Physics chp 13 liquids
If an object suspended by a scale shows a weight of 3 N in air, and 2 N when submerged in water, the buoyant force on the submerged object is __________.
1 N
on which of these blocks submerged in water is the buoyant force the greatest? 1 kg lead, 1 kg aluminum, 1 kg of uranium, all the same
1 kg aluminum
What is the mass of 1 L of water? Its weight in newtons?
1.0 kg
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?
1//2 L and the buoyant force is equal to the weight of the container.
Suppose that we repeat the experiment shown in the video, but we replace one of the cylinders with a cylinder that has twice the radius (and use larger containers of water). If the height of the original cylinder is h, how deeply must we submerge the new cylinder to get the same weight reduction as in the video?
1/4 h
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
100 lb woman since the area of the bottom of the heal is very small
How much air must a 100-ton blimp displace to float and neither rise nor sink?
100 tons
Pascal's Principle
A change in pressure at any point in an enclosed fluid will be transmitted equally to all parts of the fluid
Principle of flotation
A floating object displaces a weight of fluid equal to its own weight
Distinguish between adhesive and cohesive forces.
Adhesive forces are between unlike substances, whereas cohesive forces are between like substances.
If a friend commented that a hydraulic device is a common way of multiplying energy, what would you say?
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!
Archimedes' Principle
An immersed object is buoyed up by a force equal to the weight of the fluid it displaces.
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
H2O
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 50 cm2
P= F/A the force applied on the piston is 500N
how does pressure relate to force
Pressure is force per unit area.
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 also the volume of the butter.
How does the buoyant force on a submerged object compare with the weight of the water displaced?
The buoyancy force is equal to the weight of the displaced fluid
Suppose that you raise one foot when you are standing on a bathroom scale. 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, the pressure on the scale's surface 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.
A beaker more than half full of water weighs 20 N. What will be the scale reading when.(a) a 5-N block of wood floats in it? (b) an 8-N block of wood floats in it?
a) 20 N + 5 N = 25 N, (b) 20 N + 8 N = 28 N.
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 an immersed and a submerged body.
an immersed body may be wither partially or fully submerged, but a submerged body is always immersed
water pressure in a lake is greater
at the bottom
If a weighted air-filled balloon sinks in deep water, it will
be acted on by a continuously buoyant force
You know that a sharp knife cuts better than a dull knife. Do you know why this is so? Defend your answer.
because the area of contact of the sharp knife is smaller that the dull knife
Why does your body get more rest when you're lying down than when you're sitting? Is blood pressure in your legs greater?
blood pressure is lower lying down
water pressure on a submerged object is greatest against its
bottom
How is the density of a fish controlled? How is the density of a submarine controlled?
by the expansion and contraction of its air sacs while the density of a submarine is controlled by ballast tanks
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?
he buoyant force is the same on both blocks because they displace the same volume of water. For submerged objects, the buoyant force is determined by only the volume of water displaced, not the object's weight.
on a boat ride, the skipper gives you a life preserver filled with 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 styro foam preservers
he is correct
on what factors does the buoyant force acting on an object depend
height of the object
When a fish decreases the size of its air bladder, the density of the fish
increases
When a fish makes itself less dense, the buoyant force on it
increases
When a submarine takes water into its ballast tanks, its density
increases
What is the cause of surface tension?
intermolecular attraction
pressure
is a force divided by the area over which the force is exerted:
If we ignore atmospheric pressure, at twice the depth, the liquid pressure against the bottom
is twice as great; at three times the depth, the liquid pressure is threefold; and so on
It was emphasized earlier that the buoyant force does not equal an object's weight but does equal the weight of the displaced water. Now we say that the buoyant force equals the object's weight. Isn't this a grand contradiction? Explain.
it is not a contradiction in case of floating bodies
when you stand on one foot instead of two, the pressure you exert on the floor is
more
the reason a person finds it easier to float in saltwater compared to freshwater is that in salt water. The buoyant force is greater, a person feels less heavy, neither of these, none of the above
neither of these
An object having the same density as water will __________.
neither sink nor float in water
calculate the pressure a 40 N block exerts on the table it rests on if its area is 50 cm e2
p=0.8 n/cm e2
How does pressure at the bottom of a body of water relate to the weight of water above each square meter of the bottom surface?
pressure at the bottom a body in water is the weight of water per one meter square area
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?
pressure in both cases will be the same because height of fluid above datum line.
