PHSC 170- Exam 2 Ch. 6-9

What are the temperatures for freezing water on the Celsius and Fahrenheit scales? For boiling water at sea level?

0°C, 32°F; 100°C, 212°F.

What is the mass in kilograms of 1 L of water? What is its weight in newtons?

1 kilogram; 10 newtons.

What is the magnitude of the gravitational force between the Earth and a 1-kilogram body?

10 N, or more precisely 9.8 N.

What is the efficiency of a machine that miraculously converts all the input energy to useful output energy?

100%—the efficiency of an ideal machine.

How much power is required to raise a 100-kg crate a vertical distance of 3 m in a time of 2 s?

1500 W.

Which of these temperatures could result from mixing a container of water at 20°C with one at 24°C?

22°C.

What is the temperature of melting ice on the Kelvin scale? Of boiling water at atmospheric pressure?

273°C; 373°C.

At what temperature do the combined effects of contraction and expansion produce the smallest volume for water?

4°C.

If you drop a ball it will fall 5 m in the first second of fall. If you instead toss it sideways, in 1 second it will fall beneath its straight-line path a vertical distance of...

5 m.

What is the magnitude of gravitational force between two 1-kilogram bodies that are 1 meter apart?

6.67 × 10-11 N.

What is the minimum speed for orbiting the Earth in close orbit? The maximum speed? What happens above this speed?

8 km/s; 11.2 km/s; above this speed the satellite leaves the Earth permanently.

Cite an example where a force is exerted on an object without doing work on the object.

A force is needed to hold a barbell overhead, but this force does no work on the barbell while the barbell is at rest.

Compared to some original speed, how much work must the brakes of a car supply to stop a four-times-as-fast car? How will the stopping distance compare?

A four-times as fast car has 16 times as much KE and will require 16 times as much work to stop, and 16 times as much stopping distance

A moving car has kinetic energy. If it speeds up until it is going four times as fast, how much kinetic energy does it have in comparison?

A four-times as fast car has 42 or 16 times the KE.

How does the frequency of radiant energy relate to the absolute temperature of the radiating source?

As the equation says, f ~ T, so higher T means higher f.

Why does a penny become warmer when struck by a hammer?

Atoms are set into motion.

Does the buoyant force on a floating object equal the weight of fluid displaced? Or on the weight of the object? When is BF equal to both?

BF always equals the weight of fluid displaced. In the special case of floating, BF also equals the weight of the object.

A balloon that weighs 1 N is suspended in air, drifting neither up nor down. How does the buoyant force acting on it compare with its weight? What happens if the buoyant force decreases? Increases?

BF equals its weight. When BF decreases, balloon descends; When BF increases, balloon ascends.

Why does the speed of a satellite undergo change in an elliptical orbit?

Because half the time is going away from gravity, and the other half going with gravity. When going against gravity, speed decreases. When going with gravity, speed increases.

When G was first measured, the experiment was called the "weighing the Earth experiment." Why?

Because once G was known, it was a simple matter to calculate the mass of the Earth.

How does the buoyant force on a fully submerged object compare with the weight of water displaced?

Both are equal.

How does the downward pressure of the 76-cm column of mercury in a barometer compare with the air pressure at the surface of the dish of mercury that it stands in?

Both are the same.

How does the weight of mercury in a barometer tube compare with the weight of an equal cross-section of air from sea level to the top of the atmosphere?

Both are the same.

Distinguish between a joule and a calorie.

Both are units of energy; 4.18 J = 1 cal.

By how much does the density of air increase when it is compressed to half its volume? Its pressure?

Both density and pressure are doubled.

Which requires more work—lifting a 50-kg sack a vertical distance of 2 m or lifting a 25-kg sack a vertical distance of 4 m?

Both require the same work because the force × distance is the same for each.

What is meant by the statement "a thermometer measures its own temperature"?

Both the thermometer and whatever it measures reach a common temperature—this is thermal equilibrium. In this sense, a thermometer measures its own temperature.

What weight of water is displaced by a 100-ton floating ship? What is the buoyant force that acts on this ship?

Both weight of water displaced and buoyant force are 100 tons.

How does the brightness of light change when a point source of light is brought twice as far away?

Brightness is 1/4 as much, in accord with the inverse-square law.

Is cold the opposite of thermal energy or the lack of it?

Cold is the lack of thermal energy (like black is the absence of light).

When the temperature of ice-cold water is increased slightly, does it undergo a net expansion or net contraction?

Contraction (until it reaches 4°C).

