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How does an object's velocity change if it's mass suddenly changes?

  1. Sep 30, 2013 #1
    Let’s say we have two superheroes standing-off in a weightless environment. Assume that their masses are identical (100kg each). Hero-A gets catapulted at Hero-B – left to right. At the moment of collision, Hero-A’s inertia is transferred to Hero-B, and B hurtles off to the right while A is left behind (stationary) - just as it would in the pool-ball examples (conservation of momentum).

    Now… instead of bouncing off of each other, let’s say that Hero-A tackles Hero-B and hangs on, effectively DOUBLING his mass at the moment of impact and beyond.
    If the speed before impact was 100mph, does that mean that the speed is suddenly cut in half at and after impact (because the mass is doubled)?
  2. jcsd
  3. Oct 1, 2013 #2

    I'm not sure where you're going with this, but the speed isn't immediately halved because that would imply an infinite force. But the speed does drop to half extremely quickly.
  4. Oct 1, 2013 #3


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    The momentum is conserved ... so just add the momentum vectors together, and then solve for v' that goes with m'=m1+m2.
  5. Oct 1, 2013 #4


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    The velocity of both heroes combined (their common center of mass) is 50mph all the time, even before impact.
  6. Oct 1, 2013 #5
    What your describing is an elastic and an inelastic collision. The second case, where the heroes stay connected, is inelastic. Note that in that case, although momentum is conserved, some kinetic energy is converted - perhaps into heat and perhaps into the sound of a thump.
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