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Problems 209 Section 7.2 Work Done by a Constant Force A block of mass 2.50 kg is pushed 2.20 m along a friction- 1. 1, 2 , 3 = straightforward, intermediate, challenging = full solution available in the Student Solutions Manual and Study Guide = coached solution with hints available at http://www.pse6.com = computer useful in solving problem = paired numerical and symbolic problems P R O B L E M S 2. A shopper in a supermarket pushes a cart with a force of 35.0 N directed at an angle of 25.0° downward from the Batman, whose mass is 80.0 kg, is dangling on the free end of a 12.0-m rope, the other end of which is fixed 3. 1. When a particle rotates in a circle, a force acts on it di- rected toward the center of rotation. Why is it that this 2. Discuss whether any work is being done by each of the fol- lowing agents and, if so, whether the work is positive or 3. When a punter kicks a football, is he doing any work on the ball while his toe is in contact with it? Is he doing any 4. Cite two examples in which a force is exerted on an object without doing any work on the object. 6. If the dot product of two vectors is positive, does this imply that the vectors must have positive rectangular components? 7. For what values of ! is the scalar product (a) positive and (b) negative? 8. As the load on a vertically hanging spiral spring is in- creased, one would not expect the F s -versus-x graph line to remain straight, as shown in Figure 7.10d. Explain qualita- 9. A certain uniform spring has spring constant k. Now the spring is cut in half. What is the relationship between k - of each resulting smaller spring? Explain your reasoning. 11. Discuss the work done by a pitcher throwing a baseball. What is the approximate distance through which the force 10. 5. One bullet has twice the mass of a second bullet. If both 13. Two sharpshooters fire 0.30-caliber rifles using identical shells. A force exerted by expanding gases in the barrels 15. A car salesman claims that a souped-up 300-hp engine is a necessary option in a compact car, in place of the conven- 16. Can the average power over a time interval ever be equal to the instantaneous power at an instant within the inter- 17. In Example 7.15, does the required power increase or decrease as the force of friction is reduced? 18. The kinetic energy of an object depends on the frame of reference in which its motion is measured. Give an exam- 19. Words given precise definitions in physics are sometimes used in popular literature in interesting ways. For example, 20. In most circumstances, the normal force acting on an ob- ject and the force of static friction do zero work on the ob- 21. “A level air track can do no work.” Argue for or against this statement. 22. Who first stated the work – kinetic energy theorem? Who showed that it is useful for solving many practical prob- 14. 12. Q U E S T I O N S |