Calculate the density of block of mass 22 kg that has a volume of 2.6 cm3.

Density = mass/volume = 22 kg/2.6 cm3 = 2.6 kg/cm3.

Is energy added, or is it released, when boiling water is changed to steam?

Energy is added,

State Newton's law of universal gravitation in words. Then do the same with one equation.

Every mass attracts every other mass with a force that for any two masses is directly proportional to the product of the masses involved and inversely proportional to the square of the distance separating them. F =Gx m1xm2/d2

What do peaked roofs, convertible tops, and airplane wings have in common when air moves faster across their top surfaces?

Faster-moving air results in lower air pressure.

If you punch a hole in the side of a container filled with water, in what direction does the water initially flow outward from the container?

Flow is perpendicular to surface of container.

What makes the Earth round?

Gravitation pulls the parts of the Earth as close as it can and pulls in any "corners."

Why does buoyant force act upward on an object submerged in water?

Greater pressure against the bottom than against the top. Correspondingly, greater force up on the bottom than down on the top.

At what part of an elliptical orbit does a satellite have the greatest speed? The least speed?

Greatest speed nearest the Earth, and slowest speed farthest away.

What connection did Newton make between a falling apple and the Moon?

He realized they were both under the influence of Earth's gravity.

If a machine multiplies force by a factor of four, what other quantity is diminished, and how much?

Increasing force means decreasing distance, so the distance moved by the four-fold force is one fourth as much.

Does a good insulator prevent heat from getting through it, or does it simply slow its passage?

Insulation slows heat transfer; it does not stop it.

How can a projectile "fall around the Earth"?

It can fall around the earth if its tangential velocity is sufficient to ensure its curved path matches the curve of Earth.

In what sense does the Moon "fall"?

It falls beneath the straight-line path it would follow if there were no gravity.

With no air drag, what happens to the horizontal component of velocity for the batted baseball?

It remains the same.

Does a substance that heats up quickly have a high or a low specific heat capacity?

Low.

Distinguish between mass density and weight density. What are the mass density and the weight density of water?

Mass density is mass/volume; water's density is 1 kg/m3. Weight density is weight/volume; water's weight density is 62.4 lb/ft3

Does Pascal's Principle provide a way to get more energy from a machine than is put into it? Defend your answer.

No, it can multiply forces or distances, but not energy. A conservation of energy no-no!

Is a machine physically possible that has an efficiency greater than 100 percent? Discuss.

Not in the world as we know it. Such a machine would clearly violate the conservation of energy, a law that has never been violated.

What is the ultimate source of geothermal energy?

Nuclear energy from radioactive decay.

Why does a bimetallic strip bend with changes in temperature?

One side changes length more than the other, bending the strip, which can trip a switch and activate a thermal device.

If a 1-L container is immersed halfway in water, what is the volume of water displaced? What is the buoyant force on the container?

One-half liter; 5 newtons.

Calculate the watts of power expended when a force of 1 N moves a book 2 m in a time interval of 1 s.

P = (1N)x(2m)/1s = 2w.

How many joules of potential energy does a 1-N book gain when it is elevated 4 m? When it is elevated 8 m?

PE = 1 N × 4 m = 4 J. At 8 m the PE = 1N × 8 m = 8 J.

What will be the kinetic energy of pile driver ram when it undergoes a 10 kJ decrease in potential energy? (Assume no energy goes to heat.)

PE = KE = 10 kJ.

What are the two main forms of mechanical energy?

Potential energy and kinetic energy.

If both sacks in the preceding question are lifted their respective distances in the same time, how does the power required for each compare? How about for the case where the lighter sack is moved its distance in half the time?

Power for each is the same because the same work is done in the same time. Twice the power is required to do the same work on the lighter sack in half the time.

Calculate the water pressure at the base of a 30-m tall water tower.

Pressure = weight-density × depth = (1000 km/m3)(10 N/kg)(30 m) = 3 × 105 N/m3.

How does the pressure exerted by a liquid change with depth in the liquid? How does the pressure exerted by a liquid change as the density of the liquid changes?

Pressure increases with increases in depth and/or density.

Why does the direction of coastal winds change from day to night?

Reverses at night from daytime direction.

How does water pressure one meter below the surface of a small pond compare to water pressure one meter below the surface of a huge lake?

Same pressure at same depths.

What happens to the pressure in all parts of a confined fluid when the pressure in one part is increased?

Same pressure is transmitted to all parts.

Consider two tubes of equal cross-sectional area. One is a 76-cm tall tube of mercury and the other a 10.3-m tall tube of water. Which weighs more? Which exerts more pressure at the bottom of the tube?

