Chapter 13 Assessment

Your lab has two pressurized gas cylinders, each containing a different gas. Describe an experiment that could be used to determine which gas is less dense than air.

Fill a balloon with each gas and release them into the air of the lab. Whichever balloon floats contains the gas that is less dense than air.

Why is it easier to float in salt water than in fresh water?

Salt water is denser, so the water you displace is heavier, giving you a greater buoyant force.

When a driver presses on the brake pedal of a car, the car is easily brought to a stop. Explain how the car's hydraulic braking system transmits and increases the force applied to the brake pedal.

The small piston on the brake generates a very large pressure, exerting a large force on the output piston and bringing the car to a stop.

Why is it easier to pull a submerged boat anchor to the surface than it is to lift it onto the boat?

There is more buoyant force acting on the anchor in water than in air, so you perceive it to be lighter in the water.

In a hydraulic lift system, the output force is greater than the input force because a. a larger pressure acts on the output piston. b. a larger pressure acts on the input piston. c. the fluid pressure acts on areas of different sizes. d. of Bernoulli's principle.

c

A stone sinks to the bottom of a beaker. There is 10 mL overflow. What is the volume of the stone in cm^3? (Hint: 1.0 mL = 1.0 cm^3)

10 cm^3

If 1.0 cm^3 equals 1.0 mL, and 1.0 cm^3 of water has a mass of 1.0 g, calculate the buoyant force acting on said stone.

10 g

A small hydraulic jack exerts a force of 2200 N. If the area of the jack's output piston is 0.0060 m^2, what is the pressure of the hydraulic fluid inside the jack?

370,000 N

A 520-N ballet dancer is balanced on the toe of her shoe. If the toe has an area of 0.0010 m^2, what pressure does she exert on the floor?

520,000 N

Explain how a hydraulic lift operates.

An increased output force is produced because a constant fluid pressure is exerted on the larger area of the output force.

In order to save someone who has fallen through the ice of a frozen lake, rescuers often reach the victim by crawling across the ice on their stomachs. Explain why the rescuers use this technique.

Crawling on your stomach gives you greater area than walking on your feet, causing less pressure to be on the ice to prevent you from falling in.

What two forces determine whether an object floats or sinks?

Gravity and buoyant force.

Why do helium-filled balloons rise in air?

Helium is less dense than air, and the air the balloon displaces is heavier than the balloon is, so it floats.

For patients with muscles weakened due to injury, physical therapists recommend exercising in a swimming pool. Why is pool exercise preferred to exercise in a gym?

In a pool, there is more buoyant force acting on you, so your limbs are perceived to be lighter and it is easier to move them.

How is pressure different from force?

Pressure is a force distributed over an area, but force is just a force.

State Archimedes' principle in your own words.

The buoyant force on an object equals the weight of the fluid it displaces.

An empty metal can is capped, attached to weights, and thrown into a deep ocean trench. A second, identical metal can is left uncapped, attached to weights, and thrown into the same trench. Both cans sink. Predict what will happen to each can and explain your reasons.

The capped can will crumble and cave in because the pressure inside it is lower than the pressure in the ocean, but the uncapped can will stay the same shape because the pressure inside it is the same as the pressure in the ocean.

Three different liquids are layered in a beaker. Two equal-size cubes are placed in the beaker. One stays halfway submerged at the top, the other goes down to the very bottom. Which of the objects has the greater buoyant force acting on it?

The cube at the top.

Water exerts pressure on all sides of a submerged submarine. Compare and contrast the pressures acting on the submarine at a depth of 50 m to the pressures at a depth of 200 m.

The pressures acting on a submarine at 50 m are much less than the pressures at a depth of 200 m.

The high-speed winds of a hurricane sometimes cause houses to explode. Describe the pressures acting on the house and explain the areas of high and low pressure using Bernoulli's principle.

There is little pressure pushing on the house and lots of pressure pushing out of the house. The winds have low pressure, but inside there is high pressure.

What are two characteristics of fluids?

They can flow, and they can take on the shape of their containers.

A partially submerged object floats when a. the object's weight is equal to the buoyant force. b. the object's mass is equal to the buoyant force. c. the object's weight is greater than the buoyant force. d. the buoyant force is downward.

a

Blaise Pascal discovered that changes in pressure a. are transmitted equally throughout a fluid. b. increase with depth of fluid. c. decrease with depth of fluid. d. depend on area.

a

Which of the following is NOT true about Bernoulli's principle? a. The pressure within a moving fluid is greater than the pressure within a nonmoving fluid. b. As a fluid's speed increases, the pressure within it decreases. c. When a moving fluid slows, the pressure within it increases. d. It helps explain the lift of an airplane.

a

Which of the following is NOT true about the SI unit of pressure? a. It is used for fluids only. b. It is called a pascal. c. It is equal to one newton per square meter. d. It represents force per unit area.

a

A submarine changes depth by altering its a. speed. b. density. c. total area. d. shape.

b

Compared to the pressure exerted by a brick standing on its end, the pressure exerted by a brick resting on its side is a. the same. b. less. c. more. d. twice as much.

b

A resting object exerts pressure equal to a. its mass times its contact area. b. its weight times its contact area. c. its mass divided by its contact area. d. its weight divided by its contact area.

d

The amount of water pressure you experience while swimming in a pool depends on a. your buoyancy. b. the area of the pool. c. the volume of water in the pool. d. how deep you are in the water.

d

The buoyant force acting on a submerged object is equal to a. the object's mass. b. the object's volume. c. the mass of the fluid displaced by the object. d. the weight of the fluid displaced by the object.

d