# POTENTIAL ENERGY AND CONSERVATION OF ENERGY

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1 Chapter 8: POTENTIAL ENERGY AND CONSERVATION OF ENERGY 1 A good eample of kinetic energy is provided by: A a wound clock spring B the raised weights of a grandfather's clock C a tornado D a gallon of gasoline E an automobile storage battery Section: 8{2; Di±culty: E 2 No kinetic energy is possessed by: A ashootingstar B a rotating propeller on a moving airplane C a pendulum at the bottom of its swing D an elevator standing at the fth oor E a cyclone Section: 8{2; Di±culty: E 3 The wound spring of a clock possesses: A kinetic but no potential energy B potential but no kinetic energy C both potential and kinetic energy in equal amounts D neither potential nor kinetic energy E both potential and kinetic energy, but more kinetic energy than potential energy Section: 8{2; Di±culty: E 4 A body at rest in a system is capable of doing work if: A the potential energy of the system is positive B the potential energy of the system is negative C it is free to move in such a way as to decrease its kinetic energy D it is free to move in such a way as to decrease the potential energy of the system E it is free to move in such a way as to increase the potential energy of the system Section: 8{2; Di±culty: E 5 Which one of the following ve quantities CANNOT be used as a unit of potential energy? A watt second B gram cm/s 2 C joule D kg m 2 /s 2 E ft lb Section: 8{2; Di±culty: E Chapter 8: POTENTIAL ENERGY AND CONSERVATION OF ENERGY 115

2 6 Suppose that the fundamental dimensions are taken to be: force (F), velocity (V) and time (T) The dimensions of potential energy are then: A F/T B FVT C FV/T D F/T 2 E FV 2 /T 2 Section: 8{2; Di±culty: E 7 Only if a force on a particle is conservative: A is its work zero when the particle moves eactly once around any closed path B is its work always equal to the change in the kinetic energy of the particle C does it obey Newton's second law D does it obey Newton's third law E is it not a frictional force ans: A Section: 8{3; Di±culty: E 8 A nonconservative force: A violates Newton's second law B violates Newton's third law C cannot do any work D must be perpendicular to the velocity of the particle on which it acts E none of the above Section: 8{3; Di±culty: E 9 A force on a particle is conservative if: A its work equals the change in the kinetic energy of the particle B it obeys Newton's second law C it obeys Newton's third law D its work depends on the end points of every motion, not on the path between E it is not a frictional force Section: 8{3; Di±culty: E 116 Chapter 8: POTENTIAL ENERGY AND CONSERVATION OF ENERGY

3 10 Two particles interact by conservative forces In addition, an eternal force acts on each particle They complete round trips, ending at the points where they started Which of the following must have the same values at the beginning and end of this trip? A the total kinetic energy of the two-particle system B the potential energy of the two-particle system C the mechanical energy of the two-particle system D the total linear momentum of the two-particle system E none of the above Section: 8{3; Di±culty: E 11 Two objects interact with each other and with no other objects Initially object A has a speed of 5 m=s and object B has a speed of 10 m=s In the course of their motion they return to their initial positions Then A has a speed of 4 m=s andbhasaspeedof7m=s We can conclude: A the potential energy changed from the beginning to the end of the trip B mechanical energy was increased by nonconservative forces C mechanical energy was decreased by nonconservative forces D mechanical energy was increased by conservative forces E mechanical energy was decreased by conservative forces Section: 8{3; Di±culty: E 12 A golf ball is struck by a golf club and falls on a green three meters above the tee The potential energy of the Earth-ball system is greatest: A just before the ball is struck B just after the ball is struck C just after the ball lands on the green D when the ball comes to rest on the green E when the ball reaches the highest point in its ight Section: 8{4; Di±culty: E 13 A 2-kg block is thrown upward from a point 20 m above Earth's surface At what height above Earth's surface will the gravitational potential energy of the Earth-block system have increased by 500 J? A 5 m B 25 m C 46 m D 70 m E 270 m Section: 8{4; Di±culty: E Chapter 8: POTENTIAL ENERGY AND CONSERVATION OF ENERGY 117