Same weights and same pressures.

How does the volume of a fully submerged object compare with the volume of water displaced?

Same.

If the man in Figure 6.22 pulls 1 m of rope downward with a force of 100 N, and the load rises 1/7 as high (about 14 cm), what is the maximum load that can be lifted?

Seven times as much, 700 N.

What is the ultimate source of energies for the burning of fossil fuels, dams, and windmills?

Solar energy

What are streamlines? Is pressure greater or less in regions of crowded streamlines?

Streamlines are imaginary smooth paths of fluid flow. When crowded, pressure there is less; when far apart, pressure there is more.

What does it mean to say something moving in a curve has a tangential velocity?

Tangential velocity is velocity that is parallel to (tangent) the curve at every point.

What happens to the temperature of air when it expands?

Temperature drops.

Distinguish between temperature and heat.

Temperature measured in degrees. It is a measure of average translational KE per/molecule. Heat is a flow of energy measured in joules.

What is terrestrial radiation? How does it differ from solar radiation?

Terrestrial radiation comes from Earth; solar from the Sun. Both are electromagnetic waves. Earth's waves are lower frequency than Sun's.

What will happen to air bubbles in a narrowing pipe of moving water?

The air bubbles will get bigger.

How does the force of gravity between two bodies change when the distance between them is doubled?

The force is 1/4 as much, in accord with the inverse-square law.

How does the thickness of paint sprayed on a surface change when the sprayer is held twice as far away?

The paint is 1/4 as thick, in accord with the inverse-square law.

With no gravity, a horizontally-moving projectile follows a straight-line path. With gravity, how far below the straight-line path does a projectile fall compared with the distance of free fall?

The same.

Two cars are lifted to the same elevation in a service station. If one car is twice as massive as the other, how do their potential energies compare?

The twice-as-massive car has twice the PE

A ball is batted upward at an angle. What happens to the vertical component of its velocity as it rises? As it falls?

The vertical component changes the same way a ball tossed straight upward changes, and changes on the way down just as a stone does in free fall.

When you touch a cold surface, does "coldness" travel from the surface to your hand or does thermal energy travel from your hand to the cold surface? Explain.

Thermal energy, not "coldness," can travel. So when your hand touches a cold surface, thermal energy travels from hand to the cold surface.

A car is lifted a certain distance in a service station and therefore has potential energy relative to the floor. If it were lifted twice as high, how much potential energy would it have?

Twice.

Calculate the work done when a force of 2 N moves a book 3 m.

W = (2 N)x(3 m) = 6 J.

The energy we require for existence comes from the chemically stored potential energy in food, which is transformed into other forms when it is metabolized. What happens to a person whose work output is less than the energy he or she consumes? Whose work output is greater than the energy he or she consumes? Can an undernourished person perform extra work without extra food? Briefly discuss.

When work output is less than input energy, the person gains weight. When work output is more than input energy, weight loss results. An undernourished person performing extra work without extra food violates energy conservation—and perishes.

Why is the West Coast of the United States warmer in winter than the East Coast?

Winds carrying energy from ocean water blow westerly over the West Coast. This is not true of the East Coast where winds blow over land.

What is the explanation for a barefoot firewalker being able to walk safely on red-hot wooden coals?

Wood has low conductivity, even when red hot. Very little heat conducts to John's feet.

A force sets an object in motion. When the force is multiplied by the time of its application, we call the quantity impulse, which changes the momentum of that object. What do we call the quantity force distance?

Work.

Exactly what is it that a body having energy is capable of doing?

Work.

Are the planets of the solar system simply projectiles falling around and around the Sun?

Yes indeed!

Can a machine multiply input force? Input distance? Input energy? (If your three answers are the same, seek help, for the last question is especially important.)

Yes; yes; NO! (Machines are good at multiplying force or distance, but no machine can multiply energy—a conservation of energy no-no!)

What do we call the gravitational force between the Earth and your body?

Your weight.

When you compress a loaf of bread you increase its...

density.

If the Sun were half as massive, its pull on Mars would be...

half.

Consider a U-shaped tube, thin on one side and thick on the other, filled with water with pistons at each end. When the downward force at the thin end is increased, the upward force on the piston at the thicker end will...

increase more.

The warmer an object is, the greater is its...

thermal energy.

Raising a heavy load requires work. Raising it twice as high requires...

twice the work.

If you double the depth of a lake, the pressure at the bottom due to the weight of water above is...

twice.