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the physical universe

Questions and Answers of The Physical Universe

Why is it unlikely that dark matter consists of brown dwarfs, burned-out white dwarfs, and/or black holes? Why is it unlikely that it consists of neutrinos?
There is no day-night difference in cosmic-ray intensity. How does this observation bear on the possibility of a solar origin for cosmic rays?
Cosmic-ray primaries are mostly protons, but few protons are in the cosmic rays that reach the earth’s surface. Why?
What is Hubble’s law?
What is a quasar?
The spectra of quasars exhibit red shifts, never blue shifts. Why does this suggest that quasars are not members of our galaxy?
What is the evidence for the idea that dark energy fills the universe? Is the nature of dark energy known at present? How does the amount of dark energy compare with the amount of ordinary matter in
To what event in the history of the universe can the cosmic microwave background radiation be traced? Was the radiation always in the form of microwaves? If not, what happened to change the
Suppose the spectral lines of distant galaxies were shifted toward the blue end. What would this suggest about the universe?
The sun and the giant outer planets contain hydrogen and helium in abundance; the inner planets, very little. Why?
Did the planets come into being before, during, or after the formation of the sun?
In what way or ways, if any, is the sun an unusual star?
Why is it reasonable to suppose that many stars besides the sun have planets orbiting them? Why does it seem unlikely that spacecraft from the earth will ever visit other planetary systems?
The brakes of a car moving at 14 m/s are applied, and the car comes to a stop in 4 s.(a) What was the car’s acceleration?(b) How long would the car take to come to a stop starting from 20 m/s with
The driver of a train moving at 20 m/s applies the brakes when it passes an amber signal. The next signal is 1 km down the track and the train reaches it 75 s later. Find the train’s acceleration
When a football is thrown, it follows a curved path through the air like the ones shown in Fig. 2-18. Where in its path is the ball’s speed greatest? Where is it least?
A crate is dropped from an airplane flying horizontally at constant speed. How does the path of the crate appear to somebody on the airplane? To somebody on the ground?
(a) Imagine that Charlotte drops a ball from a window on the twentieth floor of a building while at the same time Fred drops another ball from a window on the nineteenth floor of that building. As
How fast must a ball be thrown upward to reach a height of 12 m?
A ball dropped from the roof of a building takes 4 s to reach the street. How high is the building?
When will a stone thrown vertically upward at 9.8 m/s reach the ground?
The air resistance experienced by a falling object is not an important factor until a speed of about half its terminal speed is reached. The terminal speed of a golf ball is 40 m/s. How much time is
A rifle is aimed directly at the bull’s-eye of a target 50 m away. If the bullet’s speed is 350 m/s, how far below the bull’s-eye does the bullet strike the target?
A ball is thrown horizontally from the roof of a building 20 m high at 30 m/s. At what speed will the ball strike the ground?
An airplane whose speed is 60 m/s is flying at an altitude of 500 m over the ocean toward a stationary sinking ship.At what horizontal distance from the ship should the crew of the airplane drop a
A force of 20 N gives an object an acceleration of 5 m/s2.(a) What force would be needed to give the same object an acceleration of 1 m/s2? (b) What force would be needed to give it an
A 12,000-kg airplane launched by a catapult from an aircraft carrier is accelerated from 0 to 200 km/h in 3 s.(a) How many times the acceleration due to gravity is the airplane’s
A 430-g soccer ball at rest on the ground is kicked with a force of 600 N and flies off at 15 m/s. How long was the toe of the person kicking the ball in contact with it?
Before picking up the passengers, the driver of the car of Exercise 49 shifts into neutral when the car is moving at 80 km/h and finds that its speed has dropped to 65 km/h after 10 s. What was the
When a force equal to its weight is applied to an object free to move, what is its acceleration?
A mass of 8 kg and another of 12 kg are suspended by a string on either side of a frictionless pulley. Find the acceleration of each mass.
How much force is needed to give a 5-kg box an upward acceleration of 2 m/s2?
A person in an elevator suspends a 1-kg mass from a spring balance. What is the nature of the elevator’s motion when the balance reads 9.0 N? 9.8 N? 10.0 N?
A 60-kg person stands on a scale in an elevator. How many newtons does the scale read(a) when the elevator is ascending with an acceleration of 1 m/s2;(b) when it is descending with an acceleration
What is the relationship, if any, between the first and second laws of motion? Between the second and third laws of motion?
