Lecture 6; Work and Kinetic energy

Sara carries a heavy box of books down a flight of stairs from the third floor to the first floor. Which one of the following statements best describes the work done on the box by Sara? A. Sara does a large amount of negative work on the heavy box of books. B. Sara does a large amount of positive work on the heavy box of books. C. Sara does a small amount of positive work on the heavy box of books. D. Sara doesn't do any work on the heavy box of books. E. Sara does a small amount of negative work on the heavy box of books

A. Sara does a large amount of negative work on the heavy box of books

Which one of the following choices is an example of a non-conservative force? A. kinetic frictional force B. electrical force C. elastic spring force D. gravitational force

A. kinetic frictional force

Complete the following statement: A force that acts on an object is said to be conservative if: A. the work it does on the object is independent of the path of the motion. B. the work it does on the object is equal to the increase in the object's kinetic energy C. it results in a change in the object's kinetic energy. D. it always acts in the direction of motion of the object. E. it obeys Newton's laws of motion.

A. the work it does on the object is independent of the path of the motion.

Which one of the following statements concerning the principle of the conservation of mechanical energy is true? A. Mechanical energy is always conserved in situations where external conservative forces do no work. B. Mechanical energy is always conserved in situations where external non-conservative forces do no work. C. Mechanical energy is always conserved in situations where the kinetic energy is constant. D. Mechanical energy is always conserved in situations where the gravitational potential energy is zero joules. E. Mechanical energy is always conserved in situations where the gravitational potential energy is constant.

B. Mechanical energy is always conserved in situations where external non-conservative forces do no work.

Two balls of equal size are dropped from the same height from the roof of a building. One ball has twice the mass of the other. When the balls reach the ground, how do the kinetic energies of the two balls compare? A. The lighter one has one fourth as much kinetic energy as the other does. B. The lighter one has one half as much kinetic energy as the other does. C. The lighter one has twice as much kinetic energy as the other does. D. The lighter one has four times as much kinetic energy as the other does E. The lighter one has the same kinetic energy as the other does.

B. The lighter one has one half as much kinetic energy as the other does.

Complete the following statement: In an isolated system, the total mechanical energy remains constant if: A. all exerted forces are nonconservative B. all exerted forces are conservative. C. the work done by nonconservative forces is negative. D. an object returns to its starting position. E. the work done by nonconservative forces is positive.

B. all exerted forces are conservative.

Complete the following statement: The net work done by a conservative force acting on an object: A. is always equal to zero joules. B. depends only on the starting and ending points. C. is always greater than zero joules. D. depends only on the velocity of the object. E. depends only on the time that the force is exerted.

B. depends only on the starting and ending points.

Which one of the following choices is an example of a conservative force? A. static frictional force B. elastic spring force C. motor propulsion force D. normal force E. tension

B. elastic spring force

Complete the following statement: In situations involving non-conservative external forces, the work done by these forces: A. can be either positive or negative. B. is always negative. C. is always positive D. is always equal to zero. E. usually cannot be determined.

B. is always negative.

Complete the following statement: The total mechanical energy of a system remains unchanged if: A. the external forces acting on the system are nonconservative. B. there are no external forces acting on the system C. the kinetic energy is constant. D. the external forces acting on the system are conservative. E. the potential energy is constant

B. there are no external forces acting on the system

An arrow is launched straight up from the surface of the Earth. Which one of the following statements describes the energy transformation of the arrow as it rises? Neglect air resistance. A. Both the kinetic energy of the arrow and its potential energy remain constant. B. Both the potential energy of the arrow and its total energy increase. C. The kinetic energy of the arrow decreases as the potential energy increases. D. The kinetic energy of the arrow increases and its potential energy decreases. E. The total energy of the arrow increases.

C. The kinetic energy of the arrow decreases as the potential energy increases

An elevator supported by a single cable descends a shaft at a constant speed. The only forces acting on the elevator are the tension in the cable and the gravitational force. Which one of the following statements is true? A. The work done by the tension force is zero joules B. The magnitude of the work done by the gravitational force is larger than that done by the tension force. C. The net work done by the two forces is zero joules D. The work done by the gravitational force is zero joules. E. The magnitude of the work done by the tension force is larger than that done by the gravitational force.

C. The net work done by the two forces is zero joules

A donkey pulls a crate up a rough, inclined plane at constant speed. Which one of the following statements concerning this situation is false? A. The donkey does "positive" work in pulling the crate up the incline. B. The net work done by all the forces acting on the crate is zero joules. C. The work done on the object by gravity is zero joules. D. The gravitational potential energy of the crate is increasing. E. The work done on the crate by the normal force of the plane is zero joules.

C. The work done on the object by gravity is zero joules.

You are investigating the safety of a playground slide. You are interested in finding out what the maximum speed will be of children sliding on it when the conditions make it very slippery (assume frictionless). The height of the slide is 2.5 m. What is that maximum speed of a child if she starts from rest at the top? A. 1.9 m/s B. 2.5 m/s C. 9.8 m/s D. 7.0 m/s E. 4.9 m/s

D. 7.0 m/s

Which one of the following situations is an example of a conservative force acting? A. A normal force pushes upward on a book as it sits on a table. B. The static frictional force between the tires of a car and the road. C. The tension force in the rope increased as the piano was lifted off the ground. D. A child jumping on a trampoline is pushed upward by an elastic spring force. E. A boat motor produces a propulsion force that moves the boat across a lake.

D. A child jumping on a trampoline is pushed upward by an elastic spring force.

Which one of the following statements concerning the principle of conservation of energy is false? A. Kinetic energy may be converted into gravitational potential energy and heat. B. Thermal energy may be converted into kinetic energy and kinetic energy may be converted into thermal energy. C. Chemical energy may be converted into thermal energy and gravitational potential energy. D. The net work done on an object must be zero joules since energy is neither created nor destroyed in the process. E. The total energy in the universe has a constant value.

D. The net work done on an object must be zero joules since energy is neither created nor destroyed in the process

In which one of the following circumstances does the principle of conservation of mechanical energy apply, even though a nonconservative force acts on the moving object? A. The nonconservative force has a component that points in the same direction as the displacement of the object. B. The nonconservative force has a direction that is opposite to the displacement of the object. C. The nonconservative force points in the same direction as the displacement of the object. D. The nonconservative force is perpendicular to the displacement of the object. E. The nonconservative force has a component that points opposite to the displacement of the object

D. The nonconservative force is perpendicular to the displacement of the object.

Which one of the following choices is an example of a non-conservative force? A. electrical force B. elastic spring force C. gravitational force D. kinetic frictional force

D. kinetic frictional force

In which one of the following systems is there a decrease in gravitational potential energy? A. water is forced upward through a pipe B. a crate rests at the bottom of an inclined plane C. a car ascends a steep hill D. a boy stretches a horizontal spring E. a girl jumps down from a bed

E. a girl jumps down from a bed

The power due to a force depends on which of the following parameters? A. the mass of the object and the velocity of the object B. the mass of the object and the displacement of the object C. the work done on the object and its displacement D. the magnitude of the force and the displacement of the object E. the magnitude of the force and the velocity of the object

E. the magnitude of the force and the velocity of the object