Problems & Exercises

7.1 Work: The Scientific Definition


How much work does a supermarket checkout attendant do on a can of soup he pushes 0.600 m horizontally with a force of 5.00 N? Express your answer in joules and kilocalories.


A 75.0-kg person climbs stairs, gaining 2.50 meters in height. Find the work done to accomplish this task.


(a) Calculate the work done on a 1,500-kg elevator car by its cable to lift it 40.0 m at constant speed, assuming friction averages 100 N. (b) What is the work done on the lift by the gravitational force in this process? (c) What is the total work done on the lift?


Suppose a car travels 108 km at a speed of 30.0 m/s, and uses 2.0 gal of gasoline. Only 30 percent of the gasoline goes into useful work by the force that keeps the car moving at constant speed despite friction (See Table 7.1 for the energy content of gasoline). (a) What is the magnitude of the force exerted to keep the car moving at constant speed? (b) If the required force is directly proportional to speed, how many gallons will be used to drive 108 km at a speed of 28.0 m/s?


Calculate the work done by an 85.0-kg man who pushes a crate 4.00 m up along a ramp that makes an angle of 20.0º20.0º size 12{"20" "." 0°} {} with the horizontal (see Figure 7.36). He exerts a force of 500 N on the crate parallel to the ramp and moves at a constant speed. Be certain to include the work he does on the crate and on his body to get up the ramp.

A person is pushing a heavy crate up a ramp. The force vector F applied by the person is acting parallel to the ramp.
Figure 7.36 A man pushes a crate up a ramp.

How much work is done by the boy pulling his sister 30.0 m in a wagon, as shown in Figure 7.37? Assume no friction acts on the wagon.

A child is sitting inside a wagon and being pulled by a boy with a force F at an angle thirty degrees upward from the horizontal. F is equal to fifty newtons, the displacement vector d is horizontal in the direction of motion. The magnitude of d is thirty meters.
Figure 7.37 The boy does work on the system of the wagon and the child when he pulls them as shown.

A shopper pushes a grocery cart 20.0 m at constant speed on level ground, against a 35.0 N frictional force. He pushes in a direction 25.0º25.0º size 12{"25" "." 0°} {} below the horizontal. (a) What is the work done on the cart by friction? (b) What is the work done on the cart by the gravitational force? (c) What is the work done on the cart by the shopper? (d) Find the force the shopper exerts, using energy considerations. (e) What is the total work done on the cart?


Suppose the ski patrol lowers a rescue sled and victim, having a total mass of 90.0 kg, down a 60.0º60.0º size 12{"60" "." 0°} {} slope at constant speed, as shown in Figure 7.38. The coefficient of friction between the sled and the snow is 0.100. (a) How much work is done by friction as the sled moves 30.0 m along the hill? (b) How much work is done on the sled by the rope in this distance? (c) What is the work done on the sled by the gravitational force? (d) What is the total work done?

A person on a rescue sled is shown being pulled up a slope. The slope makes an angle of sixty degrees from the horizontal. The weight of the person is shown by vector w acting vertically downward. The tension in the rope depicted by vector T is along the incline in the upward direction; vector f depicting frictional force is also acting in the same direction.
Figure 7.38 A rescue sled and victim are lowered down a steep slope.

7.2 Kinetic Energy and the Work-Energy Theorem


Compare the kinetic energy of a 20,000-kg truck moving at 110 km/h with that of an 80.0-kg astronaut in orbit moving at 27,500 km/h.


(a) How fast must a 3,000-kg elephant move to have the same kinetic energy as a 65.0-kg sprinter running at 10.0 m/s? (b) Discuss how the larger energies needed for the movement of larger animals would relate to metabolic rates.


Confirm the value given for the kinetic energy of an aircraft carrier in Table 7.1. You will need to look up the definition of a nautical mile (1 knot = 1 nautical mile/h).


(a) Calculate the force needed to bring a 950-kg car to rest from a speed of 90.0 km/h in a distance of 120 m—a fairly typical distance for a non-panic stop. (b) Suppose instead the car hits a concrete abutment at full speed and is brought to a stop in 2.00 m. Calculate the force exerted on the car and compare it with the force found in part (a).


