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Unformatted text preview: CHAPTER 6 WORK AND ENERGY ANSWERS TO FOCUS ON CONCEPTS QUESTIONS 1. (e) When the force is perpendicular to the displacement, as in C, there is no work. When the force points in the same direction as the displacement, as in B, the maximum work is done. When the force points at an angle with respect to the displacement but has a component in the direction of the displacement, as in A, the work has a value between zero and the maximum work. 2. (b) Work is positive when the force has a component in the direction of the displacement. The force shown has a component along the x and along the + y axis. Therefore, displacements in these two directions involve positive work. 3. (c) The work is given by W = ( F cos ) s , which is zero when F = 0 N, s = 0 m, or = 90. 4. 78 kgm 2 /s 2 5. (a) The kinetic energy is 2 1 2 KE mv = . Since the velocity components are perpendicular, the Pythagorean theorem indicates that 2 2 2 East North v v v = + . Therefore, ( 29 ( 29 ( 29 ( 29 2 2 2 2 1 1 East North 2 2 KE 3.00 kg 5.00 m/s 8.00 m/s m v v = + = + 6. 115 m/s 7. (e) The workenergy theorem states that W = KE f KE . Since work is done, the kinetic energy changes. Since kinetic energy is 2 1 2 KE mv = , the speed v must also change. Since the instantaneous speed is the magnitude of the instantaneous velocity, the velocity must also change. 8. (d) The workenergy theorem indicates that when the net force acting on the particle does negative work, the kinetic energy decreases. Since each force does negative work, the work done by the net force must be negative, and the kinetic energy must decrease. But when the kinetic energy decreases, the speed must also decrease, since the kinetic energy is proportional to the square of the speed. Since it is stated that the speed increases, this answer is not possible. 9. (b) The workenergy theorem states that the net work done on the particle is equal to the change in the particles kinetic energy. However, the speed does not change. Therefore, the kinetic energy does not change, because kinetic energy is proportional to the square of the 26 WORK AND ENERGY speed. According to the workenergy theorem, the net work is zero, which will be the case if W 1 = W 2 . 10. (d) Since the block starts from rest, the work energy theorem is ( 29 2 2 2 1 1 1 f f 2 2 2 2.70 kg W mv mv v = = The final speed can be obtained from this expression, since the work done by the net force is W = (75.0 N) (cos 38.0) (6.50 m) + (54.0 N) (cos 0.0) (6.50 m) + (93.0 N) (cos 65.0) (6.50 m) 11. (c) The gravitational force is a conservative force, and a conservative force does no net work on an object moving around a closed path....
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 Fall '11
 rollino
 Physics, Energy, Force, Potential Energy, Work, WorkEnergy Theorem, kef, mgh

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