Quiz 5: Work and Energy
Physics & Astronomy
Q 1Q 1
(a) As a weightlifter lifts a barbell from the floor in the "clean" procedure (Fig. a), has he done work Why or why not (b) In raising the barbell above his head in the "jerk" procedure, is he doing work Explain. (c) In holding the barbell above his head (Fig. b), is he doing more work, less work, or the same amount of work as in lifting the barbell Explain. (d) If the weightlifter drops the barbell, is work done on the barbell Explain what happens in this situation.
FIGURE Man at work See Conceptual.
Free
Essay
(a)
Yes. The weight lifter does positive work on the barbell.
Work done on an object is equal to the product of the force
applied on the object and the displacement
of the object.
Here,
is the angle between force and displacement.
Here, in this case the weight lifter applied force on the barbell in the upward direction which is against the gravitational force and the displacement of the barbell is also in the direction of application of force. Thus, the weight lifter does positive work in this clean procedure.
(b)
Yes, the weight lifter had done work in lifting the bare bell above his head.
Here, also the displacement of the Barbell is in the direction of application of force. Thus, the weight lifter does positive work.
(c)
No work is done by the weight lifter is holding the bare bell above his head.
Holding the barbell at the position which is above his head implies there is no displacement of the barbell even though the force is applied. Thus, no work is done on it.
Thus, the work done is holding the bare bell above the lifter's head is less when compared to the work done by the lifter in raising the bare bell above his head.
(d)
Yes, work is done on the bare bell when it is falling down. The gravitational force on the barbell does positive work because its displacement is in the direction gravitational force.
Q 2Q 2
If a person does 50 J of work in moving a 30-kg box over a 10-m distance on a horizontal surface, what is the minimum force required
Free
Essay
The work done is defined as the force times the displacement and it is expressed as follows:
Here, F is the net force acting on the system and x is the displacement.
The expression for the work done on the suitcase is,
Here, F is the net force exerted on the suitcase and x is the displacement covered by the suitcase.
Rearrange the equation
for F.
Substitute
for
and
for
.
Therefore, the force exerted on the suitcase is
. The force is pointed along the displacement vector.
Free
Multiple Choice
Concept:
Work is the scalar product of force and displacement. And the word done by constant force on an object is equal to the product of the magnitude of the displacement and the component of force.
Units of work are
or Joule.
If we express force as product if mass and acceleration
Units of work done are
.
Therefore the choice is ( d ).
Q 4Q 4
You are carrying a backpack across campus. What is the work done by your vertical carrying force on the backpack Explain.
Free
Essay
Q 5Q 5
A 5.0-kg box slides a 10-m distance on ice. If the coefficient of kinetic friction is 0.20, what is the work done by the friction force
Free
Essay
Q 6Q 6
For a particular force and displacement, the most work is done when the angle between them is (a) 30°, (b) 60°, (c) 90°, (d) 180°.
Free
Multiple Choice
Q 7Q 7
A jet plane flies in a vertical circular loop. In what regions of the loop is the work done by the force of gravity on the plane positive and/or negative Is the work constant If not, are there maximum and minimum instantaneous values Explain.
Free
Essay
Q 8Q 8
A passenger at an airport pulls a rolling suitcase by its handle. If the force used is 10 N and the handle makes an angle of 25° to the horizontal, what is the work done by the pulling force while the passenger walks 200 m
Free
Essay
Q 9Q 9
A pitcher throws a fastball. When the catcher catches it, (a) positive work is done, (b) negative work is done, (c) the net work is zero.
Free
Multiple Choice
Q 10Q 10
When walking up stairs, it is easier to do so in a zigzag path rather than going straight up. Why is this ( Hint : Think of an inclined plane.)
Free
Essay
Q 11Q 11
A 3.00-kg block slides down a frictionless plane inclined 20° to the horizontal. If the length of the plane's surface is 150 m, how much work is done, and by what force
Free
Essay
Q 12Q 12
Work done in free fall (a) is only positive, (b) is only negative, or (c) can be either positive or negative.
