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physics
mechanics

Physics 5th edition James S. Walker - Solutions

A box is placed on a conveyor belt that moves with a constant speed of 1.25 m/s. The coefficient of kinetic friction between the box and the belt is 0.780. (a) How much time does it take for the box to stop sliding relative to the belt? (b) How far does the box move in this time?
To move a large crate across a rough floor, you push on it with a force F at an angle of 21° below the horizontal, as shown in FIGURE 6-34. Find the force necessary to start the crate moving, given that the mass of the crate is 32 kg and the coefficient of static friction between the crate and
As part of a circus act, a person drives a motorcycle with constant speed v around the inside of a vertical track of radius r, as indicated in FIGURE 6-75. If the combined mass of the motorcycle and rider is m, find the normal force exerted on the motorcycle by the track at the points(a) A,(b)
On the straight-line segment II in Figure 6-76 (b) we see that increasing the applied mass from 20 g to 25 g results in a reduction of the end-to-end distance from 42 mm to 24 mm. What is the force constant in N/m on segment II?A. 0.28 N/mB. 0.37 N/mC. 2.7 N/mD. 3.6 N/m
Rank the straight segments I, II, and III in order of increasing "stiffness" of the nasal strips.
In use on a typical human nose, the end-to-end distance is measured to be 30 mm. What force does the strip exert on the nasal passages in this situation?A. 0.23 NB. 0.69 NC. 9.0 ND. 2 N
Suppose the coefficients of static and kinetic friction between the crate and the truck bed are 0.415 and 0.382, respectively. (a) Does the crate begin to slide at a tilt angle that is greater than, less than, or equal to 23.2°? (b) Verify your answer to part (a) by determining the angle at which
The crate begins to slide when the tilt angle is 17.5°. When the crate reaches the bottom of the flatbed, after sliding a distance of 2.75 m, its speed is 3.11 m/s.(a) The coefficient of static friction(b) The coefficient of kinetic friction between the crate and the flatbed.
Suppose that the mass on the frictionless tabletop has the value m1 = 2.45 kg.(a) Find the value of m2 that gives an acceleration of 2.85 m/s2.(b) What is the corresponding tension, T, in the string?(c) Calculate the ratio T/m2g and show that it is less than 1, as expected.
(a) At what speed will the force of static friction exerted on the car by the road be equal to half the weight of the car? The mass of the car is m = 1200 kg, the radius of the corner is r = 45 m, and the coefficient of static friction between the tires and the road is μs = 0.82. (b) Suppose that
A 37-kg crate is placed on an inclined ramp. When the angle the ramp makes with the horizontal is increased to 23°, the crate begins to slide downward.(a) What is the coefficient of static friction between the crate and the ramp?(b) At what angle does the crate begin to slide if its mass is
Engine 1 produces twice the power of engine 2. Is it correct to conclude that engine 1 does twice as much work as engine 2? Explain.
A friend makes the statement, "Only the total force acting on an object can do work." Is this statement true or false? If it is true, state why; if it is false, give a counterexample.
A ski boat moves with constant velocity. Is the net force acting on the boat doing work? Explain.
A pendulum bob swings from point I to point II along the circular arc indicated in FIGURE 7-19.(a) Is the work done on the bob by gravity positive, negative, or zero? Explain.(b) Is the work done on the bob by the string positive, negative, or zero? Explain.
A 57-kg packing crate is pulled with constant speed across a rough floor with a rope that is at an angle of 37° above the horizontal. If the tension in the rope is 142 N, how much work is done on the crate to move it 6.1 m?
To clean a floor, a janitor pushes on a mop handle with a force of 50.0 N.(a) If the mop handle is at an angle of 55° above the horizontal, how much work is required to push the mop 0.50 m?(b) If the angle the mop handle makes with the horizontal is increased to 65°, does the work done by the
A small plane tows a glider at constant speed and altitude. If the plane does 2.00 × 105 J of work to tow the glider 145 m and the tension in the tow rope is 2560 N, what is the angle between the tow rope and the horizontal?
