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7 – 1
Chapter 7: Work and Kinetic Energy
Answers to Even-Numbered Conceptual Questions
False. Any force acting on an object can do work. The work done by different forces may add to produce agreater net work, or they may cancel to some extent. It follows that the net work done on an object can bethought of in the following two equivalent ways: (i) The sum of the works done by each individual force; or(ii) the work done by the net force.
If the net work done on an object is zero, it follows that its change in kinetic energy is also zero. Therefore,its speed remains the same.
Frictional forces do negative work whenever they act in a direction that opposes the motion. For example,friction does negative work when you push a box across the floor, or when you stop your car.
The fact that the ski boat’s velocity is constant means that its kinetic energy is also constant. Therefore, thenet work done on the boat is zero. It follows that the net force acting on the boat does no work. (In fact, thenet force acting on the boat is zero, since its velocity is constant.)
No. What we can conclude, however, is that the net force acting on the object is zero.
No. Power depends both on the amount of work done by the engine, and the amount of time during whichthe work is performed. For example, if engine 2 does its work in less than half the time of engine 1, it canproduce more power even if the amount of work the engines do is the same.
Solutions to Problems and Conceptual Exercises
Picture the Problem
: The International Space Station orbits the Earth in an approximately circular orbit at a height of above the Earth’s surface.
According to equation 7-3, the work done on an object is positive if the force and the displacement are alongthe same direction, but zero if the force is perpendicular to the displacement.
The gravitational force exerted by the Earth is radial (toward the center of the circular orbit), but thedisplacement is tangential, perpendicular to the force. We conclude that
the work done by the Earth on the space stationis zero.
If the Earth were to do any work on the space station, its energy would change. Of course the rockets thatlifted the parts of the space station into space
do work on them, increasing their energy sufficiently to achieve orbit.
Picture the Problem
: A pendulum bob swings from point I to point IIalong the circular arc indicated in the figure at right.
Apply equation 7-3, which says that the work done on an objectis positive if the force and the displacement are along the same direction, but zero if the force is perpendicular to the displacement.
As the pendulum bob swings from point I to point II, theforce of gravity points downward and a component of the displacement isdownward. Therefore, the work done on the bob by gravity is positive.
As the pendulum bob swings, the force exerted by the string is radial (toward the pivot point) but thedisplacement is tangential, perpendicular to the force. We conclude that the work done on the bob by the string is zero.
The work done by the Earth is negative if the bob swings from point II to point III because a component of thedisplacement is upward but the force is downward.
Да нет, сэр, - попыталась она сгладить неловкость. - Не в этом дело… - Да в этом. - Он все еще посмеивался.