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Oct16-10, 02:05 PM
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P: 2,031
you have the right answer for part a but I am not sure how you got there.In the first 0.8 second the collar does not move.In the next 1.2 seconds it does move being subjected to a resultant force which varies from 0 to 6 in 1.2 seconds.The force then remains constant at 6 for a further 1 sec.Look at the graph you presented in post 1 and resketch it by drawing a line parallel to the x axis and meeting the y axis at 4.The change of momentum is then equal to the area under the graph going down to the horizontal line you have just drawn.