A car’s bumper is designed to withstand a 4.0-km/h (1.1-m/s) collision with an immovable object without damage to the body of the car. The bumper cushions the shock by absorbing the force over a distance. Calculate the magnitude of the average force on a bumper that collapses 0.200 m while bringing a 900-kg car to rest from an initial speed of 1.1 m/s.


Boxing gloves are padded to lessen the force of a blow. (a) Calculate the force exerted by a boxing glove on an opponent’s face, if the glove and face compress 7.50 cm during a blow in which the 7.00-kg arm and glove are brought to rest from an initial speed of 10.0 m/s. (b) Calculate the force exerted by an identical blow in the gory old days when no gloves were used and the knuckles and face would compress only 2.00 cm. (c) Discuss the magnitude of the force with gloves on. Does it seem high enough to cause damage even though it is lower than the force with no gloves?


Using energy considerations, calculate the average force a 60.0-kg sprinter exerts backward on the track to accelerate from 2.00 to 8.00 m/s in a distance of 25.0 m, if he encounters a headwind that exerts an average force of 30.0 N against him.

7.3 Gravitational Potential Energy


A hydroelectric power facility converts the gravitational potential energy of water behind a dam to energy (see Figure 7.39). (a) What is the gravitational potential energy relative to the generators of a lake of volume 50.0 km350.0 km3 size 12{"50" "." 0`"km" rSup { size 8{3} } } {} (mass=5.00×1013kg )mass=5.00×1013kg ) size 12{"mass"=5 "." "00" times "10" rSup { size 8{"13"} } `"kg " \) } {}, given that the lake has an average height of 40.0 m above the generators? (b) Compare this with the energy stored in a 9-megaton fusion bomb.

A dam with water flowing down its gates.
Figure 7.39 Hydroelectric facility. (Credit: Denis Belevich, Wikimedia Commons)

(a) How much gravitational potential energy (relative to the ground on which it is built) is stored in the Great Pyramid of Cheops, given that its mass is about 7 × 109 kg7 × 109 kg size 12{7 times "10" rSup { size 8{9} } " kg"} {} and its center of mass is 36.5 m above the surrounding ground? (b) How does this energy compare with the daily food intake of a person?


Suppose a 350-g kookaburra—a large kingfisher bird—picks up a 75-g snake and raises it 2.5 m from the ground to a branch. (a) How much work did the bird do on the snake? (b) How much work did it do to raise its own center of mass to the branch?


In Example 7.7, we found that the speed of a roller coaster that had descended 20.0 m was only slightly greater when it had an initial speed of 5.00 m/s than when it started from rest. This implies that ΔPE >> KEiΔPE >> KEi size 12{D"PE>>KE" rSub { size 8{i} } } {}. Confirm this statement by taking the ratio of ΔPEΔPE size 12{D"PE"} {} to KEiKEi size 12{"KE" rSub { size 8{i} } } {}. Note that mass cancels.


An object of mass m slides down a frictionless inclined plane that makes an angle θ with the horizontal. Write expressions for both the kinetic and potential energies as a function of time, assuming both are equal to zero at time t = 0. How do the kinetic and potential energies, as well as the total energy, depend on the construction of the inclined plane?


A 100-g toy car is propelled by a compressed spring that starts it moving. The car follows the curved track in Figure 7.40. Show that the final speed of the toy car is 0.687 m/s if its initial speed is 2.00 m/s and it coasts up the frictionless slope, gaining 0.180 m in altitude.

A toy car is moving up a curved track.
Figure 7.40 A toy car moves up a sloped track. (Credit: Leszek Leszczynski, Flickr)

In a downhill ski race, surprisingly, little advantage is gained by getting a running start. This absence of advantage is because the initial kinetic energy is tiny compared with to gain in gravitational potential energy on even small hills. To test this theory, find the final speed and the time taken for a skier who skies 70.0 m along a 30º30º size 12{"30"°} {} slope, neglecting friction. . (a) Calculate the speed and time with the skier starting at rest. (b) Now, calculate the speed and time with the skier starting at an initial speed of 2.50 m/s. (c) Does the answer surprise you? Discuss why it is still advantageous to get a running start in very competitive events.


The following is a contour plot showing isolines of gravitational potential for a collection of three point objects: object A ( 1,0 ) ( 1,0 ) , object B ( 0,1 ) ( 0,1 ) , and object C ( 1,0 ) ( 1,0 ) . List the three objects in order of increasing mass.

