Potential Energy And Conservation Of Energy - Chapter No 7

Right Answers have been shown below in red color.

1. Only if a force on a particle is conservative:

A. is its work zero when the particle moves exactly 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

2. A non conservative 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

3. The sum of the kinetic and potential energies of a system of objects is conserved:


A. only when no external force acts on the objects
B. only when the objects move along closed paths
C. only when the work done by the resultant external force is zero
D. always
E. none of the above

4. 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

5. Two particles interact by conservative forces. In addition, an external 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. themechanical energy of the two-particle system
D. the total linear momentum of the two-particle system
E. none of the above

6. Two objects interact with each other and with no other objects. Initially object A has a speedof 5 m/s and object B has a speed of 10 m/s. In the course of their motion they return to theirinitial positions. Then A has a speed of 4 m/s and B has a speed of 7 m/s. We can conclude:


A. the potential energy changed from the beginning to the end of the trip
B. mechanical energy was increased by non conservative forces
C. mechanical energy was decreased by non conservative forces
D. mechanical energy was increased by conservative forces
E. mechanical energy was decreased by conservative forces

7. A good example 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

8. No kinetic energy is possessed by:

A. a shooting star
B. a rotating propeller on a moving airplane
C. a pendulum at the bottom of its swing
D. an elevator standing at the fifth floor
E. a cyclone

9. 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

10. 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

11. Which one of the following five quantities CANNOT be used as a unit of potential energy?

A. watt·second
B. gram·cm/s2
C. joule
D. kg·m2/s2
E. ft·lb

12. 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/T2
E. FV2/T

13. The graphs below show the magnitude of the force on a particle as the particle moves along the positive x axis from the origin to x = x1. The force is parallel to the x axis and is conservative. The maximum magnitude F1 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).

A. 1, 2, 3
B. 1, 3, 2
C. 2, 3, 1
D. 3, 2, 1
E. 2, 1, 3

14. 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 flight

15. A ball is held at a height H above a floor. It is then released and falls to the floor. If air resistance can be ignored, which of the five graphs below correctly gives the mechanical energy E of the Earth-ball system as a function of the altitude y of the ball?

E is the Right Answer

16. A 6.0-kg block is released from rest 80 m above the ground. When it has fallen 60 m its kinetic energy is approximately:

A. 4800 J
B. 3500 J
C. 1200 J
D. 120 J
E. 60 J

17. 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

18. 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 five 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

19. A projectile of mass 0.50 kg is fired with an initial speed of 10 m/s at an angle of 60◦ above the horizontal. The potential energy of the projectile-Earth system (relative potential energy
when the projectile is at ground level) is:

A. 25 J
B. 18.75 J
C. 12.5 J
D. 6.25 J
E. none of these

20. 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:

 

A. gh
B. mgh
C. gh/2
D. mgh/2
E. 2mgh

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