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Homework Help
Introductory Physics Homework Help
Motion with gravitational, elastic, and friction forces
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[QUOTE="henry3369, post: 4987944, member: 538766"] [h2]Homework Statement [/h2] A 2000-kg elevator with broken cables in a test rig is falling at when it contacts a cushioning spring at the bottom of the shaft. The spring is intended to stop the elevator, compressing 2.00 m as it does so. During the motion a safety clamp applies a constant 17,000-N frictional force to the elevator. What is the necessary force constant k for the spring? [h2]Homework Equations[/h2] W[SUB]other[/SUB] + K[SUB]i[/SUB]+U[SUB]i[/SUB] = K[SUB]f[/SUB]+U[SUB]f[/SUB] [h2]The Attempt at a Solution[/h2] W[SUB]other[/SUB] + K[SUB]i[/SUB] = U[SUB]f[/SUB] (-17000 * 2) + (1/2)(2000)(4[SUP]2[/SUP]) = (1/2)k(2[SUP]2[/SUP]) + (2000)(-9.8)(-2) So the correct answer to the problem is k = 10600 N/m. My book solves this by making the final potential energy negative so that it is (2000)(9.8)(-2) and then solving for the spring constant. What I don't understand is why the force of gravity is not negative to make the entire final potential energy positive. The negative 2 clearly states that down is negative so why is gravity not negative if it is also pointing down? [/QUOTE]
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Introductory Physics Homework Help
Motion with gravitational, elastic, and friction forces
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