This is a graph of a contour plot, showing isolines of gravitational potential for a collection of three point objects: object A (-1, 0), object B (0, 1), and object C (1, 0). The lines are most widely spaced for object B, and most closely spaced for object C.
Figure 7.41 A contour plot showing the gravitational potential of three masses.

The mass of Earth is approximately 5.98× 10 24 5.98× 10 24 kg; the mass of the Moon is roughly 7.35× 10 22 7.35× 10 22 kg; and the distance between Earth and the moon is about 3.63× 10 5 3.63× 10 5 km. What is the gravitational potential at a distance 98 percent of the way from Earth to the Moon?


A point object with mass M is located at point x=a x=a on the x-axis. Which of the following cases will have the same gravitational potential at the origin? Justify your answers.

  • A pair of point objects, each with mass M/2 M/2 , located at x=a x=a and x=a x=a , respectively.

  • A point object with mass 2M located at point x=2a x=2a on the x-axis.

  • A spherical shell of negligible thickness with mass M centered at the origin.

  • A hemispherical shell of negligible thickness with mass M with its center at the origin.

Assuming that the gravitational potential approaches zero as the distance from a collection of objects with mass approaches infinity, is it possible to arrange the collection of objects so that there is a point of zero gravitational potential in the vicinity of the objects? Justify your answer.

7.4 Conservative Forces and Potential Energy


A 5.00×105-kg5.00×105-kg size 12{5 "." "00" times "10" rSup { size 8{5} } "-kg"} {} subway train is brought to a stop from a speed of 0.500 m/s in 0.400 m by a large spring bumper at the end of its track. What is the force constant kk size 12{k} {} of the spring?


A pogo stick has a spring with a force constant of 2.50×104 N/m2.50×104 N/m size 12{2 "." "50" times "10" rSup { size 8{4} } " N/m"} {}, which can be compressed 12.0 cm. To what maximum height can a child jump on the stick using only the energy in the spring, if the child and stick have a total mass of 40.0 kg? Explicitly show how you follow the steps in the Figure 7.21.

7.5 Nonconservative Forces


A 60.0-kg skier with an initial speed of 12.0 m/s coasts up a 2.50-m-high, rise as shown in Figure 7.42. Find her final speed at the top, given that the coefficient of friction between her skis and the snow is 0.0800. Hint—Find the distance traveled up the incline assuming a straight-line path, as shown in Figure 7.42.

A skier is about to go up an inclined slope with some initial speed v sub i shown by an arrow towards right. The slope makes a thirty-five-degree with the horizontal. The height of the point where the slope ends from the skiers’ starting position is two point five meters. Final speed of the skier at the end of the inclined slope is unknown.
Figure 7.42 The skier’s initial kinetic energy is partially used in coasting to the top of a rise.

(a) How high a hill can a car coast up with the engine disengaged if work done by friction is negligible and the car's initial speed is 110 km/h? (b) If, in actuality, a 750-kg car with an initial speed of 110 km/h is observed to coast up a hill to a height 22.0 m above its starting point, how much thermal energy was generated by friction? (c) What is the average force of friction if the hill has a slope 2.2. size 12{2 "." 5°} {} above the horizontal?

7.6 Conservation of Energy


Using values from Table 7.1, how many DNA molecules could be broken by the energy carried by a single electron in the beam of an old-fashioned TV tube? Understand that these electrons were not dangerous in themselves, but they did create dangerous X-rays. Later model tube TVs had shielding that absorbed X-rays before they escaped and exposed viewers.


Using energy considerations and assuming negligible air resistance, show that a rock thrown from a bridge 20.0 m above water with an initial speed of 15.0 m/s strikes the water with a speed of 24.8 m/s independent of the direction thrown.


If the energy in fusion bombs were used to supply the energy needs of the world, how many of the 9-megaton variety would be needed for a year’s supply of energy? Use data from Table 7.1? This is not as far-fetched as it may sound—there are thousands of nuclear bombs, and their energy can be trapped in underground explosions and converted to electricity, as natural geothermal energy is converted to electricity.