A car with its engine running and in forward gear goes up a hill and then down on the other side. What forces cause it to move upward? Downward?
Two children wish to break a string. Are they more likely to succeed if each takes one end of the string and they pull against each other, or if they tie one end of the string to a tree and both pull
Where should you stand on the earth’s surface to experience the most centripetal force? The least?
A person swings an iron ball in a vertical circle at the end of a string. At what point in the circle is the string most likely to break? Why?
When you whirl a ball at the end of a string, the ball seems to be pulling outward away from your hand. When you let the string go, however, the ball moves along a straight path perpendicular to the
The greatest force a level road can exert on the tires of a certain 2000-kg car is 4 kN. What is the highest speed the car can round a curve of radius 200 m without skidding?
Some people believe that aliens from elsewhere in the universe visit the earth in spacecraft that travel faster than jet airplanes and can turn in their own lengths. Calculate the centripetal force
An airplane flying at a constant speed of 160 m/s pulls out of a dive in a circular arc. The 80-kg pilot presses down on his seat with a force of 3000 N at the bottom of the arc. What is the radius
If the moon were half as far from the earth as it is today, how would the gravitational force it exerts on the earth compare with the force it exerts today?
A hole is bored to the center of the earth and a stone is dropped into it. How do the mass and weight of the stone at the earth’s center compare with their values at the earth’s surface?
A 2-kg mass is 1 m away from a 5-kg mass. What is the gravitational force (a) that the 5-kg mass exerts on the 2-kg mass, and (b) that the 2-kg mass exerts on the 5-kg mass? (c) If
A bull and a cow elephant, each of mass 2000 kg, attract each other gravitationally with a force of 2 × 10⁻5 N. How far apart are they?
Two satellites are launched from Cape Canaveral with the same initial speeds relative to the earth’s surface. One is sent toward the west, the other toward the east.Will there be any
Under what circumstances (if any) is no work done on a moving object even though a net force acts upon it?
A crate is pushed across a horizontal floor at constant speed by a horizontal force of 140 N, which is just enough to overcome the friction between the crate and the floor. (a) How much work is
A woman eats a cupcake and proposes to work off its 400-kJ energy content by exercising with a 15-kg barbell. If each lift of the barbell from chest height to overhead is through 60 cm and the
The kilowatt-hour is a unit of what physical quantity or quantities?
How much power must the legs of a 70-kg man develop in order to run up a staircase 5 m high in 9 s?
An escalator 14 m long is carrying a 70-kg person from one floor to another 8 m higher. The escalator has a speed of 1.0 m/s. (a) How much work does the escalator do in carrying the person to
A person’s metabolic processes can usually operate at a power of 6 W/kg of body mass for several hours at a time. If a 60-kg woman carrying a 12-kg pack is walking uphill with an energy-conversion
A total of 104 kg of water per second flows over a waterfall 30 m high. If half of the power this flow represents could be converted into electricity, how many 100-W lightbulbs could be supplied?
What is the speed of an 800-kg car whose KE is 250 kJ?
Is the work needed to bring a car’s speed from 0 to 10 km/h less than, equal to, or more than the work needed to bring its speed from 10 to 20 km/h? If the amounts of work are different, what is
During a circus performance, John Tailor was fired from a compressed-air cannon whose barrel was 20 m long.Mr. Tailor emerged from the cannon (twice on weekdays, three times on Saturdays and Sundays)
A 60-kg woman jumps off a wall 80 cm high and lands on a concrete road with her knees stiff. Her body is compressed by 8 cm at the moment of impact.(a) What was the average force the road exerted on
A 3-kg stone is dropped from a height of 100 m. Find its kinetic and potential energies when it is halfway to the ground.
(a) A yo-yo is swung in a vertical circle in such a way that its total energy KE + PE is constant. At what point in the circle is its speed a maximum? A minimum? Why? (b) If the yo-yo has a
A ramp 20 m long slopes down 1.2 m to the edge of a lake. A force of 300 N is needed to pull a boat on a trailer at constant speed up the ramp when they are clear of the water. If friction is
An 80-kg crate is raised 2 m from the ground by a man who uses a rope and a system of pulleys. He exerts a force of 220 N on the rope and pulls a total of 8 m of rope through the pulleys while
A person sitting under a coconut palm is struck by a 1-kg coconut that fell from a height of 20 m. (a) Find the kinetic energy of the coconut when it reaches the person.(b) Find the average
A force of 10 N is used to lift a 600-g ball from the ground to a height of 1.8 m, when it is let go. What is the speed of the ball when it is let go?