Free
Multiple Choice
Free
Essay
Q 14Q 14
Suppose the coefficient of kinetic friction between the block and the plane in Exercise is 0.275. What would be the net work done in this case
Exercise
A 3.00-kg block slides down a frictionless plane inclined 20° to the horizontal. If the length of the plane's surface is 150 m, how much work is done, and by what force
Free
Essay
Free
Multiple Choice
Q 16Q 16
Does it take twice the work to stretch a spring 2 cm from its equilibrium position as it does to stretch it 1 cm from its equilibrium position
Free
Essay
Q 17Q 17
A father pulls his young daughter on a sled with a constant velocity on a level surface a distance of 10 m, as illustrated in Fig. a. If the total mass of the sled and the girl is 35 kg and the coefficient of kinetic friction between the sled runners and the snow is 0.20, how much work does the father do
FIGURE Fun and work See Exercises.
Free
Essay
Q 18Q 18
The work done by a variable force of the form F = kx is equal to (a) kx 2 , (b) kx , (c) kx 2 , (d) none of the preceding.
Free
Multiple Choice
Q 19Q 19
If a spring is compressed 2.0 cm from its equilibrium position and then compressed an additional 2.0 cm, how much more work is done in the second compression than in the first Explain.
Free
Essay
Q 20Q 20
A father pushes horizontally on his daughter's sled to move it up a snowy incline, as illustrated in Fig. b. If the sled moves up the hill with a constant velocity, how much work is done by the father in moving it from the bottom to the top of the hill (Some necessary data are given in Exercise.)
FIGURE Fun and work See Exercise.
Exercise
A father pulls his young daughter on a sled with a constant velocity on a level surface a distance of 10 m, as illustrated in Fig. a. If the total mass of the sled and the girl is 35 kg and the coefficient of kinetic friction between the sled runners and the snow is 0.20, how much work does the father do
FIGURE Fun and work See Exercise.
Free
Essay
Q 21Q 21
Which of the following is a scalar quantity: (a) work, (b) force, (c) kinetic energy, or (d) both a and c
Free
Multiple Choice
Q 22Q 22
You want to decrease the kinetic energy of an object as much as you can. You can do so by either reducing the mass by half or reducing the speed by half. Which option should you pick, and why
Free
Essay
Q 23Q 23
A block on a level frictionless surface has two horizontal forces applied, as shown in Fig. (a) What force F 2 would cause the block to move in a straight line to the right (b) If the block moves 50 cm, how much work is done by each force (c) What is the total work done by the two forces
FIGURE Make it go straight See Exercise.
Free
Essay
Q 24Q 24
If the angle between the net force and the displacement of an object is greater than 90°, (a) kinetic energy increases, (b) kinetic energy decreases, (c) kinetic energy remains the same, (d) the object stops.
Free
Multiple Choice
Q 25Q 25
A certain amount of work W is required to accelerate a car from rest to a speed v. How much work is required to accelerate the car from rest to a speed of v /2
Free
Essay
Q 26Q 26
A 0.50-kg shuffleboard puck slides a distance of 3.0 m on the board. If the coefficient of kinetic friction between the puck and the board is 0.15, what work is done by the force of friction
Free
Essay
Q 27Q 27
Two identical cars, A and B, traveling at 55 mi/h collide head-on. A third identical car, C, crashes into a brick wall going 55 mi/h. Which car has the least damage: (a) car A, car B, (c) car C, or (d) all the same
Free
Multiple Choice
Q 28Q 28
A certain amount of work W is required to accelerate a car from rest to a speed v. If instead an amount of work equal to 2 W is done on the car, what is the car's speed
Free
Essay
Q 29Q 29
A crate is dragged 3.0 m along a rough floor with a constant velocity by a worker applying a force of 500 N to a rope at an angle of 30° to the horizontal. (a) How many forces are acting on the crate (b) How much work does each of these forces do (c) What is the total work done on the crate
Free
Essay
Q 30Q 30
Which of the following objects has the least kinetic energy: (a) an object of mass 4 m and speed v , (b) an object of mass 3 m and speed 2 v , (c) an object of mass 2 m and speed 3 v , or (d) an object of mass m and speed 4 v
Free
Multiple Choice
Q 31Q 31
Car B is traveling twice as fast as car A, but car A has four times the mass of car B. Which car has the greater kinetic energy
Free
Essay
Q 32Q 32
A hot-air balloon ascends at a constant rate. (a)The weight of the balloon does (1) positive work, (2) negative work, (3) no work. Why (b) A hot-air balloon with a mass of 500 kg ascends at a constant rate of 1.50 m/s for 20.0 s. How much work is done by the upward buoyant force (Neglect air resistance.)
Free
Essay
Q 33Q 33
A change in gravitational potential energy (a) is always positive, (b) depends on the reference point, (c) depends on the path, (d) depends only on the initial and final positions.