(a) Use of hydrogen fusion to supply energy is a dream that may be realized in the next century. Fusion would be a relatively clean and almost limitless supply of energy, as can be seen in Table 7.1. To illustrate this, calculate how many years the present energy needs of the world could be supplied by one millionth of the oceans’ hydrogen fusion energy. (b) How does this time compare with historically significant events, such as the duration of stable economic systems?

7.7 Power


The Crab Nebula pulsar is the remnant of a supernova that occurred in 1054 A.D. (see Figure 7.43). Using data from Table 7.3, calculate the approximate factor by which the power output of this astronomical object has declined since its explosion.

A supernova explosion.
Figure 7.43 Crab Nebula. (Credit: ESO, via Wikimedia Commons)

Suppose a star 1,000 times brighter than our Sun—that is, emitting 1,000 times the power—suddenly goes supernova. Using data from Table 7.3: (a) By what factor does its power output increase? (b) How many times brighter than our entire Milky Way Galaxy is the supernova? (c) Based on your answers, discuss whether it should be possible to observe supernovas in distant galaxies. Note that there are on the order of 10111011 size 12{"10" rSup { size 8{"11"} } } {} observable galaxies, the average brightness of which is somewhat less than our own galaxy.


A person in good physical condition can put out 100 W of useful power for several hours at a stretch, perhaps by pedaling a mechanism that drives an electric generator. Neglecting any problems of generator efficiency and practical considerations such as resting time: (a) How many people would it take to run a 4.00-kW electric clothes dryer? (b) How many people would it take to replace a large electric power plant that generates 800 MW?


What is the cost of operating a 3.00-W electric clock for a year if the cost of electricity is $0.0900 per kWhkWh size 12{"kW" cdot h} {}?


A large household air conditioner may consume 15.0 kW of power. What is the cost of operating this air conditioner 3.00 h per day for 30.0 d if the cost of electricity is $0.110 per kWhkWh size 12{"kW" cdot h} {}?


(a) What is the average power consumption in watts of an appliance that uses 5.00 kWh5.00 kWh size 12{5 "." "00 kW" cdot h} {} of energy per day? (b) How many joules of energy does this appliance consume in a year?


(a) What is the average useful power output of a person who does 6.00×106J6.00×106J size 12{6 "." "00" times "10" rSup { size 8{6} } " J"} {} of useful work in 8.00 h? (b) Working at this rate, how long will it take this person to lift 2,000 kg of bricks 1.50 m to a platform? Work done to lift his body can be omitted because it is not considered useful output here.


A 500-kg dragster accelerates from rest to a final speed of 110 m/s in 400 m, about a quarter of a mile, and encounters an average frictional force of 1,200 N. What is its average power output in watts and horsepower if this takes 7.30 s?


(a) How long will it take an 850-kg car with a useful power output of 40.0 hp (1 hp = 746 W) to reach a speed of 15.0 m/s, neglecting friction? (b) How long will this acceleration take if the car also climbs a 3.00-m-high hill in the process?


(a) Find the useful power output of an elevator motor that lifts a 2,500-kg load a height of 35.0 m in 12.0 s, if it also increases the speed from rest to 4.00 m/s. Note that the total mass of the counterbalanced system is 10,000 kg—so that only 2500 kg is raised in height, but the full 10,000 kg is accelerated. (b) What does it cost, if electricity is $0.0900 per kWhkWh size 12{"kW" cdot h} {}?


(a) What is the available energy content, in joules, of a battery that operates a 2.00-W electric clock for 18 months? (b) How long can a battery that can supply 8.00×104J8.00×104J size 12{8 "." "00" times "10" rSup { size 8{4} } " J"} {} run a pocket calculator that consumes energy at the rate of 1.00×103 W1.00×103 W size 12{1 "." "00" times "10" rSup { size 8{-3} } " W"} {}?


(a) How long would it take a 1.50×1051.50×105 size 12{1 "." "50" times "10" rSup { size 8{5} } } {}-kg airplane with engines that produce 100 MW of power to reach a speed of 250 m/s and an altitude of 12.0 km if air resistance were negligible? (b) If it actually takes 900 s, what is the power? (c) Given this power, what is the average force of air resistance if the airplane takes 1,200 s? Hint—You must find the distance the plane travels in 1,200 s, assuming constant acceleration.