As a snowboarder descends a mountain slope, gravity, the normal force, and friction combine to exert a net force of 320 N in the downhill direction. The net force does 4700 J of work on her during one segment of her descent. If her path cuts across the face of the mountain at a 65° angle from the
A young woman on a skateboard is pulled by a rope attached to a bicycle. The velocity of the skateboarder is v(vector) = (4.1 m/s) x̂ and the force exerted on her by the rope is F(vector) = (17 N) x̂ + (12 N)ŷ.(a) Find the work done on the skateboarder by the rope in 25 seconds.(b) Assuming the
To keep her dog from running away while she talks to a friend, Susan pulls gently on the dog’s leash with a constant force given by F(vector) = (2.2 N)x̂ + (1.1 N)ŷ. How much work does she do on the dog if its displacement is(a) D(vector) = (0.25 m)x̂,(b) D(vector) = (0.25 m)ŷ,(c) D(vector)
Water skiers often ride to one side of the center line of a boat, as shown in FIGURE 7-20. In this case, the ski boat is traveling at 12 m/s and the tension in the rope is 91 N. If the boat does 3800 J of work on the skier in 50.0 m, what is the angle u between the tow rope and the center line of
A pitcher throws a ball at 90 mi/h and the catcher stops it in her glove. (a) Is the work done on the ball by the pitcher positive, negative, or zero? Explain. (b) Is the work done on the ball by the catcher positive, negative, or zero? Explain.
How much work is needed for a 73-kg runner to accelerate from rest to 7.7 m/s?
When Skylab reentered the Earth's atmosphere on July 11, 1979, it broke into a myriad of pieces. One of the largest fragments was a 1770-kg lead-lined film vault, and it landed with an estimated speed of 120 m/s. What was the kinetic energy of the film vault when it landed?
A pendulum bob swings from point II to point III along the circular arc indicated in Figure 7-19.(a) Is the work done on the bob by gravity positive, negative, or zero? Explain. (b) Is the work done on the bob by the string positive, negative, or zero? Explain.
A 9.50-g bullet has a speed of 1.30 km/s. (a) What is its kinetic energy in joules? (b) What is the bullet's kinetic energy if its speed is halved? (c) If its speed is doubled?
The energy required to increase the speed of a certain car from 18 m/s to 24 m/s is 190 kJ. (a) What is the mass of the car? (b) At what speed is the car's kinetic energy equal to 190 kJ?
The work W0 accelerates a car from 0 to 50 km/h. (a) Is the work required to accelerate the car from 50 km/h to 150 km/h equal to 2W0, 3W0, 8W0, or 9W0? (b) Choose the best explanation from among the following: I. The work to accelerate the car depends on the speed squared. II. The final speed is
Car A has a mass m and a speed v, car B has a mass m/2 and a speed 3v, car C has a mass 3m and a speed v/2, and car D has a mass 4m and a speed v/2. Rank the cars in order of increasing kinetic energy. Indicate ties where appropriate.
A 0.14-kg pinecone falls 16 m to the ground, where it lands with a speed of 13 m/s. (a) With what speed would the pinecone have landed if there had been no air resistance? (b) Did air resistance do positive work, negative work, or zero work on the pinecone? Explain.
In the previous problem, In Previous problem A 0.14-kg pinecone falls 16 m to the ground, where it lands with a speed of 13 m/s. (a) How much work was done on the pinecone by air resistance? (b) What was the average force of air resistance exerted on the pinecone?
At t = 1.0 s, a 0.55-kg object is falling with a speed of 6.5 m/s. At t = 2.0 s, it has a kinetic energy of 38 J. (a) What is the kinetic energy of the object at t = 1.0 s? (b) What is the speed of the object at t = 2.0 s? (c) How much work was done on the object between t = 1.0 s and t = 2.0 s?
After hitting a long fly ball that goes over the right fielder's head and lands in the outfield, the batter decides to keep going past second base and try for third base. The 62.0-kg player begins sliding 3.40 m from the base with a speed of 4.35 m/s. If the player comes to rest at third base, (a)
A 1100-kg car coasts on a horizontal road with a speed of 19 m/s. After crossing an unpaved, sandy stretch of road 32 m long, its speed decreases to 12 m/s. (a) Was the net work done on the car positive, negative, or zero? Explain. (b) Find the magnitude of the average net force on the car in the
A 65-kg bicyclist rides his 8.8-kg bicycle with a speed of 14 m/s.(a) How much work must be done by the brakes to bring the bike and rider to a stop?(b) How far does the bicycle travel if it takes 4.0 s to come to rest?(c) What is the magnitude of the braking force?
A farmhand pushes a 26-kg bale of hay 3.9 m across the floor of a barn. If she exerts a horizontal force of 88 N on the hay, how much work has she done?
A block of mass m and speed v collides with a spring, compressing it a distance Δx. What is the compression of the spring if the force constant of the spring is increased by a factor of four?
A spring with a force constant of 3.5 × 104 N/m is initially at its equilibrium length.(a) How much work must you do to stretch the spring 0.050 m?(b) How much work must you do to compress it 0.050 m?
