Deck 10: Energy and Work

Question 1

A crane lifts a 425 kg steel beam vertically upward a distance of 117 m.How much work does the crane do on the beam if the beam accelerates upward at 1.8 m/s²? Neglect frictional forces.

A)5.8 × 105 J
B)3.4 × 105J
C)4.0 × 105 J
D)4.9 × 105 J

Answer 1

5.8 × 105 J

Question 2

The frame of a cart has a mass of 45 kg,and the cart rolls on four railroad car wheels,each with a mass of 22 kg.The cart is being ridden at a speed of 5.0 m/s by a 75 kg man.What is the total kinetic energy? (Model the railroad car wheels as solid uniform disks of radius 20 cm. )

A)3200 J
B)1900 J
C)2100 J
D)2200 J

Answer 2

3200 J

Question 3

A massless spring hangs from the ceiling.How much does its potential energy increase,if a 28.8-kg mass is attached to it? The spring constant is 4800 N/m.

A)8.3 J
B)8.3 kJ
C)17 kJ
D)17 J

Answer 3

8.3 J

Question 4

Calculate the kinetic energy of a 0.300 kg baseball thrown at a velocity of 44 m/s.

A)290 J
B)580 J
C)440 J
D)510 J

Question 5

An airplane is observed to be 2.68 km from its takeoff point,at a horizontal distance of 2.66 km.What is the gravitational potential energy of the plane with respect to the ground? The plane's mass is 23.0 metric tons.

A)73.7 MJ
B)600 MJ
C)600 J
D)73.7 J

Question 6

A block starts from rest at the top of a 31.0° inclined plane and encounters a spring,of constant 3.4 kN/m,rigidly attached to the plane.If the block's mass is 33.0 kg and it compresses the spring by 37.0 cm,find the distance the block travelled before it encountered the spring.

A)1.0 m
B)137 cm
C)74 m
D)82.6 cm

Question 7

4.00 × 10⁵ J of work are done on a 1110 kg car while it accelerates from 10.0 m/s to some final velocity.Find this final velocity.

A)28.6 m/s
B)22.9 m/s
C)25.7 m/s
D)31.5 m/s

Question 8

Find the net work done by friction between the ground and the body of a snake slithering in a complete circle of 3.93 m radius.The coefficient of friction between the ground and the snake is 0.25,and the snake's weight is 54.0 N.

A)330 J
B)0 J
C)3300 J
D)670 J

Question 9

A child does 350 J of work while pulling a box from the ground up to his tree house with a rope.The tree house is 8.4 m above the ground.What is the mass of the box?

A)4.3 kg
B)2.6 kg
C)3.2 kg
D)3.9 kg

Question 10

You carry a 7.0 kg bag of groceries 1.2 m above the ground at constant velocity across a 2.3 m room.How much work do you do on the bag in the process?

A)0.0 J
B)82 J
C)158 J
D)134 J

Question 11

A car drives 5.0 km north,then 7.3 km east,then 3.4 km northeast,all at a constant velocity.If the car had to perform 2.6 × 10⁶ J of work during this trip,what was the magnitude of the average frictional force on the car?

A)1.7 × 10² N
B)2.4 × 10² N
C)3.2 × 10² N
D)3.6 × 10² N

Question 12

A spring stretches by 21.0 cm when a 135 N object is attached.What is the weight of a fish that would stretch the spring by 31.0 cm?

A)199 N
B)91 N
C)145 N
D)279 N

Question 13

A tennis ball bounces on the floor three times.If each time it loses 22.0% of its energy due to heating,how high does it bounce after the third time,provided we released it 2.3 m from the floor?

A)110 cm
B)11 cm
C)110 mm
D)140 cm

Question 14

A helicopter descends vertically to land with a speed of 4.0 m/s.If the shock absorbers have an initial length of 0.50 m,they compress to 79% of their original length and the air in the tires absorbs 21% of the initial energy as heat,what is the ratio of the spring constant to the helicopter's mass? Assume one landing gear in the craft.

A)1.1 kN/kg-m
B)1.1 N/kg-m
C)11 N/kg-m
D)0.11 kN/kg-m

Question 15

A traveler pulls on a suitcase strap at an angle 36° above the horizontal.If 908 J of work are done by the strap while moving the suitcase a horizontal distance of 15 m,what is the tension in the strap?