Calculate the power output needed for a 950-kg car to climb a 2.00º2.00º slope at a constant 30.0 m/s while encountering wind resistance and friction totaling 600 N. Explicitly show how you follow the steps in the Problem-Solving Strategies for Energy.


(a) Calculate the power per square meter reaching Earth’s upper atmosphere from the Sun. Take the power output of the Sun to be 4.00×1026 W.4.00×1026 W. (b) Part of this is absorbed and reflected by the atmosphere, so that a maximum of 1.30 kW/m21.30 kW/m2 reaches Earth’s surface. Calculate the area in km2km2 of solar energy collectors needed to replace an electric power plant that generates 750 MW if the collectors convert an average of 2.00 percent of the maximum power into electricity. This small conversion efficiency is due to the devices themselves, and the fact that the Sun is directly overhead only briefly. With the same assumptions, what area would be needed to meet the United States’ energy needs (1.05×1020 J)?(1.05×1020 J)? size 12{ \( 1 "." "05" times "10" rSup { size 8{"20"} } " J" \) ?} {} Australia’s energy needs (5.4×1018 J)?(5.4×1018 J)? size 12{ \( 5 "." 4 times "10" rSup { size 8{"18"} } " J" \) ?} {} China’s energy needs (6.3×1019 J)?(6.3×1019 J)? size 12{ \( 6 "." 3 times "10" rSup { size 8{"19"} } " J" \) ?} {} These energy consumption values are from 2006.

7.8 Work, Energy, and Power in Humans


(a) How long can you rapidly climb stairs (116/min) on the 93.0 kcal of energy that you gained from a 10.0-g pat of butter? (b) How many flights is this if each flight has 16 stairs?


(a) What is the power output in watts and horsepower of a 70.0-kg sprinter who accelerates from rest to 10.0 m/s in 3.00 s? (b) Considering the amount of power generated, do you think a well-trained athlete could do this repetitively for long periods of time?


Calculate the power output in watts and horsepower of a shot putter who takes 1.20 s to accelerate the 7.27-kg shot from rest to 14.0 m/s, while raising it 0.800 m. Do not include the power produced to accelerate his body.

A close view of a shot-putter throwing the shot.
Figure 7.44 Shot putter at the Dornoch Highland Gathering in 2007. (Credit: John Haslam, Flickr)

(a) What is the efficiency of an out-of-condition professor who does 2.10×105 J2.10×105 J size 12{2 "." "10" times "10" rSup { size 8{5} } " J"} {} of useful work while metabolizing 500 kcal of food energy? (b) How many food calories would a well-conditioned athlete metabolize in doing the same work with an efficiency of 20 percent?


Energy that is not utilized for work or heat transfer is converted to the chemical energy of body fat containing about 39 kJ/g. How many grams of fat will you gain if you eat 10,000 kJ (about 2,500 kcal) one day and do nothing but sit relaxed for 16.0 h and sleep for the other 8.00 h? Use data from Table 7.5 for the energy consumption rates of these activities.


Using data from Table 7.5, calculate the daily energy needs of a person who sleeps for 7.00 h, walks for 2.00 h, attends classes for 4.00 h, cycles for 2.00 h, sits relaxed for 3.00 h, and studies for 6.00 h. Studying consumes energy at the same rate as sitting in class.


What is the efficiency of a subject on a treadmill who puts out work at the rate of 100 W while consuming oxygen at the rate of 2.00 L/min? Hint—See Table 7.5.


Shoveling snow can be extremely taxing because the arms have such a low efficiency in this activity. Suppose a person shoveling a footpath metabolizes food at the rate of 800 W. (a) What is her useful power output? (b) How long will it take her to lift 3000 kg of snow 1.20 m? This could be the amount of heavy snow on 20 m of footpath. (c) How much waste heat transfer in kilojoules will she generate in the process?


Very large forces are produced in joints when a person jumps from some height to the ground. (a) Calculate the magnitude of the force produced if an 80.0-kg person jumps from a 0.600-m-high ledge and lands stiffly, compressing joint material 1.50 cm as a result. Be certain to include the weight of the person. (b) In reality, the knees bend almost involuntarily to help extend the distance over which you stop. Calculate the magnitude of the force produced if the stopping distance is 0.300 m. (c) Compare both forces with the weight of the person.


