ME 242 Lecture 14 02.25

ME 242 Lecture 14 02.25 - ME 242 Dynamics February 25, 2009...

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1 ME 242 Dynamics February 25, 2009 Eric Wang Review • Revisit Angular momentum example • Midterm preview Recall (prob. 3.5.40) Given: m =10 kg Find: F = 4 j r A / O = (4 i + 1 j ) v A = 10 j H A / O ˙ H A / O N m m/s Last time we found: H A / O = r A / O × mv A = (4 i + 1 j ) × 10(10 j ) = 400 k kg . m 2 /s ˙ H A / O = r A / O × F = (4 i + 1 j ) × (4 j ) = 16 k Nm I Got Confused!! • How can both be true? H A / O = 400 k kg . m 2 /s ˙ H A / O = 16 k Nm “Two men say they’re Jesus one of them must be wrong” - Industrial Disease by Dire Straits “whatever remains, however improbable, must be the truth.” - Sherlock Holmes
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2 Both cannot be true: H A / O = r A / O × mv A = (4 i + 1 j ) × 10(10 j ) ˙ H A / O = r A / O × F = (4 i + 1 j ) × (4 j ) Both are true, but only one is constant! constant because v is changing due to F H A / O = 400 k Kg . m 2 /s ˙ H A / O = 16 k Nm H A / O Right now: Just to Clarify: M O = d dt xyz ( H O ) = d dt ijk r × mv ( ) = d dt ijk
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ME 242 Lecture 14 02.25 - ME 242 Dynamics February 25, 2009...

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