Quick QuizzesAnswers to Even Numbered Conceptual QuestionsAnswers to Even Numbered ProblemsProblem SolutionsChapter 4 The Laws of Motion Quick Quizzes 1. (a) True. Motion requires no force. Newton’s first law says an object in motion continues to move by itself in the absence of external forces. (b) False. It is possible for forces to act on an object with no resulting motion if the forces are balanced. 2. (a) True. If a single force acts on an object, it must accelerate. From Newton's second law, = mΣaFGG, and a single force must represent a non-zero net force. (b) True. If an object accelerates, at least one force must act on it. (c) False. If an object has no acceleration, you cannot conclude that no forces act on it. In this case, you can only say that the net force on the object is zero. 3. False. If the object begins at rest or is moving with a velocity with only an x component, the net force in the x direction causes the object to move in the x direction. In any other case, however, the motion of the object involves velocity components in directions other than x. Thus, the direction of the velocity vector is not generally along the x axis. What we can say with confidence is that a net force in the x direction causes the object to accelerate in the x direction. 4. (a). Because the value of g is smaller on the Moon than on the Earth, more mass of gold would be required to represent 1 newton of weight on the Moon. Thus, your friend on the Moon is richer, by about a factor of 6! 5. (c) and (d). Newton’s third law states that the car and truck will experience equal magnitude (but oppositely directed) forces. Newton’s second law states that acceleration is inversely proportional to mass when the force is constant. Thus, the lower mass vehicle (the car) will experience the greater acceleration. 6. (c). The scale is in equilibrium in both situations, so it experiences a net force of zero. Because each person pulls with a force F and there is no acceleration, each person is in equilibrium. Therefore, the tension in the ropes must be equal to F. In case (i), the person on the right pulls with force F on a spring mounted rigidly to a brick wall. The resulting tension F in the rope causes the scale to read a force F. In case (ii), the person on the left can be modeled as simply holding the rope tightly while the person on the right pulls. Thus, the person on the left is doing the same thing that the wall does in case (i). The resulting scale reading is the same whether there is a wall or a person holding the left side of the scale. 99100 CHAPTER 4 7. (c). The tension in the rope has a vertical component that supports part of the total weight of the child and sled. Thus, the upward normal force exerted by the ground is less than the total weight. 8. (b). Friction forces are always parallel to the surfaces in contact, which, in this case, are the wall and the cover of the book. This tells us that the friction force is either upward or downward. Because the tendency of the book is to fall due to gravity, the friction force must be in the upward direction. 9. (b). The static friction force between the bottom surface of the crate and the surface of the truck bed is the net horizontal force on the crate that causes it to accelerate. It is in the same direction as the acceleration, to the east. 10. (b). It is easier to attach the rope and pull. In this case, there is a component of your applied force that is upward. This reduces the normal force between the sled and the snow. In turn, this reduces the friction force between the sled and the snow, making it easier to move. If you push from behind, with a force with a downward component, the normal force is larger, the friction force is larger, and the sled is harder to move.The Laws of Motion 101 Answers to Even Numbered Conceptual Questions 2. If the car is traveling at constant velocity, it has zero acceleration. Hence, the resultant force acting on it is zero. 4. The force causing the ball to rebound upward is the normal force exerted on the ball by the floor. 6. w = mg and g decreases with altitude. Thus, to get a good buy, purchase it in Denver. If gold was sold by mass, it would not matter where you bought it. 8. If it has a large mass, it will take a large force to alter its motion even when floating in space. Thus, to avoid injuring himself, he should push it gently toward the storage compartment. 10. The net force acting on the object decreases as the resistive force increases. Eventually, the resistive force becomes equal to the weight of the object, and the net force goes to zero. In this condition, the object stops accelerating, and the velocity stays constant. The rock has reached its terminal velocity. 12. The barbell always exerts a downward force on the lifter equal in magnitude to the upward force that she exerts on the barbell. Since the lifter is in equilibrium, the magnitude of the upward force exerted on her by the scale (that is, the scale reading) equals the sum of her weight and the downward force exerted by the barbell. As the barbell goes through the bottom of the cycle and is being lifted upward, the scale reading exceeds the combined weights of the lifter and the barbell. At the top of the motion and as the barbell is allowed to move back downward, the scale reading is less than the combined weights. If the barbell is moving upward, the lifter can declare she has thrown it just by letting go of it for a moment. Thus, the case is included in the previous answer. 14. While the engines operate, their total upward thrust exceeds the weight of the rocket, and the rocket experiences a net upward force. This net force causes the upward velocity of the rocket to increase in magnitude (speed). The upward thrust of the engines is constant, but the remaining mass of the rocket (and hence, the downward gravitational force or weight) decreases as the rocket consumes its fuel. Thus, there is an increasing net upward force acting on a diminishing mass. This yields an acceleration that increases in time. 16. The truck’s skidding distance can be shown to be 202kvxgµ= where kµ is the coefficient of kinetic friction and is the initial velocity of the truck. This equation demonstrates that the mass of the truck does not affect the skidding distance, but halving the velocity will decrease the skidding distance by a factor of four. 0v18. Because the mass of the truck is decreasing, the