one foot instead of two, the pressure you exert on the floor is A) less. B) the same. C) more. D) None of the above.

C) more. Twice as much, in fact!

The reason a life jacket helps you float is A) if you sink, the jacket sinks. B) the jacket makes you weigh less. C) the density of both you and the jacket together is less than your density alone. D) the jacket has the same density as an average human. E) the jacket repels water.

C) the density of both you and the jacket together is less than your density alone.

In a hydraulic device, it is impossible for the A) output piston to move farther than the input piston. B) force output to exceed the force input. C) output piston's speed to exceed the input piston's speed. D) energy output to exceed energy input.

D) energy output to exceed energy input.

pressure

Force per surface area where the force is normal (perpendicular) to the surface; measured in pascals.

adhesion

Molecular attraction between two surfaces making contact.

buoyant force

Net upward force exerted by a fluid on a submerged or immersed object.

If a friend commented that a hydraulic device is a common way of multiplying energy. what would you say?

No, no, no! Although a hydraulic device, like a mechanical lever, can multiply force it always does so at the expense of distance. Energy is the product of force and distance, if you increase one,you decrease the other. No device has ever been found that can multiply energy!

A recipe calls for a specific amount of butter. How does the displacement method relate to the use of a kitchen measuring cup?

Put some water in the cup before you add the butter. Note the water-level reading on the side of the cup. Then add the butter and you'll note the water level rise. Because butter floats. poke it beneath the surface. When you subtract the lower level reading from the higher-level reading, you know not only the volume of water displaced, but the volume of the butter.

Archimedes' principle

Relationship between buoyancy and displaced fluid: An immersed object is buoyed up by a force equal to the weight of the fluid it displaces.

capillarity

Rise of a liquid in a fine, hollow tube or in a narrow space.

pascal (Pa)

SI unit of pressure. One pascal of pressure exerts a normal force of 1 newton per square meter. A kilopascal (kPa) is 1000 pascals.

Why is it easier for you to float in saltwater than in freshwater?

Saltwater is denser than freshwater, which means you don't"sink" as far when displacing your weight. You'd float even higher in mercury (density 13.6 g/(m~), and you'd sink completely in alcohol (density 0.8 g/cm3).

A 1-L container completely filled with lead has a mass of 11.3 kg and is submerged in water. What is the buoyant force acting on it?

The buoyant force equals the weight of the liter of water displaced-not the weight of the lead! One L of water has a mass of 1 kg and weighs 10 N. So the buoyant force on it is 10 N.

Two solid blocks of identical size are submerged in water. One block is lead and the other is aluminum. Upon which is the buoyant force greater?

The buoyant force is the same on each block because they displace the same volume of water. For submerged objects, buoyant force is determined only by the volume of water displaced, not the object's weight.

As the automobile on a lift is being lifted, how does the change in oil level in the reservoir compare with the distance the automobile moves?

The car moves up a greater distance than the oil level drops, since the area of the piston is smaller than the surface area of the oil in the reservoir.

Archimedes' principle tells us that any object that displaces 10 N of liquid will be buoyed up with 10 N. A) True B) False

True. It's only the weight of the displaced liquid that counts.

Pressure

force / area Pressure may be measured in any unit of force divided by any unit of area. The standard international (SI) unit of pressure, the newton per square meter, is called the pascal (Pa) . A pressure of 1 Pa is very small and approximately equals the pressure exerted by a dollar bill resting flat on a table. Science types more often use kilopascals (1 kPa = 1000 Pa).