A)75 N
B)61 N
C)85 N
D)92 N

Question 16

A horizontal spring-mass system oscillates on a frictionless table.If the ratio of the mass to the spring constant is 0.038 kg · m/N,and the maximum speed of the mass was measured to be 17.60 m/s,find the maximum extension of the spring.

A)3.4 m
B)3.4 cm
C)0.67 m
D)67 cm

Question 17

A 1000.0 kg car experiences a net force of 9500 N while decelerating from 30.0 m/s to 23.4 m/s.How far does it travel while slowing down?

A)19 m
B)17 m
C)20 m
D)22 m

Question 18

You do 174 J of work while pulling your sister back on a swing,whose chain is 5.10 m long,until the swing makes an angle of 32.0° with the vertical.What is your sister's mass?

A)22.9 kg
B)19.5 kg
C)26.3 kg
D)28.4 kg

Question 19

A prankster drops a water balloon from the top of a building on an unsuspecting person on the sidewalk below.If the balloon is traveling at 30.3 m/s when it strikes a person's head (1.5 m above the ground),how tall is the building? Neglect air resistance.

A)48 m
B)47 m
C)50 m
D)51 m

Question 20

A block slides down a frictionless inclined ramp.If the ramp angle is 17.0° and the length of it is 30.0 m,find the speed of the block as it reaches the end of the ramp,assuming it started sliding from rest at the top.

A)13.1 m/s
B)172 m/s
C)9.26 m/s
D)24.0 m/s

Question 21

A 2.3 kg object moving at 7.3 m/s collides inelastically with a 4.0 kg object which is initially at rest.What percentage of the initial kinetic energy of the system is lost during the collision?

A)63 %
B)74 %
C)57 %
D)52 %

Question 22

A 3.5 kg mass is pressed against a spring of spring constant 90.0 N/m,compressing it 25.0 cm.The greatest amount of kinetic energy this mass could gain after it is released is 2.81 J.

Question 23

A cannon recoils after it fires a shell.If you measure the range of the shell,can you find the energy released by the gunpowder?

Question 24

A rubber ball is dropped from a height h above the ground.Describe the motion of the ball for many bounces if the collision is completely elastic.

Question 25

Joe and Bill throw identical balls vertically upward.Joe throws his ball with an initial speed of twice as high as Bill's ball.The maximum height of Joe's ball will be

A)four times that of Bill's ball.
B)two times that of Bill's ball.
C)equal to that of Bill's ball.
D)eight times that of Bill's ball.
E)roughly 1.3 times that of Bill's ball.

Question 26

If a force acts perpendicular to an object's direction of motion,that force cannot change the object's kinetic energy or momentum.

Question 27

A 2.0 kg stone slides from rest down a hill 5.0 m high and is observed to be moving at 6.0 m/s at the bottom.During this process,62 J of thermal energy have been produced.

Question 28

Two cars collide,lock bumpers,and eventually slide to a stop due to friction with the roadway.During this entire process,mechanical energy and momentum are both conserved.

Question 29

A person stands on the edge of a cliff.She throws three identical rocks with the same speed.Rock X is thrown vertically upward,rock Y is thrown horizontally,and rock Z is thrown vertically downward.Assuming the elevation loss of the three rocks is the same (the base of the cliff is flat),which rock hits the ground with the highest speed?

A)They all hit the ground with the same speed.
B)Rock X
C)Rock Y
D)Rock Z

Question 30

A car needs to generate 75.0 hp in order to maintain a constant velocity of 27.3 m/s on a flat road.What is the magnitude of the total resistive force acting on the car (due to friction,air resistance,etc. )?

A)2.05 × 10³ N
B)2.75 N
C)1.03 × 10³ N
D)2.87 × 10³ N

Question 31

Two pendulums of equal length l = 0.45 m are suspended from the same point.The pendulum bobs are steel spheres.The first bob is drawn back to make a 35° angle with the vertical.If the first bob has mass 0.25 kg and the second has mass 0.85 kg,how high will the second bob rise above its initial position when struck elastically by the first bob after it is released?

