Thinking Critically Answer the following questions. Use complete sentences. 1. Graphite is frequently used as a lubricant to increase the efficiency of machines. Why does a lubricant increase efficiency? 2. Some discussions of simple machines consider only the inclined plane and the lever to be simple machines. These discussions classify the other simple machines as types of inclined planes or levers. Classify each of the following machines as a type of inclined plane or lever: screw, wedge, pulley, and wheel and axle. Explain your answers. 3. Two students are moving 90-N cartons of books from the floor to a platform. One student moves 11 cartons in 6 minutes. The other student moves 11 cartons in 5 minutes. Which student does more work on her cartons? Explain your answer. 4. If a machine is used to produce a greater effort force, what other quantity has been decreased? 5. Explain the following statements in terms of simple machines: When a screwdriver is used to drive a screw, the diameter ofthe screwdriver's handle is more important than the screwdriver's length. When a screwdriver is used to pry open a lid on a paint can, the screwdriver's length is more important than the diameter of the screwdriver's handle. B. For an ideal machine, efficiency is always 100 percent. a. close to c. greater than b. equal to d. less than 9. If the mechanical advantage of a machine is greater than 1, effort force is _ resistance force. a. close to c. greater than b. equal to d. less than 10. In an ideal machine, input work is output work. a. close to c. greater than b. equal to d. less than 11. Which of the following items relates to power but not to work? a. distance c. mass h. force d. time 12. The pedals of a bicycle act as a _. a. lever c. wedge h. pulley d. wheel and axle For each statement beiow, write true or rewrite the italicized part to make the statement true. 13. An object has kinetic energy because of its motion and mass. 14. When a constant force acts at an angle to the direction of motion, the equation W = Fa sin ii is used to calculate the work done on the object. 15. The work done by friction has a positive value. 16. If several forces act on a system, the product of the work done by each force equals the work done on the system. 17. One way to calculate power is to multiply force by veiocity. 13. In a pulley system, the resistance force is exerted by the person who is pullingthe rope. 19. In all real machines, MA is less than iMA. 2|]. The efficiency of a machine can be found from the ideai mechanicai advantage aione. 21. The resistanceforce is the force exerted on a machine. 22. The work-energy theorem states that the work done on a system is equal to the change in kinetic energy ofthe system. WORK, ENERGY, AND MACHINES Understanding Physics Concepts Circle the letter of the choice that best completes the statement or answers the question. 1. Any object that has energy has the ability to produce a change in a. itself c. both itself and the world around it b. the world around it d. neither itself not the world around it 2. If the external world does work on a system, the a. amount of work done on the system has a negative value b. energy of the system decreases c. energy of the system increases d. external world becomes warmer 3. Work is done on an object when a force is exerted on the object causing the object to be displaced a. opposite the direction of the force b. in a circle c. perpendicular to the force d. in the direction of the force 4. In which of the following situations is no work done on a book? a. carrying the book down the hall c. picking up the book b. dropping the book d. pushing the book across the desk 5. In which of the following situations is work done on a rock by a gravitational force? a. carrying the rock across the yard c. picking up the rock b. dropping the rock d. pushing the rock across the driveway 6. One definition of is "energy transformed per unit time." a. efficiency c. mechanical advantage b. ideal mechanical advantage d. power 7. An ideal machine has an ideal mechanical advantage that is _ the displacement of the effort force divided by the displacement of the load. a. close to c. greater than b. equal to d. less than