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Homework Help: Change in linear momentum

  1. Oct 23, 2005 #1
    A 0.118 kg mass is moving horizontally with a speed of 8.49 m/s when it strikes a vertical wall. The mass rebounds with a speed of 1.07 m/s. What is the magnitude of the change in linear momentum of the mass?

    isn't the change just the difference in momentum? I did .118(8.49 - 1.07) to get .87556 kg m/s. this is wrong so i think the answer is .118(8.49+1.07) because it would be minus a negative velocity. so i got 1.128 kg m/s. but i don't know if this number should be negative or not.
  2. jcsd
  3. Oct 23, 2005 #2


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    Homework Helper

    The change is, as you say, just the differnce in momentum.

    But momentum is mv. And v (the velocity) is a vector and has a direction.
    So deltaP = mv1 - mv2, but v1 and v2 are in opposite directions, so you have to (numerically) add them.

    Since you are asked for a differnce, then take the absolute value, i.e. positive.
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