A)1.7 cm
B)1.4 cm
C)2.0 cm
D)2.3 cm

Question 32

A child pulls on a wagon with a force of 75 N.If the wagon moves a total of 42 m in 3 min,what is the average power generated by the child,in watts?

A)18 W
B)22 W
C)24 W
D)27 W

Question 33

Kinetic energy is conserved only in perfectly elastic collisions,but momentum is conserved in all collisions.

Question 34

Describe the energy transfers that take place as you use a slingshot to shoot a rock into the air,it rises to a peak,then falls,and finally hits the ground and stops without bouncing.

Question 35

Particle 1 has the mass of Particle 2,yet they both have the same kinetic energy.Compare the momenta of two particles.

Question 36

Is energy conserved during a two-car collision?

Question 37

A child does work to climb to the top of a slide,thus giving herself potential energy.Explain what has happened to all this potential energy as the child,still moving,nears the bottom of the (not frictionless)slide.

Question 38

Calculate the average power output necessary for a 55.8 kg person to run up a 12.0 m long hillside,which is inclined at 25.0° above the horizontal,in 3.00 s.Express your answer in horsepower.

A)1.24 hp
B)2.93 hp
C)1.86 hp
D)0.74 hp

Question 39

When a moving object hits a stationary object and causes it to move,some of the moving object's kinetic energy is transformed into momentum in the object that was at rest.

Question 40

As a stone slides down a frictionless,ice-covered hill,its mechanical energy is conserved but its momentum is not conserved.

Question 41

A pendulum in a grandfather clock swings back and forth.Does the force of gravity do positive work on the pendulum at any point in its trajectory? If so,where?

Question 42

Two cyclists,who weigh the same and have identical bicycles,ride up the same mountain,both starting at the same time.Joe rides straight up the mountain,and Bob rides up the longer road that has a lower grade.Joe gets to the top before Bob.Which statement is true?

A)Ignoring friction and wind resistance,the amount of work done by Joe is equal to the amount of work done by Bob,but the average power exerted by Joe is greater than that of Bob.
B)Ignoring friction and wind resistance,the amount of work done by Joe is greater than the amount of work done by Bob,and the average power exerted by Joe is greater than that of Bob.
C)Ignoring friction and wind resistance,Bob and Joe exerted the same amount of work,and the average power of each cyclist was also the same.
D)Ignoring friction and wind resistance,the average power exerted by Bob and Joe was the same,but Joe exerted more work in getting there.

Question 43

A child swings on a ball of mass m on a rope in a uniform,vertical circle of radius r at a constant speed v.How much work is done by the centripetal force on the ball during one complete revolution?

Question 44

A ball can be rolled down one of four different ramps,as shown below.The final elevation loss of each of the ramps is the same.Neglecting friction,for which ramp will the speed of the ball be the highest at the bottom?

A)The speed of the ball will be the same for all ramps.
B)Ramp X
C)Ramp Y
D)Ramp Z

Question 45

Viscous friction allows oil to reduce the heating and wear of moving machine parts.But viscous friction depends on velocity.Is that a nonconservative force?

Question 46

Three cars (car F,car G,and car H)are moving with the same velocity,and slam on the brakes.The most massive car is car F,and the least massive is car H.Assuming all three cars have identical tires,which car travels the longest distance to skid to a stop?

A)They all travel the same distance in stopping.
B)Car F.
C)Car G.
D)Car H.

Question 47

You push a heavy box of mass m up a frictionless inclined plane of length l and height h in some time t₀.Later,you push the same box up a similar inclined plane of length l/2 and height h in the same time t₀.What can you say about the relationship between your power output(s)during these two processes? Assume the box is moved at a constant velocity in both cases.

Question 48

Two lumps of clay are thrown at each other such that they come completely to rest after the collision.Obviously,kinetic energy is not conserved.Where did the energy go?

Question 49

If one lifts a box and walks up a couple of floors,how would the change in potential energy be accounted for?

Question 50

A person drops one rock from rest and it falls 100 meters down a cliff before hitting the ground.The person then throws an identical rock downward.The amount of kinetic energy gained by the rock dropped from rest is _____ that of the second rock,and the magnitude of the change in speed (between hitting the ground and its initial speed)of the rock dropped from rest is _____ that of the second rock.

A)equal to,greater than
B)equal to,equal to
C)less than,less than
D)equal to,less than