6). A crane lowers a steel beam into place at a construction site. The beam move

Question

6). A crane lowers a steel beam into place at a construction site. The beam moves downwards with constant speed. Consider the work W, done by gravity and the work Wr done by the tension in the cable. Which of the following is correct? A) Wi is positive and Wr is positive. B) We is positive and W'r is negative. C) Wg is negative and W'r is negative. D) Wi and W'r are both zero. E) None of the above 7). No work is done by gravity on a bowling ball that rolls along a bowling alley because A) no force acts on the ball B) no distance is covered by the ball C) the force on the ball is perpendicular to the ball's motion D) all of the above E) none of the above 8) The figure shows four situations in which a force acts on a box while the box slides rightward a distance d across a frictionless floor. The magnitudes of the forces are identical; their orientations are as shown. Rank the situations according to the work done on the box during the displacement, from most positive to most negative. A) d, c, b, a B) a, b, c, d C) a, e, d, b D) e, d, b, a E) none of the above 9). Two balls of equal size are dropped from the same height from the roof of a building. One ball has twice the mass of the other. When the balls reach the ground, how do the werk done by weight on the two balls compare? A) The work done by weight on the lighter one is one fourth as much as the other does. B) The work done by weight on the lighter one is as much as the other does. C) The work done by weight on the lighter one is one half as much as the other does. D) The work done by weight on the lighter one is twice as much as the other does. E) The work done by weight on the lighter one is four times as much as the other does. 10). An elevator supported by a single cable descends a shaft at aconstantspeedThe only forces acting on the elevator are the tension in the cable and the gravitational force. Which one of the following statements is true? B) The net work done by the two forces is zero joules A) The work done by the tension force is zero joules. C) The work done by the gravitational force is zero joules. D) The magnitude of the work done by the gravitational force is larger than that done by the tension force. E) The magnitude of the work done by the tension force is larger than that done by the gravitational force.

Explanation / Answer

6.

B

tension force acts Up and gravity force acts down, the displacement is in down direction. since tension force and displacement are in opposite direction , work done by tension force is negative . while work done by gravity force is positive

7.

C

since the force is perpendicular to the displacement , the work done is zero

8.

work is most positive when force is in same direction as the displacement. force is in right direction in case "d". hence

A)

9)

C)

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