When an ice cube in a glass of water melts, the water level
remains the same
a block of wood with a piece of iron tied to the top of it floats in a bucket of water. If the wood and iron are turned over so that the iron is submerged beneath the wood, the water level at the side of the bucket
remains the same
when a load of wood is thrown overboard from a boat in a swimming pool, the pool level
remains unchanged
What geometrical shape has the smallest surface area for a given volume?
spherical shape has the least surface area
water pressure provided by a water tower is greater if the tower is
taller
Does the buoyant force on a submerged object depend on the volume of the object or on the weight of the object?
the buoyant fore on a submerged object depends upon the volume of the object
surface tension
the force that acts on the surface of a liquid and that tends to minimize the area of the surface
How does the height to which water is lifted in a capillary tube relate to adhesion and the weight of the water lifted?
the height of water rises slightly when adhesion takes place between the liquid and capillary tube. The rise in height of the liquid is equal to the adhesive forces between the molecules in the capillary tube
if the liquid is two or three times as dense
the liquid pressure is correspondingly two or three times as great for any given depth
Suppose you increase the depth of the object WITHOUT changing its height. What will happen to the net buoyant force?
the net buoyant force will increase
what happens to the pressure in all parts of a confined fluid if the pressure in one isincreased
the pressure everywhere increases by the same amount
What is the relationship between liquid pressure and the depth of a liquid? Between liquid pressure and weight density?
the pressure from the fluid is directly proportional to the density of the fluid, as well as the height of the fluid above the datum line.
how does pressure at the bottom of a body of water relate to the weight above each square meter of the bottom surface
the pressure is the weight of the water divided by 1m e2
What happens to the pressure in all parts of a confined fluid if the pressure in one part is increased?
the pressure will be equally increased in all parts of the confined fluid.
when you stand on one foot instead of 2, the force you exert on the floor is
the same
buoyant force
the upward force exerted by a fluid on a submerged object
How does the volume of a completely submerged object compare with the volume of water displaced?
the volume of the completely submerged object is equal to the volume of displaced water.
A stone submerged in water displaces the _________.
the volume of water
If you punch a hole in a container filled with water, in what direction does the water initially flow outward from the container
the water will flow in a horizontal direction away from the container
What is the condition in which the buoyant force on an object does equal the weight of the object?
the weight of fluid displaced is equal to buoyancy force
Why do the gondolas of the Falkirk Wheel (see Figure 13.19) have the same weight whether or not they carry boats
they maintain the same amount of water displaced as the weight of the boats
How many forces act on a submerged body at rest in a fluid?
two buoyancy and the force due to gravity
Why is blood pressure measured in the upper arm, at the elevation of your heart?
upper arm is at the same level of the heart. the blood pressure in the upper arm is equal to the pressure of the heart
a completely submerged object always displaces its own
volume of fluid
Why are persons who are confined to bed less likely to develop bedsores on their bodies if they rest on a waterbed rather than on an ordinary mattress?
waterbeds have large contact area that help reduce pressure points
You place an object in a container that is full to the brim with water on a scale. The object floats, but some water spills out. How does the weight of the object compare with the weight of the water displaced?
weight of object is equal to weight of water displaced
Why does buoyant force act upward on an object submerged in water?
when an object is submerged in water, the weight of the object will be directed downwards. This weight of the object on the water pushes the water downwards, increasing in the pressure of the water in the downward directions produces an upward force called buoyant force which acts on a counter balance
An object less dense than the fluid in which it is immersed
will float
An object that has a density equal to the density of the fluid in which it is immersed
will neither sink nor float
An object more dense than the fluid in which it is immersed
will sink
If you swim beneath the surface in saltwater, will the pressure be greater than in fresh water at the same depth?
yes because the density of saltwater is greater that the density of fresh water
Lobsters live on the bottom of the ocean. The density of a lobster is
greater than the density of sea water
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.
As a boulder thrown into a deep lake sinks deeper and deeper into the water, does the buoyant force on it increase or decrease?
The buoyant force remains unchanged as the boulder sinks because the boulder displaces the same volume and the same weight of water at any depth.
As the automobile in Figure 13.23 is being lifted, how does the oil-level change 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.
Why isn't there a horizontal buoyant force on a submerged object?
The horizontal component of the weight of the object is zero, such that the horizontal component force opposite by the fluid also zero that is the horizontal buoyant force is zero.
capillarity
This rise of a liquid in a fine hollow tube or in a narrow space is called
True or false? Archimedes' principle tells us that any object that displaces 10 N of liquid will be buoyed up with 10 N
True. It's only the weight of the displaced liquid that counts. (Also look at it in a Newton's third law way: If the immersed object pushes 10 N of fluid aside, the displaced fluid reacts by pushing back on the immersed object with 10 N.) Question 2
If a fish makes itself denser, it will sink; if it makes itself less dense, it will rise. In terms of the buoyant force, why is this so?
When the fish makes itself more dense by decreasing its volume, it displaces less water, so the buoyant force decreases. When the fish makes itself less dense by expanding its volume, more water is displaced and the buoyant force increases.
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>a>b
The operation of a hydraulic press best illustrates
conservation of energy
When a fish expands its air bladder, the density of the fish
decreases
When a submerged submarine expels water from its ballast tanks, its density
decreases
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?
depends upon the weight of the fluid displaced
Volume is not the key
depth is
A cook who measures a specific amount of butter by placing it in a measuring cup with water in it is using the
displacement rule
the pressure increases on a block resting on a table when you increase the
downward force on the block
in a hydraulic device, it is impossible for
energy output to exceed energy input
The buoyant force on a floating object is
equal to the object's weight
When scrap iron in a boat is thrown overboard in a swimming pool, the pool level
falls
The concept of pressure involves both
force and area
How does pressure relate to force?
force is directly related to the product of its pressure and the cross section of area
If you were submerged in a liquid more dense than water, the pressure would be
greater