Free
Multiple Choice
Q 34Q 34
If a spring changes its position from x o to x , what is the change in potential energy then proportional to (Express the answer in terms of x o and x.)
Free
Essay
Q 35Q 35
A hockey puck with a mass of 200 g and an initial speed of 25.0 m/s slides freely to rest in the space of 100 m on a sheet of horizontal ice. How many forces do nonzero work on it as it slows: (a) (1) none, (2) one, (3) two, or (4) three Explain. (b) Determine the work done by all the individual forces on the puck as it slows.
Free
Essay
Q 36Q 36
The change in gravitational potential energy can be found by calculating mg h and subtracting the reference point potential energy: (a) true, (b) false.
Free
Multiple Choice
Q 37Q 37
A lab notebook sits on a table 0.75 m above the floor. Your lab partner tells you the book has zero potential energy, and another student says it has 8.0 J of potential energy. Who is correct
Free
Essay
Q 38Q 38
An eraser with a mass of 100 g sits on a book at rest. The eraser is initially 10.0 cm from any edge of the book. The book is suddenly yanked very hard and slides out from under the eraser, hi doing so, it partially drags the eraser with it, although not enough to stay on the book. The coefficient of kinetic friction between the book and the eraser is 0.150. (a) The sign of the work done by the force of kinetic friction of the book on the eraser is (1) positive, (2) negative, or (3) zero work is done by kinetic friction. Explain. (b) How much work is done by the book's frictional force on the eraser by the time it falls off the edge of the book
Free
Essay
Q 39Q 39
The reference point for gravitational potential energy may be (a) zero, (b) negative, (c) positive, (d) all of the preceding.
Free
Multiple Choice
Q 40Q 40
An object is said to have a negative potential energy. Because you prefer not to work with negative numbers, how could you make the object to have a positive potential energy without moving it
Free
Essay
Q 41Q 41
A 500-kg, light-weight helicopter ascends from the ground with an acceleration of 2.00 m/s 2. Over a 5.00-s interval, what is (a) the work done by the lifting force, (b) the work done by the gravitational force, and (c) the net work done on the helicopter
Free
Essay
Free
Multiple Choice
Q 43Q 43
For a classroom demonstration, a bowling ball suspended from a ceiling is displaced from the vertical position to one side and released from rest just in front of the nose of a student (Fig.). If the student doesn't move, why won't the bowling ball hit his nose
FIGURE In your face See Conceptual Question.
Free
Essay
Q 44Q 44
A man pushes horizontally on a desk that rests on a rough wooden floor. The coefficient of static friction between the desk and floor is 0.750 and the coefficient of kinetic friction is 0.600. The desk's mass is 100 kg. He pushes just hard enough to get the desk moving and continues pushing with that force for 5.00 s. How much work does he do on the desk
Free
Essay
Q 45Q 45
If a nonconservative force acts on an object, and does work, then (a) the object's kinetic energy is conserved, (b) the object's potential energy is conserved, (c) the mechanical energy is conserved, (d) the mechanical energy is not conserved.
Free
Multiple Choice
Q 46Q 46
When you throw an object into the air, is its initial speed the same as its speed just before it returns to your hand Explain by applying the concept of conservation of mechanical energy.
Free
Essay
Q 47Q 47
A student could either pull or push, at an angle of 30° from the horizontal, a 50-kg crate on a horizontal surface, where the coefficient of kinetic friction between the crate and surface is 0.20. The crate is to be moved a horizontal distance of 15 m. (a) Compared with pushing, pulling requires the student to do (1) less, (2) the same, or (3) more work. (b) Calculate the minimum work required for both pulling and pushing.
Free
Essay
Q 48Q 48
The speed of a pendulum is greatest (a) when the pendulum's kinetic energy is a minimum, (b) when the pendulum's acceleration is a maximum, (c) when the pendulum's potential energy is a minimum, (d) none of the preceding.
Free
Multiple Choice
Q 49Q 49
A student throws a ball vertically upward so it just reaches the height of a window on the second floor of a dormitory. At the same time that the ball is thrown upward, a student at the window drops an identical ball. Are the mechanical energies of the balls the same at half the height of the window Explain.
Free
Essay
Q 50Q 50
To measure the spring constant of a certain spring, a student applies a 4.0-N force, and the spring stretches by 5.0 cm. What is the spring constant
Free
Essay
Q 51Q 51
Two springs are identical except for their force constants, k 2 k 1. If the same force is used to stretch the springs, (a) spring 1 will be stretched farther than spring 2, (b) spring 2 will be stretched farther than spring 1, (c) both will be stretched the same distance.