(a) When an object at rest explodes into two parts that fly apart, must they move in exactly opposite directions? (b) When a moving object strikes a stationary one, must they move off in exactly
A golf ball and a Ping-Pong ball are dropped in a vacuum chamber. When they have fallen halfway to the bottom, how do their speeds compare? Their kinetic energies? Their potential energies? Their
Is it possible for an object to have more kinetic energy but less momentum than another object? Less kinetic energy but more momentum?
The speed of an airplane doubles in flight. (a) How is the law of conservation of momentum obeyed in this situation? (b) The law of conservation of energy?
What, if anything, happens to the speed of a fighter plane when it fires a cannon at an enemy plane in front of it?
A railway car is at rest on a frictionless track. A man at one end of the car walks to the other end. (a) Does the car move while he is walking? (b) If so, in which direction? (c) What
A boy throws a 4-kg pumpkin at 8 m/s to a 40-kg girl on roller skates, who catches it. At what speed does the girl then move backward?
A 70-kg man and a 50-kg woman are in a 60-kg boat when its motor fails. The man dives into the water with a horizontal speed of 3 m/s in order to swim ashore. If he changes his mind, can he swim back
A 40-kg skater moving at 4 m/s overtakes a 60-kg skater moving at 2 m/s in the same direction and collides with her. The two skaters stick together. (a) What is their final speed? (b) How
A 1000-kg car moving east at 80 km/h collides head-on with a 1500-kg car moving west at 40 km/h, and the two cars stick together. (a) Which way does the wreckage move and with what initial speed? (b)
All helicopters have two rotors. Some have both rotors on vertical axes but rotating in opposite directions, and the rest have one rotor on a horizontal axis perpendicular to the helicopter body at
The earthquake that caused the Indian Ocean tsunami of 2004 (see Fig. 14.40) led to changes in the earth’s crust that reduced its diameter slightly. What effect, if any, do you think this reduction
What are the two postulates from which Einstein developed the special theory of relativity?
What physical quantity will all observers always find the same value for?
The potential energy of a golf ball in a hole is negative with respect to the ground. Under what circumstances (if any) is the ball’s kinetic energy negative? Its rest energy?
A certain walking person uses energy at an average rate of 300 W. All of this energy has its ultimate origin in the sun. How much matter is converted to energy in the sun per hour to supply this
When 1 g of gasoline is burned in an engine, about 48 kJ of heat is produced. How much mass is lost in the process? Do you think this mass change could be directly measured?
Approximately 4 × 109 kg of matter is converted into energy in the sun per second. Express the power output of the sun in watts.w
Even if the developed countries stabilize or reduce their energy consumption in years to come, worldwide energy consumption will increase. What are the two main reasons for this?
List the fossil fuels in the order in which they will probably be used up.
What energy sources cannot be traced to sunlight falling on the earth?
When a mercury-in-glass thermometer is heated, its mercury column goes down briefly before rising. Why?
Would it be more efficient to warm your bed on a cold night with a hot water bottle that contains 1 kg of water at 50°C or with a 1-kg gold bar at 50°C? Why?
Every body of matter radiates light. What is characteristic of light radiated by something very hot, such as the sun? Of light radiated by something at ordinary temperatures, such as the earth’s
List the chief greenhouse gases in the atmosphere. What property do they share?
(a) Why is deforestation so important in global warming?(b) In round numbers, which proportion of worldwide greenhouse gas emissions is due to deforestation:5 percent, 10 percent, 20 percent, 40
What fuel liberates the most energy per gram when it burns? What is produced when it burns?
How do the oil reserves in tar sands compare with the reserves of ordinary crude oil? What are some of the disadvantages of tar sands oil?
What is the chief reason why the average fuel efficiency of cars in the United States is the lowest in the world?
Why is natural gas rarely used as a vehicle fuel?
What are the chief advantages of coal as a fuel? The chief disadvantage?
Coal smoke contains sulfur and mercury. Why are they harmful?
(a) List the desirable aspects of coal gasification, the process in which coal is turned into a mixture of gases called syngas.(b) Is there any way to prevent the CO2 emitted when coal is burned from
What role does uranium play in nuclear energy production? What is the uranium supply situation?

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