Fluids - Conceptual Questions Flashcards
Imagine holding two bricks under water. Brick A is just beneath the surface of the water, while brick B is at a greater depth. The force needed to hold brick B in place is
- larger
- the same as
- smaller
than the force required to hold brick A in place.
Answer: 2.The buoyant force on each brick is equal to the weight of water it displaces and does not depend on depth.
When a hole is made in the side of a container holding water, water flows out and follows a parabolic trajectory. If the container is dropped in free fall, the water flow
- diminishes.
- stops altogether.
- goes out in a straight line.
- curves upward.
Answer: 2.When the container is at rest with respect to Earth, there is pressure on the walls of the container due to the water. The pressure depends on the depth and is equal to ρgh, with ρ being the density of water.
When the container is in free fall, both the water and the container have an acceleration of zero, not g, in the container frame of reference. In this frame, the pressure of the water on the walls of the container is zero, so
there is no outward flow.
A 200-ton ship enters the lock of a canal. The fit between the sides of the lock and the ship is tight so that the weight of the water left in the lock after it closes is much less than 200 tons. Can the ship still float if the quantity of water left in the lock is much less than the ship’s weight?
- Yes, as long as the water gets up to the ship’s waterline.
- No, the ship touches bottom because it weighs more than the water in the lock.
Answer: 1.What matters is not the weight of the water left in the lock, but the weight of the water forced out of the lock by the ship. As long as the density of the ship is less than that of water and the water gets to the waterline,
it floats.
Two cups are filled to the same level with water. One of the two cups has ice cubes floating in it. Which weighs more?
- The cup without ice cubes.
- The cup with ice cubes.
- The two weigh the same.
Answer: 3. The ice cubes displace exactly their own weight in water, so the two cups weigh the same amount.
Two cups are filled to the same level with water. One of the two cups has ice cubes floating in it. When the ice cubes melt, in which cup is the level of the water higher?
- The cup without ice cubes.
- The cup with ice cubes.
- It is the same in both.
Answer: 3.When the ice cubes melt, they turn into an amount of water having the same weight. This weight is also equal to the weight of the water originally displaced by the cubes. Since the densities of the melted ice and the surrounding water are identical, the volume occupied by the melted ice exactly equals the volume of displaced water.
Two cups are filled to the same level with water. One of the two cups has plastic balls floating in it. If the density of the plastic balls is less than that of ice, which of the two cups weighs more?
- The cup without plastic balls.
- The cup with plastic balls.
- The two weigh the same.
Answer: 3.The plastic balls displace exactly their own weight in water, so the two cups weigh the same amount.
A lead weight is fastened on top of a large solid piece of Styrofoam that floats in a container of water. Because of the weight of the lead, the water line is flush with the top surface of the Styrofoam. If the piece of Styrofoam is turned upside down so that the weight is now suspended underneath it,
- the arrangement sinks.
- the water line is below the top surface of the Styrofoam.
- the water line is still flush with the top surface of the
Styrofoam.
Answer: 2. Since both the weight and the Styrofoam are solid, the arrangement floats in both orientations. Therefore, turning the arrangement upside down does not change the buoyant force; however, since the weight is now submerged, it displaces a volume of water. To maintain the same buoyant force, the Styrofoam must therefore displace less water than before.
A lead weight is fastened to a large solid piece of Styrofoam that floats in a container of water. Because of the weight of the lead, the water line is flush with the top surface of the Styrofoam. If the piece of Styrofoam is turned upside down, so that the weight is now suspended underneath it, the water level in the container
- rises.
- drops.
- remains the same.
Answer: 3. The buoyant force is unchanged, so the volume of water displaced is unchanged and the water level stays the same.
A boat carrying a large boulder is floating on a lake. The boulder is thrown overboard and sinks. The water level in the lake (with respect to the shore)
- rises.
- drops.
- remains the same.
Answer: 2.When it is inside the boat, the boulder displaces its weight in water .When it is thrown overboard, it only displaces its volume in water so the water level of the lake with respect to the shore goes down.
Consider an object that floats in water but sinks in oil. When the object floats in water, half of it is submerged. If we slowly pour oil on top of the water so it completely covers the object, the object
- moves up.
- stays in the same place.
- moves down.
Answer: 1.With the oil atop the water, there is an additional buoyant force on the object equal to the weight of the displaced oil. The effect of this additional force is to displace the object upward.
Consider an object floating in a container of water. If the container is placed in an elevator that accelerates upward,
- more of the object is below water.
- less of the object is below water.
- there is no difference.
Answer: 3.The acceleration effectively increases g, changing the apparent weight of objects inside the elevator. However, there is no difference in the
amount that the object is submerged because the acceleration affects the water and the object in the same amount.
A circular hoop sits in a stream of water, oriented
perpendicular to the current. If the area of the hoop is doubled, the flux (volume of water per unit time) through it
- decreases by a factor of 4.
- decreases by a factor of 2.
- remains the same.
- increases by a factor of 2.
- increases by a factor of 4.
Answer: 4.The flux is equal to the flow velocity times the cross-sectional area through which the fluid flows. If the area is doubled, so is the flux.
Blood flows through a coronary artery that is partially blocked by deposits along the artery wall. Through which part of the artery is the flux (volume of blood per unit time) largest?
- The narrow part.
- The wide part.
- The flux is the same in both parts.
Answer: 3. Because liquids, such as blood, are incompressible and because no blood accumulates in (or leaks out of) the artery, the flux is the same everywhere.
Blood flows through a coronary artery that is partially blocked by deposits along the artery wall. Through which part of the artery is the flow speed largest?
- The narrow part.
- The wide part.
- The flux is the same in both parts.
Answer: 1. Because liquids, such as blood, are incompressible and because no blood accumulates in (or leaks out of) the artery, the flux is the same everywhere. Since the flux is equal to the flow speed times the cross-sectional area of the artery, the blood must flow faster in the narrow part.
Two hoses, one of 20-mm diameter, the other of 15-mm diameter are connected one behind the other to a faucet. At the open end of the hose, the flow of water measures 10
liters per minute. Through which pipe does the water flow faster?
- the 20-mm hose
- the 15-mm hose
- The flow rate is the same in both cases.
- The answer depends on which of the two hoses comes
first in the flow.
Answer: 2. Unless the hose is leaking, the flux, or volume of fluid per unit time through each hose is the same. Since the flux is equal to the flow speed times the cross-sectional area of the hose, the water must flow faster in the narrower hose.
