Chapter 4
Questions
Q4.2 When the bus starts moving, the mass of Claudette is accelerated by the force of the back of the seat on
her body. Clark is standing, however, and the only force on him is the friction between his shoes and
the floor of the bus. Thus, when the bus starts moving, his feet start accelerating forward, but the rest
of his body experiences almost no accelerating force (only that due to his being attached to his
accelerating feet!). As a consequence, his body tends to stay almost at rest, according to Newton's first
law, relative to the ground. Relative to Claudette, however, he is moving toward her and falls into her
lap. (Both performers won Academy Awards.)
Q4.10 Mistake one: The car might be momentarily at rest, in the process of reversing forward into backward motion. In this case, the forces on it add to a large backward resultant.
Mistake two: There are no cars in interstellar space. If the car is remaining at rest, there are some large forces on it, including its weight and some force or forces of support.
Mistake three: The statement reverses cause and effect, like a politician who thinks that his getting
elected was the reason for people to vote for him.
Q4.13 Some physics teachers do this by asking a beefy student to pull on the ends of a cord supporting a can of soup at its center. Some get two burly young men pulling on opposite ends of a strong rope, while the smallest person in class gleefully mashes the center of the rope down to the table. Point out the beauty of sagging suspension-bridge cables. With a laser and optical lever, demonstrate that the mayor makes the table sag when he sits on it, and the judge bends the bench. Give them “I make the floor sag” buttons. Estimate the cost of an infinitely strong cable, and the truth will always win.
Q4.14 Suppose the rope is horizontal and has negligible weight. Its tension is 200 N. Along with a vertical force equal to his weight, each person must exert a 200 N horizontal force on the ground.
Problems
