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Collision between moving and stationary object

  1. Dec 16, 2012 #1
    Consider a feral elephant that charges a hovering stationary fly with
    a speed of 5m/s at a hovering fly. What is the speed with which the fly will recoil?

    Well I know the answer's 10m/s as the mass of the fly is negligible with respect to the mass of the elephant... But how?
  2. jcsd
  3. Dec 16, 2012 #2


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    Since the fly mass is negligible, the center of mass is coincident with the elephant. In the c.m. system (stationary elephant) the fly hits the elephant at 5 m/s, bounces off and recoils at 5 m/s in the opposite direction. Add the c. m. speed and the fly will be going 10 m/s.
  4. Dec 17, 2012 #3
    Thank you... :)
  5. Dec 17, 2012 #4


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    I prefer the model of a ball bearing being fired at an oncoming express locomotive with a thick steel plate on the front. Same principle but the resulting high speed ball bearing is somehow a more attractive idea than a squashed fly.

    Also, a really useful version of this is the way you can use slingshot orbits around large planets to speed up space vehicles. It's all to do with transfer of momentum.
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