4 14 A particle moves along the ais under the in uence of a stationary object The net force on the particle, which is conservative, is given by F =(8N=m 3 ) 3 If the potential energy is takentobezerofor = 0 then the potential energy is given by: A (2 J=m 4 ) 4 B ( 2J=m 4 ) 4 C (24 J=m 2 2 D ( 24 J=m 2 ) 2 E 5 J (2 J=m 4 ) 4 Section: 8{4; Di±culty: M 15 A force of 10 N holds an ideal spring with a 20-N/m spring constant in compression The potential energy stored in the spring is: A 0:5J B 2:5J C 5 J D 10 J E 200 J Section: 8{4; Di±culty: E 16 A 0:50-kg block attached to an ideal spring with a spring constant of 80 N=m oscillates on a horizontal frictionless surface The total mechanical energy is 0:12 J The greatest etension of the spring from its equilibrium length is: A 1: m B 3: m C 0:039 m D 0:054 m E 18 m Section: 8{4; Di±culty: E 118 Chapter 8: POTENTIAL ENERGY AND CONSERVATION OF ENERGY

5 17 The graphs below show the magnitude of the force on a particle as the particle moves along the positive ais from the origin to = 1 The force is parallel to the ais and is conservative The maimum magnitude F 1 has the same value for all graphs Rank the situations according to the change in the potential energy associated with the force, least (or most negative) to greatest (or most positive) F F F F 1 1 F F A 1, 2, 3 B 1, 3, 2 C 2, 3, 1 D 3, 2, 1 E 2, 1, 3 Section: 8{2, 4; Di±culty: E 18 The sum of the kinetic and potential energies of a system of objects is conserved: A only when no eternal force acts on the objects B only when the objects move along closed paths C only when the work done by the resultant eternal force is zero D always E none of the above Section: 8{5; Di±culty: E 19 A 0:20-kg particle moves along the ais under the in uence of a stationary object The potential energy is given by U() =(8:0J=m 2 ) 2 +(2:0J=m 4 ) 4 ; where is in coordinate of the particle If the particle has a speed of 5:0m=s whenitisat =1:0 m, its speed when it is at the origin is: A 0 B 2:5m=s C 5:7m=s D 7:9m=s E 11 m=s Section: 8{5; Di±culty: M Chapter 8: POTENTIAL ENERGY AND CONSERVATION OF ENERGY 119

6 20 A ball is held at a height H above a oor It is then released and falls to the oor If air resistance can be ignored, which of the ve graphs below correctly gives the mechanical energy E of the Earth-ball system as a function of the altitude y of the ball? y ² H 0 E E E A y H B y H C H y E E Section: 8{5; Di±culty: E D H y E H y 21 A 6:0-kg block is released from rest 80 m above the ground When it has fallen 60 m its kinetic energy is approimately: A 4800 J B 3500 J C 1200 J D 120 J E 60 J Section: 8{5; Di±culty: E 120 Chapter 8: POTENTIAL ENERGY AND CONSERVATION OF ENERGY

7 22 An elevator is rising at constant speed Consider the following statements: I the upward cable force is constant II the kinetic energy of the elevator is constant III the gravitational potential energy of the Earth-elevator system is constant IV the acceleration of the elevator is zero V the mechanical energy of the Earth-elevator system is constant A all ve are true B only II and V are true C only IV and V are true D only I, II, and III are true E only I, II, and IV are true Section: 8{4, 5; Di±culty: E 23 A projectile of mass 0:50 kg is red with an initial speed of 10 m=s at an angle of 60 ± above the horizontal The potential energy of the projectile-earth system when the projectile is at its highest point (relative to the potential energy when the projectile is at ground level) is: A 25 J B 18:75 J C 12:5J D 6:25 J E none of these 24 For a block of mass m to slide without friction up the rise of height h shown, it must have a minimum initial kinetic energy of: m ~v h A gh B mgh C gh=2 D mgh=2 E 2mgh Section: 8{4, 5; Di±culty: E Chapter 8: POTENTIAL ENERGY AND CONSERVATION OF ENERGY 121