Free
Multiple Choice
Q 52Q 52
If you chock your electricity bill, you will note that you are paying the power company for so many kilowatt- hours (kWh). Are you really paying for power Explain.
Free
Essay
Q 53Q 53
A spring has a spring constant of 30 N/m. How much work is required to stretch the spring 2.0 cm from its equilibrium position
Free
Essay
Q 54Q 54
If the two springs in Exercise are compressed the same distance, on which spring is more work done: (a) spring 1, (b) spring 2, or (c) equal work on both Exercise
Two springs are identical except for their force constants, k 2 k 1. If the same force is used to stretch the springs, (a) spring 1 will be stretched farther than spring 2, (b) spring 2 will be stretched farther than spring 1, (c) both will be stretched the same distance.
Free
Multiple Choice
Q 55Q 55
(a) Does efficiency describe how fast work is done Explain. (b) Does a more powerful machine always perform more work than a less powerful one Explain.
Free
Essay
Free
Essay
Q 57Q 57
Two identical stones are thrown from the top of a tall building. Stone 1 is thrown vertically downward with an initial speed v , and stone 2 is thrown vertically upward with the same initial speed. Neglecting air resistance, which stone hits the ground with a greater speed: (a) stone 1, (b) stone 2, or (c) both have the same speed
Free
Multiple Choice
Q 58Q 58
Two students who weigh the same start at the same ground floor location at the same time to go to the same classroom on the third floor by different routes. If they arrive at different times, which student will have expended more power Explain.
Free
Essay
Q 59Q 59
If a 10-N force is used to compress a spring with a spring constant of 4.0 × 10 2 N/m, what is the resulting spring compression
Free
Essay
Q 60Q 60
In Exercise, if air resistance is taken into account, which stone hits the ground with a greater speed: (a) stone 1, (b) stone 2, or (c) both have the same speed Exercise
A Gaussian surface surrounds an object with a net charge of 5.0 µ C. Which of the following is true: more electric field lines will point outward than inward; (b) more electric field lines will point inward than outward; (c) the net number of field lines through the surface is zero; or (d) there must be only field lines passing inward through the surface
Free
Multiple Choice
Q 61Q 61
A certain amount of work is required to stretch a spring from its equilibrium position. (a) If twice the work is performed on the spring, the spring will stretch more by a factor of (1)
(2) 2, (3) 1/
(4)
. Why (b) If 100 J of work is done to pull a spring 1.0 cm, what work is required to stretch it 3.0 cm
Free
Essay
Free
Multiple Choice
Q 63Q 63
Compute the work done by the variable force in the graph of F versus x in Fig. [ Hint : The area of a triangle is A =
altitude × base]
FIGURE How much work is done See Exercise.
Free
Essay
Q 64Q 64
Consider a 2.0-hp motor and a 1.0-hp motor. Compared to the 2.0-hp motor, for a given amount of work, the 1.0-hp motor can (a) do twice as much work in half the time, (b) half the work in the same time, (c) one quarter of the work in three quarters of the time, (d) none of the preceding.
Free
Multiple Choice
Q 65Q 65
A spring with a force constant of 50 N/m is to be stretched from 0 to 20 cm. (a) The work required to stretch the spring from 10 cm to 20 cm is (1) more than, (2 the same as, (3) less than that required to stretch it from 0 to 10 cm. (b) Compare the two work values to prove your answer to part (a).
Free
Essay
Q 66Q 66
In gravity-free interstellar space, a spaceship fires its rockets to speed up. The rockets are programmed to increase thrust from zero to 1.00 × 10 4 N with a linear increase over the course of 18.0 km. Then the thrust decreases linearly back to zero over the next 18.0 km. Assuming the rocket was stationary to start, (a) during which segment will more work (magnitude) be done: (1) the first 60 s, (2) the second 60 s, or (3) the work done is the same in both segments Explain your reasoning.(b) Determine quantitatively how much work is done in each segment.
