ps6b15 - Physics 111.01 October 8, 2008 PS 6b (15) Problem...

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Physics 111.01 October 8, 2008 PS 6b (15) Problem 1 = Walker P 8-21. A 0.26 kg rock is thrown vertically upward from the top of a cliff that is 28 m high. When it hits the ground at the base of the cliff the rock has a speed of 28 m/s. (a) Assuming that air resistance can be ignored, find the initial speed of the rock. m/s (b) Find the greatest height of the rock as measured from the base of the cliff. m Solution. This problem can be done using forces and acceleration. But it is much easier to do just using conservation of energy. We will evaluate the total energy at three points: (1) is the start of the motion; (2) is the highest point; and (3) is at the base of the cliff. g h v 1 2 3 y y=0 H (a) We compare start and finish. () 13 11 33 22 2 2 2 0 2 28 m/s 2 9.8 m/s 28 m EE KU KU mv mgh mv vvg h 15.34 m/s = +=+ += + =− = (b) Now compare highest and lowest points: 23 2 1 3 2 2 3 2 2 00 2 28 m/s 2 9.8 m/s KU mgH mv v H g 40.0 m = + = = =
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Problem 2 = Walker P 8-52. A 74 kg skier encounters a dip in the snow's surface that has a circular cross section with a radius of curvature of 12 m. If the skier's speed at point
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ps6b15 - Physics 111.01 October 8, 2008 PS 6b (15) Problem...

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