Chapter_7_solved_problems_Physics_1301_2009 - Chapter 7...

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Chapter 7 solved problems 24. Picture the Problem : The pine cone falls straight down under the influence of gravity. Strategy: The work done by gravity equals the change in kinetic energy according to equation 7-7. The work done by gravity is always W = mgh as indicated in Example 7-2 and Conceptual Checkpoint 7-1. Solution: 1. (a) The work done by gravity on the pine cone equals the increase in its kinetic energy. Set the energies equal and solve for v : ( 29 ( 29 2 1 2 2 2 2 9.81 m/s 16 m 18 m/s W K mgh mv v gh = ∆ = = = = = 2. (b) Air resistance did negative work because the speed and therefore the kinetic energy of the pine cone when it landed was reduced. Air resistance removed energy from the pine cone. Insight: Kinetic friction always does negative work because the force is always opposite to the direction of motion. 33. Picture the Problem : The compressed spring pushes the block from rest horizontally on a frictionless surface. The block slides to the left as indicated in the figure.
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Chapter_7_solved_problems_Physics_1301_2009 - Chapter 7...

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