Initially sliding with a speed of 4.1 m/s, a 1.5-kg block collides with a spring and compresses it 0.27 m before coming to rest. What is the force constant of the spring?
The force shown in FIGURE 7-21 moves an object from x = 0 to x = 0.75 m.(a) How much work is done by the force?(b) How much work is done by the force if the object moves from x = 0.15 m to x = 0.60 m?
An object is acted on by the force shown in FIGURE 7-22. What is the final position of the object if its initial position is x = 0.40 m and the work done on it is equal to(a) 0.21 J,(b) -0.19 J?
To compress spring 1 by 0.20 m takes 150 J of work. Stretching spring 2 by 0.30 m requires 210 J of work. Which spring is stiffer?
It takes 180 J of work to compress a certain spring 0.15 m.(a) What is the force constant of this spring?(b) To compress the spring an additional 0.15 m, does it take 180 J, more than 180 J, or less than 180 J? Verify your answer with a calculation.
The force shown in Figure 7-22 acts on a 1.3-kg object whose initial speed is 0.35 m/s and initial position is x = 0.27 m. (a) Find the speed of the object when it is at the location x = 0.99 m. (b) At what location would the object's speed be 0.25 m/s?
A block is acted on by a force that varies as (2.0 × 104 N/m) x for 0 ≤ x ≤ 0.21 m, and then remains constant at 4200 N for larger x. How much work does the force do on the block in moving it (a) From x = 0 to x = 0.30 m, (b) From x = 0.10 m to x = 0.40 m?
Force F1 does 5 J of work in 10 seconds, force F2 does 3 J of work in 5 seconds, force F3 does 6 J of work in 18 seconds, and force F4 does 25 J of work in 125 seconds. Rank these forces in order of increasing power they produce. Indicate ties where appropriate.
Children in a tree house lift a small dog in a basket 3.85 m up to their house. If it takes 201 J of work to do this, what is the combined mass of the dog and basket?
A new record for running the stairs of the Empire State Building was set on February 4, 2003. The 86 flights, with a total of 1576 steps, was run in 9 minutes and 33 seconds. If the height gain of each step was 0.20 m, and the mass of the runner was 70.0 kg, what was his average power output during
Calculate the power output of a 14-mg fly as it walks straight up a windowpane at 2.3 cm/s.
Your car produces about 34 kW of power to maintain a constant speed of 31 m/s on the highway. What average force does the engine exert?
You raise a bucket of water from the bottom of a deep well. If your power output is 108 W, and the mass of the bucket and the water in it is 5.00 kg, with what speed can you raise the bucket? Ignore the weight of the rope.
As Chinook salmon swim upstream in the Yukon River to spawn, they must travel about 3000 km in about 90 days, and in the process each fish must do about 1.7 × 106 J of work. (a) What is the average power output of a Chinook salmon as it swims upstream? (b) What average force does the salmon exert
In order to keep a leaking ship from sinking, it is necessary to pump 12.0 lb of water each second from below deck up a height of 2.00 m and over the side. What is the minimum horsepower motor that can be used to save the ship?
A kayaker paddles with a power output of 35.0 W to maintain a steady speed of 2.24 m/s.(a) Calculate the resistive force exerted by the water on the kayak.(b) If the kayaker doubles her power output, and the resistive force due to the water remains the same, by what factor does the kayaker's speed
Human-powered aircraft require a pilot to pedal, as in a bicycle, and produce a sustained power output of about 0.30 hp. The Gossamer Albatross flew across the English Channel on June 12, 1979, in 2 h 49 min. (a) How much energy did the pilot expend during the flight? (b) What is the minimum number
A sailboat can be propelled into the wind by a maneuver called beating to windward. Beating requires the sailboat to travel in a zigzag pattern at an angle to the wind that is greater than the no-go zone, which is shaded red in FIGURE 7-23. When a sailboat is just outside the no-go zone (boats B in
Early one October, you go to a pumpkin patch to select your Halloween pumpkin. You lift the 3.2-kg pumpkin to a height of 1.2 m, then carry it 50.0 m (on level ground) to the check-out stand.(a) Calculate the work you do on the pumpkin as you lift it from the ground.(b) How much work do you do on
A grandfather clock is powered by the descent of a 4.35-kg weight. (a) If the weight descends through a distance of 0.760 m in 3.25 days, how much power does it deliver to the clock? (b) To increase the power delivered to the clock, should the time it takes for the mass to descend be increased or
A certain car can accelerate from rest to the speed v in T seconds. If the power output of the car remains constant,(a) How much time does it take for the car to accelerate from v to 2v?(b) How fast is the car moving at 2T seconds after starting?