Free
Essay
Q 67Q 67
A particular spring has a force constant of 2.5 × 10 3 N/m. (a) How much work is done in stretching the relaxed spring by 6.0 cm (b) How much more work is done in stretching the spring an additional 2.0 cm
Free
Essay
Q 68Q 68
For the spring in Exercise, how much mass would have to be suspended from the vertical spring to stretch it (a) the first 6.0 cm and (b) the additional 2.0 cm
Exercise
A particular spring has a force constant of 2.5 × 10 3 N/m. (a) How much work is done in stretching the relaxed spring by 6.0 cm (b) How much more work is done in stretching the spring an additional 2.0 cm
Free
Essay
Q 69Q 69
In stretching a spring in an experiment, a student inadvertently stretches it past its elastic limit; the force- versus-stretch graph is shown in Fig. Basically, after it reaches its limit, the spring begins to behave as if it were considerably stiffen How much work was done on the spring Assume that on the force axis, the tick marks are every 10 N, and on the x -axis, they are every 10 cm or 0.10 m.
FIGURE Past the limit See Exercise.
Free
Essay
Q 70Q 70
A spring (spring 1) with a spring constant of 500 N/in is attached to a wall and connected to another weaker spring (spring 2) with a spring constant of 250 N/m on a horizontal surface. Then an external force of 100 N is applied to the end of the weaker spring (#2). How much potential energy is stored in each spring
Free
Essay
Q 71Q 71
A 0.20-kg object with a horizontal speed of 10 m/s hits a wall and bounces directly back with only half the original speed. (a) What percentage of the object's initial kinetic energy is lost: (1) 25%, (2) 50%, or (3) 75% (b) How much kinetic energy is lost in the ball's collision with the wall
Free
Essay
Q 72Q 72
A 1200-kg automobile travels at 90 km/h. (a) What is its kinetic energy (b) What network would be required to bring it to a stop
Free
Essay
Q 73Q 73
A constant net force of 75 N acts on an object initially at rest as it moves through a parallel distance of 0.60 m. (a) What is the final kinetic energy of the object (b) If the object has a mass of 0.20 kg, what is its final speed
Free
Essay
Q 74Q 74
A 2.00-kg mass is attached to a vertical spring with a spring constant of 250 N/m. A student pushes on the mass vertically upward with her hand while slowly lowering it to its equilibrium position. (a) How many forces do nonzero work on the object: (1) one, (2) two, or (3) three Explain your reasoning. (b) Calculate the work done on the object by each of the forces acting on it as it is lowered into position.
Free
Essay
Q 75Q 75
The stopping distance of a vehicle is an important safety factor. Assuming a constant braking force, use the work-energy theorem to show that a vehicle's stopping distance is proportional to the square of its initial speed. If an automobile traveling at 45 km/h is brought to a stop in 50 m, what would be the stopping distance for an initial speed of 90 km/h
Free
Essay
Q 76Q 76
A large car of mass 2 m travels at speed v. A small car of mass m travels with a speed 2 v. Both skid to a stop with the same coefficient of friction. (a) The small car will have (1) a longer, (2) the same, (3) a shorter stopping distance. (b) Calculate the ratio of the stopping distance of the small car to that of the large car. (Use the work-energy theorem, not Newton's laws.)
Free
Essay
Q 77Q 77
An out-of-control truck with a mass of 5000 kg is traveling at 35.0 m/s (about 80 mi/h) when it starts descending a steep (15°) incline. The incline is icy, so the coefficient of friction is only 0.30. Use the work-energy theorem to determine how far the truck will skid (assuming it locks its brakes and skids the whole way) before it comes to rest.
Free
Essay
Q 78Q 78
If the work required to speed up a car from 10 km/h to 20 km/h is 5.0 × 10 3 J, what would be the work required to increase the car's speed from 20 km/h to 30 km/h
Free
Essay
Q 79Q 79
How much more gravitational potential energy does a 1.0-kg hammer have when it is on a shelf 1.2 m high than when it is on a shelf 0.90 m high
Free
Essay
Q 80Q 80
You are told that the gravitational potential energy of a 2.0-kg object has decreased by 10 J. (a) With this information, you can determine (1) the object's initial height, (2) the object's final height, (3) both the initial and the final height, (4) only the difference between the two heights. Why (b) What can you say has physically happened to the object
Free
Essay
Q 81Q 81
Six identical books, 4.0 cm thick and each with a mass of 0.80 kg, lie individually on a flat table. How much work would be needed to stack the books one on top of the other
Free
Essay
Q 82Q 82
The floor of the basement of a house is 3.0 m below ground level, and the floor of the attic is 4.5 m above ground level. (a) If an object in the attic were brought to the basement, the change in potential energy will be greatest relative to which floor: (1) attic, (2) ground, (3) basement, or (4) all the same Why (b) What are the respective potential energies of 1.5-kg objects in the basement and attic, relative to ground level (c) What is the change in potential energy if the object in the attic is brought to the basement
Free
Essay
Q 83Q 83
A 0.50-kg mass is placed on the end of a vertical spring that has a spring constant of 75 N/m and eased down into its equilibrium position. (a) Determine the change in spring (elastic) potential energy of the system. (b) Determine the system's change in gravitational potential energy.