Jogging on hard surfaces with insufficiently padded shoes produces large forces in the feet and legs. (a) Calculate the magnitude of the force needed to stop the downward motion of a jogger’s leg, if his leg has a mass of 13.0 kg, a speed of 6.00 m/s, and stops in a distance of 1.50 cm. Be certain to include the weight of the 75.0-kg jogger’s body. (b) Compare this force with the weight of the jogger.


(a) Calculate the energy in kJ used by a 55.0-kg woman who does 50 deep knee bends in which her center of mass is lowered and raised 0.400 m. She does work in both directions. You may assume her efficiency is 20 percent. (b) What is the average power consumption rate in watts if she does this in 3.00 min?


Kanellos Kanellopoulos flew 119 km from Crete to Santorini, Greece, on April 23, 1988, in the Daedalus 88, an aircraft powered by a bicycle-type drive mechanism (see Figure 7.45). His useful power output for the 234-min trip was about 350 W. Using the efficiency for cycling from Table 7.2, calculate the food energy in kilojoules he metabolized during the flight.

An aircraft flying in the air shown from outside, which is powered by a bicycle-type drive mechanism.
Figure 7.45 The Daedalus 88 in flight. (Credit: NASA photo by Beasley)

The swimmer shown in Figure 7.46 exerts an average horizontal backward force of 80.0 N with his arm during each 1.80 m long stroke. (a) What is his work output in each stroke? (b) Calculate the power output of his arms if he does 120 strokes per minute.

A person swimming and his arm motion in each stroke is represented. The arm action is shown by an arc starting from where his hand enters the surface of the water and ending at the point where his hand emerges from the water. The diameter of this arc made by the person’s hand tip in one stroke is one point eight meters.
Figure 7.46 This swimmer's stroke is 1.8 meters long.

Mountain climbers carry bottled oxygen when at very high altitudes. (a) Assuming that a mountain climber uses oxygen at twice the rate for climbing 116 stairs per minute—due to low air temperature and high winds—calculate how many liters of oxygen a climber would need for 10.0 h of climbing. These are liters at sea level. Note that only 40 percent of the inhaled oxygen is utilized; the rest is exhaled. (b) How much useful work does the climber do if he and his equipment have a mass of 90.0 kg and he gains 1000 m of altitude? (c) What is his efficiency for the 10.0-h climb?


The awe-inspiring Great Pyramid of Cheops was built more than 4,500 years ago. Its square base, originally 230 m on a side, covered 13.1 acres, and was 146 m high, with a mass of about 7×109kg7×109kg. The pyramid’s dimensions are slightly different today due to quarrying and some sagging. Historians estimate that 20,000 workers spent 20 years to construct it, working 12-hour days, 330 days per year. (a) Calculate the gravitational potential energy stored in the pyramid, given its center of mass is at one-fourth its height. (b) Only a fraction of the workers lifted blocks; most were involved in support services such as building ramps, bringing food... the site (see Figure 7.47). Calculate the efficiency of the workers who did the lifting, assuming there were 1,000 of them and they consumed food energy at the rate of 300 kcal/h. What does your answer imply about how much of their work went into block-lifting, versus how much work went into friction and lifting and lowering their own bodies? (c) Calculate the mass of food that had to be supplied each day, assuming that the average worker required 3,600 kcal per day and that their diet was 5 percent protein, 60 percent carbohydrate, and 35 percent fat. These proportions neglect the mass of bulk and nondigestible materials consumed.

A model of a step pyramid is shown with ramps along the sides of each step.
Figure 7.47 Ancient pyramids were probably constructed using ramps as simple machines. (Credit: Franck Monnier, Wikimedia Commons)

(a) How long can you play tennis on the 800 kJ (about 200 kcal) of energy found in a candy bar? (b) Does this seem like a long time? Discuss why exercise is necessary but may not be sufficient to cause a person to lose weight.

7.9 World Energy Use


Integrated Concepts

(a) Calculate the force the woman in Figure 7.48 exerts to do a push-up at constant speed, taking all data to be known to three digits. (b) How much work does she do if her center of mass rises 0.240 m? (c) What is her useful power output if she does 25 push-ups in 1 min? Should work done lowering her body be included? See the discussion of useful work in Work, Energy, and Power in Humans.