8 25 A 2:2-kgblockstartsfromrestonaroughinclinedplanethatmakesanangleof25 ± with the horizontal The coe±cient of kinetic friction is 0:25 As the block goes 2:0 m down the plane, the mechanical energy of the Earth-block system changes by: A 0 B 9:8J C 9:8J D 18 J E 18 J 26 A simple pendulum consists of a 2:0-kg mass attached to a string It is released from rest at X as shown Its speed at the lowest point Y is about: ² Y X ²² 1:85 m A p0:90 m=s B 3:6m=s C 3:6m=s D 6:0m=s E 36 m=s Section: 8{4, 5; Di±culty: E 27 An ideal spring is used to re a 15:0-g pellet horizontally The spring has a spring constant of 20 N=m and is initially compressed by 7:0 cm The kinetic energy of the pellet as it leaves the spring is: A zero B 2: J C 4: J D 9: J E 1:4J 122 Chapter 8: POTENTIAL ENERGY AND CONSERVATION OF ENERGY

9 28 The long pendulum shown is drawn aside until the ball has risen 0:50 m It is then given an initial speed of 3:0m=s The speed of the ball at its lowest position is: ²² ² 0:5m A zero B 0:89 m=s C 3:1m=s D 3:7m=s E 4:3m=s Section: 8{4, 5; Di±culty: E 29 Which of the ve graphs correctly shows the potential energy of a spring as a function of its elongation? U U U A B C U U Section: 8{4, 5; Di±culty: E D E 30 A 0:50-kg block attached to an ideal spring with a spring constant of 80 N=m oscillates on a horizontal frictionless surface The total mechanical energy is 0:12 J The greatest speed of the block is: A 0:15 m=s B 0:24 m=s C 0:49 m=s D 0:69 m=s E 1:46 m=s Section: 8{4, 5; Di±culty: E Chapter 8: POTENTIAL ENERGY AND CONSERVATION OF ENERGY 123

10 31 A 0:50-kg block attached to an ideal spring with a spring constant of 80 N=m oscillates on a horizontal frictionless surface When the spring is 4:0 cm longer than its equilibrium length, the speed of the block is 0:50 m=s The greatest speed of the block is: A 0:23 m=s B 0:32 m=s C 0:55 m=s D 0:71 m=s E 0:93 m=s 32 A 0:5-kg block slides along a horizontal frictionless surface at 2 m=s It is brought to rest by compressingaverylongspringofspringconstant800n=m The maimum spring compression is: A 0 B 3 cm C 5 cm D 80 cm E 80 m 33 A block of mass m is initially moving to the right on a horizontal frictionless surface at a speed v It then compresses a spring of spring constant k At the instant when the kinetic energy of the block is equal to the potential energy of the spring, the spring is compressed a distance of: A v p m=2k B (1=2)mv 2 C (1=4)mv 2 D mv 2 =4k E (1=4) p mv=k ans: A 34 A 700-N man jumps out of a window into a re net 10 m below The net stretches 2 m before bringing the man to rest and tossing him back into the air The maimum potential energy of the net, compared to its unstretched potential energy, is: A 300 J B 710 J C 850 J D 7000 J E 8400 J 124 Chapter 8: POTENTIAL ENERGY AND CONSERVATION OF ENERGY

11 35 A toy cork gun contains a spring whose spring constant is 10:0N=m The spring is compressed 5:00cmandthenusedtopropela6:00-g cork The cork, however, sticks to the spring for 1:00 cm beyond its unstretched length before separation occurs The muzzle velocity of this cork is: spring cork ² A 1:02 m=s B 1:41 m=s C 2:00 m=s D 2:04 m=s E 4:00 m=s 36 A small object of mass m, on the end of a light cord, is held horizontally at a distance r from a ed support as shown The object is then released What is the tension force of the cord whentheobjectisatthelowestpointofitsswing? r ² ²² m A mg=2 B mg C 2mg D 3mg E mgr 37 The string in the gure is 50 cm long When the ball is released from rest, it swings along the dotted arc How fast is it going at the lowest point in its swing? 50 cm ² ²² A 2:0m=s B 2:2m=s C 3:1m=s D 4:4m=s E 6:0m=s Section: 8{4, 5; Di±culty: E Chapter 8: POTENTIAL ENERGY AND CONSERVATION OF ENERGY 125