Free
Essay
Q 84Q 84
A horizontal spring, resting on a frictionless tabletop, is stretched 15 cm from its unstretched configuration and a 1.00-kg mass is attached to it. The system is released from rest. A fraction of a second later, the spring finds itself compressed 3.0 cm from its unstretched configuration. How does its final potential energy compare to its initial potential energy (Give your answer as a ratio, final to initial.)
Free
Essay
Q 85Q 85
A student has six textbooks, each with a thickness of 4.0 cm and a weight of 30 N. What is the minimum work the student would have to do to place all the books in a single vertical stack, starting with all the books on the surface of the table
Free
Essay
Q 86Q 86
A 1.50-kg mass is placed on the end of a spring that has a spring constant of 175 N/m. The mass-spring system rests on a frictionless incline that is at an angle of 30° from the horizontal (Fig.). The system is eased into its equilibrium position, where it stays. (a) Determine the change in elastic potential energy of the system. (b) Determine the system's change in gravitational potential energy.
FIGURE Changes in potential energy See Exercise.
Free
Essay
Q 87Q 87
A 0.300-kg ball is thrown vertically upward with an initial speed of 10.0 m/s. If the initial potential energy is taken as zero, find the ball's kinetic, potential, and mechanical energies (a) at its initial position, (b) at 2.50 m above the initial position, and (c) at its maximum height.
Free
Essay
Q 88Q 88
What is the maximum height reached by the ball in Exercise
Exercise
A 0.300-kg ball is thrown vertically upward with an initial speed of 10.0 m/s. If the initial potential energy is taken as zero, find the ball's kinetic, potential, and mechanical energies (a) at its initial position, (b) at 2.50 m above the initial position, and (c) at its maximum height.
Free
Essay
Q 89Q 89
Referring to Fig. 3.13, find the speed with which the stone strikes the water using energy considerations.
Free
Essay
Q 90Q 90
A girl swings back and forth on a swing with ropes that are 4.00 m long. The maximum height she reaches is 2.00 m above the ground. At the lowest point of the swing, she is 0.500 m above the ground. (a) The girl attains the maximum speed (1) at the top, (2) in the middle, (3) at the bottom of the swing. Why (b) What is the girl's maximum speed
Free
Essay
Q 91Q 91
A 1.00-kg block ( M ) is on a flat frictionless surface (Fig.). This block is attached to a spring initially at its relaxed length (spring constant is 50.0 N/m). A light string is attached to the block and runs over a frictionless pulley to a 450-g dangling mass ( m ). If the dangling mass is released from rest, how far does it fall before stopping
FIGURE How far does it go See Exercise.
Free
Essay
Q 92Q 92
A 500-g (small) mass on the end of a 1.50-m-long string is pulled aside 15° from the vertical and shoved downward (toward the bottom of its motion) with a speed of 2.00 m/s. (a) Is the angle on the other side (1) greater than, (2) less than, or (3) the same as the angle on the initial side (15°) Explain in terms of energy. (b) Calculate the angle it goes to on the other side, neglecting air resistance.
Free
Essay
Q 93Q 93
A 0.20-kg rubber ball is dropped from a height of 1.0 m above the floor and it bounces back to a height of 0.70 m. (a) What is the ball's speed just before hitting the floor (b) What is the speed of the ball just as it leaves the ground (c) How much energy was lost and where did it go
Free
Essay
Q 94Q 94
A skier coasts down a very smooth, 10-m-high slope similar to the one shown in Fig. 5.21. If the speed of the skier on the top of the slope is 5.0 m/s, what is his speed at the bottom of the slope
Free
Essay
Q 95Q 95
A roller coaster travels on a frictionless track as shown in Fig. (a) If the speed of the roller coaster at point A is 5.0 m/s, what is its speed at point B (b) Will it reach point C (c) What minimum speed at point A is required for the roller coaster to reach point C
FIGURE Energy conversion(s) See Exercise.
