# lecture24 - Mechanical Energy More examples...

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1 Physics 1D03 Mechanical Energy Serway 8.3, 8.4 More examples Non-conservative forces Practice problems: Serway chapter 8, problems 19, 21, 23, 53, 54, 55 Physics 1D03 Mechanical Energy E = K + U = K + U gravity + U spring + ... Mechanical energy is conserved by “conservative” forces; the total mechanical energy does not change if only conservative forces do work.

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2 Physics 1D03 From what initial height must the the block be released on the frictionless track in order to complete the loop-the-loop without falling off? Example Physics 1D03 First: what minimum kinetic energy does it need at the top, in order not to fall? Quiz A) mgR B) ½ mgR C) 2 mgR D) 4 mgR E) zero
3 Physics 1D03 So, the minimum initial h is Quiz h A) ½ R B) 2 R C) 2.5 R D) 3 R E) 3.5 R Physics 1D03 Non-Conservative Forces Non-conservative forces cause the mechanical energy of the system to change. Divide work W into work by conservative forces and work by “other” forces.: Δ K = W = W c + W other W c can be replaced by potential energy terms: W c = - Δ U , so Δ K = U + W

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