A 15 kg, 1m wide door which has frictionless hinges is closed but unlocked. A 400 g ball hits the exact middle of the door at a velocity of 35 m/s and bounces off elastically, thereby causing the door to slowly swing open. How long in seconds does it take for the door to fully open (rotate 90 degrees)?
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at what height is an object that weighs 490 N gravitational potential energy is 4900 N-m/
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http://www.jiskha.com/display.cgi?id=1369042520
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