As the three small sailboats shown in FIGURE 7-24 drift next to a dock, because of wind and water currents, students pull on a line attached to the bow and exert forces of equal magnitude F. Each boat drifts through the same distance d. Rank the three boats (A, B, and C) in order of increasing work
A car is accelerated by a constant force, F. The distance required to accelerate the car from rest to the speed v is Δx.(a) Is the distance required to accelerate the car from the speed v to the speed 2v equal to Δx, 2Δx, 3Δx, or 4Δx?(b) Choose the best explanation from among the following:I.
Car 1 has four times the mass of car 2, but they both have the same kinetic energy. If the speed of car 2 is v, is the speed of car 1 equal to v/4, v/2, 2v, or 4v? Explain.
Biological muscle cells can be thought of as nanomotors that use the chemical energy of ATP to produce mechanical work. Measurements show that the active proteins within a muscle cell (such as myosin and act in) can produce a force of about 1.5 pN and displacements of 8.0 nm. How much work is done
A small motor runs a lift that raises a load of bricks weighing 836 N to a height of 10.7 m in 23.2 s. Assuming that the bricks are lifted with constant speed, what is the minimum power the motor must produce?
You push a 67-kg box across a floor where the coefficient of kinetic friction is μk = 0.55. The force you exert is horizontal. (a) How much power is needed to push the box at a speed of 0.50 m/s? (b) How much work do you do if you push the box for 35 s?
A 1300-kg elevator is lifted at a constant speed of 1.3 m/s through a height of 22 m. How much work is done(a) By the tension in the elevator cable(b) By gravity?
The work W0 is required to accelerate a car from rest to the speed v0. How much work is required to accelerate the car(a) From rest to the speed v0/2(b) From v0/2 to v0?
The coefficient of kinetic friction between a suitcase and the floor is 0.272. If the suitcase has a mass of 71.5 kg, how far can it be pushed across the level floor with 642 J of work?
After a tornado, a 0.55-g straw was found embedded 2.3 cm into the trunk of a tree. If the average force exerted on the straw by the tree was 65 N, what was the speed of the straw when it hit the tree?
You throw a glove straight upward to celebrate a victory. Its initial kinetic energy is K and it reaches a maximum height h. What is the kinetic energy of the glove when it is at the height h/2?
The water skier in Figure 7-20 is at an angle of 32.5° with respect to the center line of the boat, and is being pulled at a constant speed of 15.6 m/s. If the tension in the tow rope is 111 N,(a) How much work does the rope do on the skier in 10.0 s?(b) How much work does the resistive force of
A sled with a mass of 5.80 kg is pulled along the ground through a displacement given by d(vector) = (4.55 m)x̂. (Let the x axis be horizontal and the y axis be vertical.)(a) How much work is done on the sled when the force acting on it is F(vector) = (2.89 N) x̂ + (0.131 N)ŷ?(b) How
A 0.19-kg apple falls from a branch 3.5 m above the ground. (a) Does the power delivered to the apple by gravity increase, decrease, or stay the same during the time the apple falls to the ground? Explain. Find the power delivered by gravity to the apple when the apple is (b) 2.5 m (c) 1.5 m above
A boy pulls a bag of baseball bats across a ball field toward the parking lot. The bag of bats has a mass of 6.80 kg, and the boy exerts a horizontal force of 24.0 N on the bag. As a result, the bag accelerates from rest to a speed of 1.12 m/s in a distance of 5.25 m. What is the coefficient of
At the instant it leaves the player's hand after a jump shot, a 0.62-kg basketball is traveling 8.4 m/s. When it passes through the hoop its speed is 6.8 m/s.(a) How much work has gravity done on the ball?(b) What is the vertical component of the ball's displacement?(c) If the magnitude of the
The force shown in FIGURE 7-25 acts on an object that moves along the x axis. How much work is done by the force as the object moves from(a) X = 0 to x = 2.0 m,(b) X = 1.0 m to x = 4.0 m,(c) X = 3.5 m to x = 1.2 m?
A compound bow in archery allows the user to hold the bowstring at full draw with considerably less force than the maximum force exerted by the string. The draw force as a function of the string position x for a particular compound bow is shown in FIGURE 7-26.(a) How much work does the archer do on
The compound bow in the previous problem requires 152 J of work to draw the string back to x = 0.540 m, at which point the archer need only exert 250 N of force to hold the string in place. (a) Suppose a simple bow is used instead of a compound bow. What should its force constant k (in N/m) be so
Calculate the power output of a 0.42-g spider as it walks up a windowpane at 2.3 cm/s. The spider walks on a path that is at 25° to the vertical, as illustrated in FIGURE 7-27.