A woman is doing push-ups. Her weight w is acting on her center of gravity , shown by a vector pointing downwards. Her center of gravity  is zero point nine zero meters from her feet and reaction force F acting on her arms is shown by the vector pointing upward along her arms. The distance of reaction force from the feet is one point five zero meters.
Figure 7.48 Forces involved in doing push-ups. The woman’s weight acts as a force exerted downward on her center of gravity (CG).

Integrated Concepts

A 75.0-kg cross-country skier is climbing a 3.3. size 12{3 "." 0°} {} slope at a constant speed of 2.00 m/s and encounters air resistance of 25.0 N. Find his power output for work done against the gravitational force and air resistance. (b) What average force does he exert backward on the snow to accomplish this? (c) If he continues to exert this force and experience the same air resistance when he reaches a level area, how long will it take him to reach a velocity of 10.0 m/s?


Integrated Concepts

The 70.0-kg swimmer in Figure 7.46 starts a race with an initial velocity of 1.25 m/s and exerts an average force of 80.0 N backward with his arms during each 1.80 m long stroke. (a) What is his initial acceleration if water resistance is 45.0 N? (b) What is the subsequent average resistance force from the water during the 5.00 s it takes him to reach his top velocity of 2.50 m/s? (c) Discuss whether water resistance seems to increase linearly with velocity.


Integrated Concepts

A toy gun uses a spring with a force constant of 300 N/m to propel a 10.0-g steel ball. If the spring is compressed 7.00 cm and friction is negligible: (a) How much force is needed to compress the spring? (b) To what maximum height can the ball be shot? (c) At what angles above the horizontal may a child aim to hit a target 3.00 m away at the same height as the gun? (d) What is the gun’s maximum range on level ground?


Integrated Concepts

(a) What force must be supplied by an elevator cable to produce an acceleration of 0.800 m/s20.800 m/s2 size 12{0 "." "800 m/s" rSup { size 8{2} } } {} against a 200-N frictional force, if the mass of the loaded elevator is 1,500 kg? (b) How much work is done by the cable in lifting the elevator 20.0 m? (c) What is the final speed of the elevator if it starts from rest? (d) How much work went into thermal energy?


Unreasonable Results

A car advertisement claims that its 900-kg car accelerated from rest to 30.0 m/s and drove 100 km, gaining 3.00 km in altitude, on 1.0 gal of gasoline. The average force of friction including air resistance was 700 N. Assume all values are known to three significant figures. (a) Calculate the car’s efficiency. (b) What is unreasonable about the result? (c) Which premise is unreasonable, or which premises are inconsistent?


Unreasonable Results

Body fat is metabolized, supplying 9.30 kcal/g, when dietary intake is less than needed to fuel metabolism. The manufacturers of an exercise bicycle claim that you can lose 0.500 kg of fat per day by vigorously exercising for 2.00 h per day on their machine. (a) How many kcal are supplied by the metabolization of 0.500 kg of fat? (b) Calculate the kcal/min that you would have to utilize to metabolize fat at the rate of 0.500 kg in 2.00 h. (c) What is unreasonable about the results? (d) Which premise is unreasonable, or which premises are inconsistent?


Construct Your Own Problem

Consider a person climbing and descending stairs. Construct a problem in which you calculate the long-term rate at which stairs can be climbed considering the mass of the person, his ability to generate power with his legs, and the height of a single stair step. Also consider why the same person can descend stairs at a faster rate for a nearly unlimited time in spite of the fact that very similar forces are exerted going down as going up. This points to a fundamentally different process for descending versus climbing stairs.


Construct Your Own Problem

Consider humans generating electricity by pedaling a device similar to a stationary bicycle. Construct a problem in which you determine the number of people it would take to replace a large electrical generation facility. Among the things to consider are the power output that is reasonable using the legs, rest time, and the need for electricity 24 hours per day. Discuss the practical implications of your results.


Integrated Concepts

A 105-kg basketball player crouches down 0.400 m while waiting to jump. After exerting a force on the floor through this 0.400 m, his feet leave the floor and his center of gravity rises 0.950 m above its normal standing erect position. (a) Using energy considerations, calculate his velocity when he leaves the floor. (b) What average force did he exert on the floor? Do not neglect the force to support his weight as well as that to accelerate him. (c) What was his power output during the acceleration phase?