Questions
Q6.11 Kinetic energy is proportional to squared speed. Doubling the speed makes an object's kinetic energy
four times larger.
Q6.12 2W for each half of the spring, if it is described by Hooke's law. The spring constant for each half is
twice as large as that of the uncut spring, because the same tension stretches the cut section only half as
much. Therefore, 1/2 (2k)x^2=2(1/2 k x^2)=2W
Q6.13 No violation. Choose the book as the system. You did work and the earth did work on the book. The
average force you exerted just counterbalanced the weight of the book. The total work on the book is
zero, and is equal to its overall change in kinetic energy.
Problems
Chapter 7
Questions
Q7.1 Both agree on the change in potential energy, and the kinetic energy. They may disagree on the value
of gravitational potential energy, depending on their choice of a zero point.
Q7.5 Same amount of work. However, this is done over a much longer time (lower power output) in the
first case.
Q7.6 No. The rock has an acceleration of larger magnitude on the way up, so its upward motion occupies
less time than its downward motion. A net force of greater magnitude acts on it when it is moving up.
Then gravity and air resistance are in the same direction.
Q7.7 The original kinetic energy of the skidding can into degraded into kinetic energy of random molecular
motion in the tires and the road: it is internal energy. If the brakes are used properly, the same energy
appears as internal energy in the brake shoes and drums.
Problems
