Physics 011

A of PQ#4

 Chapter 4 : Motion and Force: Dynamics Practice Questions

 1. A packing crate slides down an inclined ramp at constant velocity. Thus we can deduce that  The correct answer: a frictional force is acting on it. If it had a net force it would be accelerating. See Section 4-2 in Giancoli. 2. You fall while skiing and one ski (of weight W) loosens and slides down an icy slope (assume no friction), which makes an angle q with the horizontal. The force that pushes it down along the hill is  The correct answer: W sin q All objects at or near the surface of the earth are acted on by gravity. See Example 4-17 in Giancoli. 3. A rope, tied between a motor and a crate, can pull the crate up a hill of ice (assume no friction) at constant velocity. The free-body diagram of the crate should contain The correct answer: three forces. 4. If two identical masses, attached by a massless cord passing over a massless, frictionless pulley of an Atwood's machine (at different heights), are released  The correct answer: the masses will not move. Review Example 4-11 in Giancoli. 5. The same horizontal force is applied to objects of different mass. Which of the following graphs illustrates the experimental results? The correct answer: 6. A train consists of a caboose (mass = 1000 kg), a car (mass 2000 kg), and an engine car (mass 2000 kg). If the train has an acceleration of 5 m/s2, then the tension force in the coupling between the middle car and the engine car is The correct answer: 15,000 N 7. A decoration, of mass M, is suspended by a string from the ceiling inside an elevator. The elevator is traveling upward with a constant speed. The tension in the string is  The correct answer: equal to Mg. This would be true if the elevator were accelerating upwards, for example. See Example 4-6 in Giancoli. 8. Two identical masses are attached by a light string that passes over a small pulley, as shown. The table and the pulley are frictionless. The system is moving The correct answer: with an acceleration less than g. 9. Two cardboard boxes full of books are in contact with each other on a frictionless table. Box H has twice the mass of box G. If you push on box G with a horizontal force F, then box H will experience a net force of  The correct answer: 2/3 F You don't need more force to accelerate less mass. Review Example 4-10 in Giancoli. 10. A toolbox, of mass M, is resting on a flat board. One end of the board is lifted up until the toolbox just starts to slide. The angle theta that the board makes with the horizontal for this to occur depends on the  The correct answer: coefficient of static friction, mu. The mass cancels out. Review Example 4-18 in Giancoli. 11. Four students perform an experiment by pulling an object horizontally across a frictionless table. They repeat the experiment on several other objects of different masses, always applying just enough force to produce the same acceleration. They each graph the results, individually. Which student's graph is correct?  The correct answer: For this to be true F must be independent of m. Review Section 4-4 in Giancoli. 12. An object is placed on an inclined plane. The angle of incline is gradually increased until the object begins to slide. The angle at which this occurs is q. What is the coefficient of static friction between the object and the plane? The correct answer: tan q 13. A mass on a string attached to a ceiling is pulled to the side and let go. When the string makes an angle q with the vertical, which of the following is not true?  The correct answer: The tension equals mg*sin(q). This is always true. Review Section 4-7 in Giancoli. 14. A roller coaster car does a loop-the-loop. When it is upside down at the very top, which of the following is true?  The correct answer: The normal force and the weight are in the same direction. The normal force is down. Review Section 4-7 in Giancoli. 15. A roller coaster car does a loop-the-loop. When it is right-side up at the very bottom, which of the following is true?  The correct answer: The normal force and the weight are in opposite directions. The normal force is perpendicular to the surface of the track. Review Section 4-7 in Giancoli. 16. A 2 kg box sits on a 3 kg box which sits on a 5 kg box. The 5 kg box rests on a table top. What is the normal force exerted on the 5 kg box by the table top?  The correct answer: 98 N Review Section 4-6 in Giancoli. 17. A 2 kg box sits on a 3 kg box which sits on a 5 kg box. The 5 kg box rests on a table top. What is the normal force exerted by the 5 kg box on the 3 kg box? The correct answer: 49 N 18. A 5 kg (4.9 N) book rests on your desk. Which of the following answers is not correct? The correct answer: The normal force on the book equals its weight because of Newton's Second Law. . 19. A 5 kg (4.9 N) book rests on your desk. What is the reaction force to the 4.9 N weight (force) of the book? The correct answer: A 4.9 N force on the Earth. This is already a reaction force to a different force. Review Section 4-5 in Giancoli. 20. You stand on a bathroom scale in an elevator accelerating upwards and it reads 200 Newtons. Which of the following is true?  The correct answer: The scale exert a 200 N force on you. Review example 4-6 in Giancoli. 21. During the investigation of a traffic accident, police find skid marks 90 m long. They determine the coefficient of friction between the car's tires and the roadway to be 0.5 for the prevailing conditions. To find the speed of the car you must The correct answer: find the acceleration of the car. 22. An object moves left to right (right is positive) with speed decreasing at a constant rate and  The correct answer: its acceleration is negative. The acceleration is to the left. Review Section 4-2 in Giancoli. 23. Money is to wealth as The correct answer: mass is to inertia. 24. A car pulls on a rope tied to a tree with a force of 1000 Newtons. Which of the following is not true. The correct answer: The tension in the rope is 2000 Newtons. 25. A 5 kg block and a 10 kg block slide down a frictionless incline and  The correct answer: they both have the same acceleration. Would this be true if the slope were completely vertical. Review Section 4-8 in Giancoli.