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Physics Ch. 13
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Gravity
"Liquids" Reading Checkpoint and HW questions
Terms in this set (34)
How does pressure relate to force?
Pressure is force per unit area.
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?
The pressure is the weight of the water divided by 1 m^2.
What is the relationship between liquid pressure and the depth of a liquid? Between liquid pressure and weight density?
Pressure is proportional to both depth and weight density.
Liquid P = wt density X depth
If you swim beneath the surface in saltwater, will the pressure be greater than in fresh water at the same depth?
The pressure will be greater.
Density of ocean water is 1027 kg/m3
(H2O is 1000 kg/m3)
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?
The pressure will be the same.
P doesn't depend on volume, but
depth
If you punch holes in the side of a container filled with water, in what direction does the water initially flow outward from the container?
It flows straight out, perpendicular to the wall.
(Force vectors act perpendicularly to the inner container surface; increase with increasing depth. Curves downward due to gravity.)
Why does the buoyant force act upward on an object submerged in water?
The pressure upward on the deeper bottom is greater than the downward pressure on the top.
(Net force upward)
Why isn't there a horizontal buoyant force on a submerged object?
Force vectors on the sides cancel one another.
(Net force = 0)
How does the volume of a completely submerged object compare with the volume of water displaced?
The volumes are equal.
(submerged stone displaces an amount of water equal to its own volume)
How does the buoyant force on a submerged object compare with the weight of the water displaced?
The buoyant force is equal to the weight of the water displaced.
Distinguish between an immersed and a submerged body.
An immersed body is partially or completely surrounded by water, whereas a submerged body is completely surrounded by fluid.
(standing in water//diving underwater)
(boat//submarine
What is the mass of 1 L of water? What is its weight in newtons?
mass 1 kg
weight 10 N
volume 1000 cm3
density 1 kg/L
wt density 10 N/L
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?
0.5 L displaced, 5 N buoyant force
(The buoyant force is equal to the weight of the water displaced.)
1/2 L immersed = 1/2 L displaced
1/2 N/L weight in = buoyant force
Is the buoyant force on a submerged object equal to the weight of the object itself or is it equal to the weight of the fluid displaced by the object?
It is equal to the weight of the fluid displaced.
What is the condition in which the buoyant force on an object does equal the weight of the object?
The object is neutrally buoyant, so it will neither sink nor float.
(Objects with density = density of fluid it is immersed in will
neither
sink nor float)
Does the buoyant force on a submerged object depend on the volume of the object or on the weight of the object?
Volume
(buoyant force depends on volume of the submerged object)
An object more dense than the fluid in which it is immersed will...
sink
(rock in water)
An object less dense than the fluid it is immersed in will...
float
(cork in water)
An object that has a density equal to the density of the fluid it is immersed in will...
neither sink nor float
(neutral buoyancy)
How is the density of a fish controlled? How is the density of a submarine controlled?
A fish changes its volume (expand/ contract air sac), whereas a submarine changes its weight (take in/ release water)
How much air must a 100-ton blimp displace to float and neither rise nor sink?
100 tons
(a floating object displaces a weight of fluid equal to its own weight.)
displace more=rise
displace less=falls
Why do the gondolas of the Falkirk Wheel (see Figure 13.19 in the textbook) have the same weight whether or not they carry boats?
The boats displace water out of the gondolas equal to their weights.
(maintains same weight)
What happens to the pressure in all parts of a confined fluid if the pressure in one part is increased?
The pressure everywhere increases by the same amount.
(exerted P on left piston of U-tube will increase P in the liquid, transmit to the right piston)
Pascal's Principle
A change in pressure at any point in an enclosed fluid at rest is transmitted undiminished to all points in the fluid.
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?
500 N
What geometrical shape has the smallest surface area for a given volume?
Sphere
(liquid drops)
surfaces tend to contract and force each drop into the shape
What is the cause of surface tension?
molecular attraction
(molecules on surface of liquid have no upward pull, thus pull the molecule into the liquid, minimize surface area)
Distinguish between adhesive and cohesive forces.
Adhesive forces are between unlike substances, whereas cohesive forces are between like substances.
(glue to paper//water to water)
*meniscus=water in cylinder
*antimeniscus= mercury in tube
How does the height to which water is lifted in a capillary tube relate to adhesion and the weight of the water lifted?
Water rises to a height where the adhesive forces equal the weight of the water lifted.
If the gravitational field of Earth were to increase, would a fish float to the surface, sink, or stay at the same depth?
...
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?)
...
Why is it inaccurate to say that heavy objects sink and that light objects float? Give exaggerated examples to support your answer.
...
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? How about if the ice cube contains many grains of heavy sand?
...
Piston problem
There is an extreme mechanical disadvantage on his side here! As discussed in Chapter 13, given Pascal's Principe (A change in pressure at any point in an enclosed fluid at rest is transmitted undiminished to all points in the fluid) and the size of each piston, the pressure change by one area affects all areas. In that case, on the larger piston, the strong man will not be able to move that small piston by his own strength! Perhaps with a hammer or other device.
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