PotW_2_solutions_Spring2012

PotW_2_solutions_Spring2012 - PHYS 172 Problem Of the Week...

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PHYS 172 Problem Of the Week - Solution #2 (Spring 2012) A falling rubber ball bounces off the floor. The velocity just before it hits the floor is <2.7,−5.2,0> m/s. Just after it hits the floor, the ball’s velocity is <2.7,5.2,0> m/s (the y axis is the vertical axis). The ball’s mass is 0.038 kg. The ball is in contact with the floor for only 1.8 × 10 -3 seconds. (a) What is the vector change in momentum of the ball from just before to just after it is in contact with the floor? Express your result as a vector. Let us call the velocity just prior to hitting the floor ° v initial and the velocity just after hitting the floor ° v final . Then we can express the change in momentum as: ( ) 0.038 (2.7 2.7),(5.2 ( 5.2)),0 0,0.40,0 / = - = - = - - - = ° ° ° ° ° final initial final initial p p p m v v kg m s Alternatively, we could find the initial and the final momentum of the ball explicity and then take the difference to find the change in momentum: (0.038 2.7),(0.038 5.2),0 0.1026, 0.1976,0 / (0.038 2.7),(0.038 5.2),0 0.1026,0.1976,0 / 0,0.40,0 / = = ⋅- = - = = = = - = ° ° ° ° ° ° ° initial initial

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