To make a batch of cookies, you mix half a bag of chocolate chips into a bowl of cookie dough, exerting a 28-N force on the stirring spoon. Assume that your force is always in the direction of motion of the spoon. (a) What power is needed to move the spoon at a speed of 0.44 m/s? (b) How much work
A pitcher accelerates a 0.14-kg hardball from rest to 42.5 m/s in 0.060 s. (a) How much work does the pitcher do on the ball? (b) What is the pitcher's power output during the pitch? (c) Suppose the ball reaches 42.5 m/s in less than 0.060 s. Is the power produced by the pitcher in this case more
The human brain consumes about 22 W of power under normal conditions, though more power may be required during exams.(a) For what amount of time can one Snickers bar (see the note following Problem 48) power the normally functioning brain?(b) At what rate must you lift a 3.6-kg container of milk
On October 9, 1992, a 27-pound meteorite struck a car in Peekskill, NY, creating a dent about 22 cm deep. If the initial speed of the meteorite was 550 m/s, what was the average force exerted on the meteorite by the car?
A pigeon in flight experiences a force of air resistance given approximately by F = bv2, where v is the flight speed and b is a constant. (a) What are the units of the constant b? (b) What is the greatest possible speed of the pigeon if its maximum power output is P? (c) By what factor does the
Two springs, with force constants k1 and k2, are connected in series, as shown in FIGURE 7-28. How much work is required to stretch this system a distance x from the equilibrium position?
Two springs, with force constants k1 and k2, are connected in parallel, as shown in FIGURE 7-29. How much work is required to stretch this system a distance x from the equilibrium position?
A block rests on a horizontal frictionless surface. A string is attached to the block, and is pulled with a force of 45.0 N at an angle Î¸ above the horizontal, as shown in FIGURE 7-30. After the block is pulled through a distance of 1.50 m, its speed is 2.60 m/s, and 50.0 J of work has
Estimate the range of flight speeds for Microraptor gui if its power output is 9.8 W.A. 0-7.7 m/sB. 7.7-15 m/sC. 15-30 m/sD. 0-15 m/s
A tow rope, parallel to the water, pulls a water skier directly behind the boat with constant velocity for a distance of 72 m before the skier falls. The tension in the rope is 140 N. (a) Is the work done on the boat by the rope positive, negative, or zero? Explain. (b) Calculate the work done by
What approximate range of flight speeds would be possible if Microraptor gui could produce 20 W of power?A. 0-25 m/sB. 25-30 m/sC. 2.5-25 m/sD. 0-2.5 m/s
How much energy would Microraptor have to expend to fly with a speed of 10 m/s for 1.0 minute?A. 8.1 JB. 81 JC. 490 JD. 600 J
Estimate the minimum force that Microraptor must exert to fly.A. 0.65 NB. 1.3 NC. 1.0 ND. 10 N
Suppose the block has a mass of 1.4 kg and an initial speed of 0.62 m/s.(a) What force constant must the spring have if the maximum compression is to be 2.4 cm?(b) If the spring has the force constant found in part (a), find the maximum compression if the mass of the block is doubled and its
In the situation shown in Figure 7-15 (d), a spring with a force constant of 750 N/m is compressed by 4.1 cm.In figure 7-15(a) If the speed of the block in Figure 7-15 (f) is 0.88 m/s, what is its mass?(b) If the mass of the block is doubled, is the final speed greater than, less than, or equal to
Suppose the car has a mass of 1400 kg and delivers 48 hp to the wheels.(a) How much time does it take for the car to increase its speed from 15 m/s to 25 m/s?(b) Would the time required to increase the speed from 5.0 m/s to 15 m/s be greater than, less than, or equal to the time found in part
A child pulls a friend in a little red wagon with constant speed. If the child pulls with a force of 16 N for 10.0 m, and the handle of the wagon is inclined at an angle of 25° above the horizontal, how much work does the child do on the wagon?
If the stretch of a spring is doubled, the force it exerts is also doubled. By what factor does the spring's potential energy increase?
If a spring is stretched so far that it is permanently deformed, its force is no longer conservative. Why?
It is a law of nature that the total energy of the universe is conserved. What do politicians mean, then, when they urge "energy conservation"?

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