Free
Essay
Q 96Q 96
A simple pendulum has a length of 0.75 m and a bob with a mass of 0.15 kg. The bob is released from an angle of 25° relative to a vertical reference line (Fig.). (a) Show that the vertical height of the bob when it is released is h = L (1 - cos 25°). (b) What is the kinetic energy of the bob when the string is at an angle of 9.0° (c) What is the speed of the bob at the bottom of the swing (Neglect friction and the mass of the string.)
FIGURE A pendulum swings See Exercise.
Free
Essay
Q 97Q 97
Suppose the simple pendulum in Exercise were released from an angle of 60°. (a) What would be the speed of the bob at the bottom of the swing (b) To what height would the bob swing on the other side (c) What angle of release would give half the speed of that for the 60° release angle at the bottom of the swing
Exercise
A simple pendulum has a length of 0.75 m and a bob with a mass of 0.15 kg. The bob is released from an angle of 25° relative to a vertical reference line (Fig.). (a) Show that the vertical height of the bob when it is released is h = L (1 - cos 25°). (b) What is the kinetic energy of the bob when the string is at an angle of 9.0° (c) What is the speed of the bob at the bottom of the swing (Neglect friction and the mass of the string.)
FIGURE A pendulum swings See Exercise.
Free
Essay
Q 98Q 98
A 1.5-kg box that is sliding on a frictionless surface with a speed of 12 m/s approaches a horizontal spring. (See Fig. 5.19.) The spring has a spring constant of 2000 N/m. If one end of the spring is fixed and the other end changes its position, (a) how far will the spring be compressed in stopping the box (b) How far will the spring be compressed when the box's speed is reduced to half of its initial speed
Free
Essay
Q 99Q 99
A 0.50-kg mass is suspended on a spring that stretches 3.0 cm. (a) What is the spring constant (b) What added mass would stretch the spring an additional 2.0 cm (c) What is the change in potential energy when the mass is added
Free
Essay
Q 100Q 100
A vertical spring with a force constant of 300 N/m is compressed 6.0 cm and a 0.25-kg ball placed on top. The spring is released and the ball flies vertically upward. How high does the ball go
Free
Essay
Q 101Q 101
A block with a mass m 1 = 6.0 kg sitting on a frictionless table is connected to a suspended mass m 2 = 2.0 kg by a light string passing over a frictionless pulley. Using energy considerations, find the speed at which m 2 hits the floor after descending 0.75 m. ( Note : A similar problem iii Example 4.6 was solved using Newton's laws.)
Free
Essay
Q 102Q 102
A hiker plans to swing on a rope across a ravine in the mountains, as illustrated in Fig. and to drop when she is just above the far edge. (a) At what horizontal speed should she be moving when she starts to swing (b) Below what speed would she be in danger of falling into the ravine Explain.
FIGURE Can she make it See Exercise.
Free
Essay
Q 103Q 103
In Exercise, if the skier has a mass of 60 kg and the force of friction retards his motion by doing 2500 J of work, what is his speed at the bottom of the slope
Exercise
A skier coasts down a very smooth, 10-m-high slope similar to the one shown in Fig. 5.21. If the speed of the skier on the top of the slope is 5.0 m/s, what is his speed at the bottom of the slope
Free
Essay
Q 104Q 104
A 1.00-kg block ( M ) is on a frictionless, 20° inclined plane. The block is attached to a spring ( k = 25 N/m) that is fixed to a wall at the bottom of the incline. A light string attached to the block runs over a frictionless pulley to a 40.0-g suspended mass. The suspended mass is given an initial downward speed of 1.50 m/s. How far does it drop before coming to rest (Assume the spring is unlimited in how far it can stretch.)