12 38 A block is released from rest at point P and slides along the frictionless track shown At point Q, its speed is: P " jj h 1 j j # Q "j h 2 j# ground level A 2g p h 1 h 2 B 2g(h 1 h 2 ) C D (h p 1 h 2 )=2g 2g(h1 h 2 ) E (h 1 h 2 ) 2 =2g Section: 8{4, 5; Di±culty: E 39 A small object of mass m starts from rest at the position shown and slides along the frictionless loop-the-loop track of radius R What is the smallest value of y such that the object will slide without losing contact with the track? m y R A R=4 B R=2 C R D 2R E zero 126 Chapter 8: POTENTIAL ENERGY AND CONSERVATION OF ENERGY

13 40 A rectangular block is moving along a frictionless path when it encounters the circular loop as shown The block passes points 1, 2, 3, 4, 1 before returning to the horizontal track At point 3: ² 3 ²1 ² 2 1 ² A its mechanical energy is a minimum B the forces on it are balanced C it is not accelerating D its speed is a minimum E it eperiences a net upward force Section: 8{4, 5; Di±culty: E 41 A ball of mass m, at one end of a string of length L, rotates in a vertical circle just fast enough to prevent the string from going slack at the top of the circle The speed of the ball at the bottom of the circle is: ² L ² m p A p 2gL B p 3gL C p 4gL D p 5gL E 7gL Chapter 8: POTENTIAL ENERGY AND CONSERVATION OF ENERGY 127

14 42 A particle is released from rest at the point = a and moves along the ais subject to the potential energy function U() shown The particle: U a b c d e A movestoapointtotheleftof = e, stops, and remains at rest B movestoapointto = e, then moves to the left C moves to in nity at varying speed D moves to = b, where it remains at rest E moves to = e and then to = d, where it remains at rest Section: 8{5, 6; Di±culty: E 43 The potential energy of a particle moving along the ais is given by U() =(8:0J=m 2 ) 2 +(2:0J=m 4 ) 4 : If the total mechanical energy is 9:0 J, the limits of motion are: A 0:96 m; +0:96m B 2:2m; +2:2m C 1:6m; +1:6m D 0:96 m; +2:2 m E 0:96 m; +1:6m ans: A Section: 8{5, 6; Di±culty: M 44 The potential energy of a 0:20-kg particle moving along the ais is given by U() =(8:0J=m 2 ) 2 +(2:0J=m 4 ) 4 : Whentheparticleisat =1:0 m it is traveling in the positive directionwithaspeedof 5:0m=s It net stops momentarily to turn around at = A 0 B 1:1m C 1:1m D 2:3m E 2:3m Section: 8{5, 6; Di±culty: M 128 Chapter 8: POTENTIAL ENERGY AND CONSERVATION OF ENERGY

15 45 Given a potential energy function U(), the corresponding force F ~ is in the positive direction if: A U is positive B U is negative C U is an increasing function of D U isadecreasingfunctionof E it is impossible to obtain the direction of F ~ from U Section: 8{6; Di±culty: E 46 Asaparticlemovesalongthe ais it is acted upon by a conservative force The potential energy is shown below as a function of the coordinate of the particle Rank the labeled regions according to the magnitude of the force, least to greatest U() A B C D A AB, BC, CD B AB, CD, BC C BC, CD, AB D BC, AB, CD E CD, BC, AB Section: 8{6; Di±culty: E Chapter 8: POTENTIAL ENERGY AND CONSERVATION OF ENERGY 129

16 47 The rst graph shows the potential energy U() for a particle moving on the ais Which of the other ve graphs correctly gives the force F eertedontheparticle? U parabola F F F A B C F F D E Section: 8{6; Di±culty: E 130 Chapter 8: POTENTIAL ENERGY AND CONSERVATION OF ENERGY

17 48 The diagram shows a plot of the potential energy as a function of for a particle moving along the ais The points of stable equilibrium are: U a b c d e A only a B only b C only c D only d E b and d Section: 8{6; Di±culty: E 49 The diagram shows a plot of the potential energy as a function of for a particle moving along the ais The points of unstable equilibrium are: U a b c d e A only a B only b C only c D only d E b and d Section: 8{6; Di±culty: E Chapter 8: POTENTIAL ENERGY AND CONSERVATION OF ENERGY 131

