mudzm2 - Muddy Card Responses Lecture M2 Can you describe...

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Muddy Card Responses Lecture M2 Can you describe what the physical meaning of a moment is, and how do you choose r in rxF. A moment (or more correctly a moment of a force) is the effect of a force acting about an axis to cause an angular acceleration (or rotation). r is to some extent arbitrary. It is the position vector of some point on the line of application of the force F. What are i and j in the force moment equations, why do you use this notation: R ij is the force acting on particle i due to particle j. I use the superscripts because I reserve a subscript to define a tensor quantity (which we will see later in the term). r i is the position vector of point i. i When we write r i × F + M i = 0 , what is M I ? It is a pure moment. In reality a pure moment usually results from a force couple (i.e. two equal and opposite forces with parallel lines of action). Given this it can be argued that the M I is superfluous. Note in the particular example I did on the board there were no pure moments applied to the system, so there were no M I terms. I missed the derivation that resulted in the sum of moments in a system being equal to the sum of the external moments? Confused as to why moments cancel? Please explain why. r 1 × R 1 3 + r 3 × R 3 1 = 0 ? Given that R 13 = R 31 and that the two forces have the same line of action this implies that r 1 × R 1 3 = r 3 × R 3 1 . Note that this is only true when the particles in the system are connected by the internal reactions. It is not true of an arbitrary system of particles where there are no
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This note was uploaded on 02/20/2012 for the course AERO 16.02 taught by Professor Charlescoleman during the Winter '12 term at MIT.

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mudzm2 - Muddy Card Responses Lecture M2 Can you describe...

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