Free
Essay
Q 105Q 105
A girl consumes 8.4 × 10 6 J (2000 food calories) of energy per day while maintaining a constant weight. What is the average power she produces in a day
Free
Essay
Q 106Q 106
A 1500-kg race car can go from 0 to 90 km/h in 5.0 s. What average power is required to do this
Free
Essay
Q 107Q 107
The two 0.50-kg weights of a cuckoo clock descend 1.5 m in a three-day period. At what rate is their total gravitational potential energy decreased
Free
Essay
Q 108Q 108
A pump lifts 200 kg of water per hour a height of 5.0 m. What is the minimum necessary power output rating of the water pump in watts and horsepower
Free
Essay
Q 109Q 109
A race car is driven at a constant velocity of 200 km/h on a straight, level track. The power delivered to the wheels is 150 kW. What is the total resistive force on the car
Free
Essay
Q 110Q 110
An electric motor with a 2.0-hp output drives a machine with an efficiency of 40%. What is the energy output of the machine per second
Free
Essay
Q 111Q 111
Water is lifted out of a well 30.0 m deep by a motor rated at 1.00 hp. Assuming 90% efficiency, how many kilograms of water can be lifted in 1 min
Free
Essay
Q 112Q 112
How much power must you exert to horizontally drag a 25.0-lcg table 10.0 m across a brick floor in 30.0 s at constant velocity, assuming the coefficient of kinetic friction between the table and floor is 0.550
Free
Essay
Q 113Q 113
A 3250-kg aircraft takes 12.5 min to achieve its cruising altitude of 10.0 km and cruising speed of 850 km/h. If the plane's engines deliver, on average, 1500 hp during this time, what is the efficiency of the engines
Free
Essay
Q 114Q 114
A sleigh and driver with a total mass of 120 kg are pulled up a hill with a 15° incline by a horse, as illustrated in Fig. (a) If the overall retarding frictional force is 950 N and the sled moves up the hill with a constant velocity of 5.0 km/h, what is the power output of the horse (Express in horsepower, of course. Note the magnitude of your answer, and explain.) (b) Suppose that in a spurt of energy, the horse accelerates the sled uniformly from 5.0 km/h to 20 km/h in 5.0 s. What is the horse's maximum instantaneous power output Assume the same force of friction.
FIGURE A one-horse open sleigh See Exercise.
Free
Essay
Q 115Q 115
A construction hoist exerts an upward force of 500 N on an object with a mass of 50 kg. If the hoist started from rest, determine the power it expended to lift the object vertically for 10 s under these conditions.
Free
Essay
Q 116Q 116
Two identical springs (neglect their masses) are used to "play catch" with a small block of mass 100 g (Fig.). Spring A is attached to the floor and compressed 10.0 cm with the mass on the end of it (loosely). Spring A is released from rest and the mass is accelerated upward. It impacts the spring attached to the ceiling, compresses it 2.00 cm, and stops after traveling a distance of 30.0 cm from the relaxed position of spring A to the relaxed position of spring B as shown. Determine the spring constant of the two springs (same since they are identical).
FIGURE Playing catch See Exercise.
Free
Essay
Q 117Q 117
A 200-g ball is launched from a height of 20.0 m above a lake. Its launch angle is 40° and it has an initial kinetic energy of 90.0 J. (a) Use energy methods to determine its maximum height above the lake surface. (b) Use projectile motion kinematics to repeat part (a). (c) Use energy methods to determine its speed just before impact with the water. (d) Repeat part (c) using projectile motion kinematics.
Free
Essay
Q 118Q 118
A 1.20-kg ball is projected straight upward with an initial speed of 18.5 m/s and reaches a maximum height of 14.7 m. (a) Show numerically that total mechanical energy is not conserved during this part of the ball's motion. (b) Determine the work done on the ball by the force of air resistance. (c) Calculate the average air resistance force on the ball and the ball's average acceleration.
Free
Essay
Q 119Q 119
An ideal spring of force constant k is hung vertically from the ceiling, and a held object of mass m is attached to the loose end. You carefully and slowly ease that mass down to its equilibrium position by keeping your hand under it until it reaches that position. (a) Show that the spring's change in length is given by
(b) Show that the work done by the spring is
(c) Show that the work done by gravity is
Explain why these two works do not add to zero. Since the overall change in kinetic energy is zero, you might think they should, no (d) Show that the work done by your hand is
and that the hand exerted an average force of half the object's weight.
Free
Essay
Q 120Q 120
A winch is capable of hauling a ton of bricks vertically two stories (6.25 m) in 19.5 s. If the winch's motor is rated at 5.00 hp, determine its efficiency during raising the load.
Free
Essay
Q 121Q 121
A 0.455-kg soccer ball is kicked off level ground at an angle of 40° with an initial speed of 30.0 m/s. Neglecting air resistance, (a) at its maximum height off the ground, its kinetic energy will be (1) less than its value at launch, but not zero, (2) more than its value at launch, (3) zero. Explain. (b) Determine its kinetic energy when it is at its maximum height above the ground and compare it to the kinetic energy at launch. [ Hint : What is its velocity at the top of its arc Review projectile motion if necessary.]
Free
Essay