18 50 The diagram shows a plot of the potential energy as a function of for a particle moving along the ais Of the labeled points, the points of neutral equilibrium are: U a b c d e A only a B only b C only c D only d E b and d Section: 8{6; Di±culty: E 51 The potential energy of a body of mass m is given by U = mg k2 The corresponding force is: A mg 2 =2+k 3 =6 B mg 2 =2 k 3 =6 C mg + k=2 D mg + k E mg k Section: 8{6; Di±culty: E 52 The potential energy of a 0:20-kg particle moving along the ais is given by U() =(8:0J=m 2 ) 2 +(2:0J=m 4 ) 4 : When the particle is at =1:0 m the magnitude of its acceleration is: A 0 B 8m=s 2 C 8 m=s 2 D 40 m=s 2 E 40 m=s 2 Section: 8{6; Di±culty: M 132 Chapter 8: POTENTIAL ENERGY AND CONSERVATION OF ENERGY

19 53 The potential energy for the interaction between the two atoms in a diatomic molecule is U = A= 12 B= 6,whereA and B are constants and is the interatomic distance The magnitudeoftheforceofoneatomontheotheris: ² ²² ²²²² ² F F Ã! A 12A=jj 13 6B=jj 7 B 13A=jj 13 +7B=jj 7 C 11A=jj 11 +5B=jj 5 D 72A=jj 12 72B=jj 6 E A=jj 13 B=jj 7 ans: A Section: 8{6; Di±culty: E 54 The thermal energy of a system consisting of a thrown ball, Earth, and the air is most closely associated with: A the gravitational interaction of Earth and the ball B the kinetic energy of the ball as a whole C motions of the individual particles within the ball D motions of individual particles within the ball and the air E the kinetic energy of Earth as a whole Section: 8{8; Di±culty: E 55 Three identical blocks move either on a horizontal surface, up a plane, or down a plane, as shown below They start with di erent speeds and continue to move until brought to rest by friction They all move the same distance Rank the three situations according to the initial speeds, least to greatest ~v ~v ~v (1) (2) (3) A The same for all cases B 1, 2, 3 C 1, then 2 and 3 tie D 3, 1, 2 E 2, 1, 3 Section: 8{8; Di±culty: E Chapter 8: POTENTIAL ENERGY AND CONSERVATION OF ENERGY 133

20 56 Objects A and B interact with each other via both conservative and nonconservative forces Let K A and K B be the kinetic energies, U be the potential energy, and E int be the thermal energy If no eternal agent does work on the objects then: A K A + U is conserved B K A + U + E int is conserved C K A + K B + E int is conserved D K A + K B + U is conserved E K A + K B + U + E int is conserved Section: 8{8; Di±culty: E 57 A block slides across a rough horizontal table top The work done by friction changes: A only the kinetic energy B only the potential energy C only the internal energy D only the kinetic and potential energies E only the kinetic and thermal energies Section: 8{8; Di±culty: E 58 A 25-g ball is released from rest 80 m above the surface of Earth During the fall the total thermal energy of the ball and air increases by 15 J Just before it hits the surface its speed is A 19 m=s B 36 m=s C 40 m=s D 45 m=s E 53 m=s ans: A Section: 8{8; Di±culty: E 59 A 5-kg projectile is red over level ground with a velocity of 200 m=s atanangleof25 ± above the horizontal Just before it hits the ground its speed is 150 m=s Overtheentiretripthe change in the thermal energy of the projectile and air is: A +19; 000 J B 19; 000 J C +44; 000 J D 44; 000 J E 0 Section: 8{8; Di±culty: E 134 Chapter 8: POTENTIAL ENERGY AND CONSERVATION OF ENERGY

21 60 A 0:75-kg block slides on a rough horizontal table top Just before it hits a horizontal ideal spring its speed is 3:5m=s It compresses the spring 5:7 cm before coming to rest If the spring constant is 1200 N=m,thethermalenergyoftheblockandthetabletopmusthave: A not changed B decreased by 1:9J C decreased by 2:6J D increased by 1:9J E increased by 2:6J Section: 8{8; Di±culty: E 61 A small object slides along the frictionless loop-the-loop with a diameter of 3 m What minimum speedmustithaveatthetopoftheloop? " j 3m j# A 1:9m=s B 3:8m=s C 5:4m=s D 15 m=s E 29 m=s Section: none; Di±culty: M Chapter 8: POTENTIAL ENERGY AND CONSERVATION OF ENERGY 135

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