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Ice skating accident problem

  1. Nov 10, 2008 #1
    1. The problem statement, all variables and given/known data
    A 35 kg ice skater traveling at 12 m/s runs head-on into a 65 kg skater traveling straight forward at 4 m/s. At what speed do the ice skaters travel if they move (stuck) together after the collision?


    2. Relevant equations
    i put it below. i dont know if youre supposed to use something else. or if theres something eaiser.


    3. The attempt at a solution
    answer from the teacher is 1.6 m/s.

    i tried this:
    m1*v1+m2*v2=m1'*v1'+m2'*v2'
    m1*v1+m2*v2=v'(m1'+m2')
    v=(m1*v1+m2*v2)/(m1'+m2')
    v=((35*12)+(65*4))/100
    v=6.8

    thats what i got. is it all wrong? i know the equation stuff isnt because thats from my teacher. but when i do the math its not the same. its supposed to be 1.6 m/s. am i missing a step?

    thank you!
    sweedeljoseph
     
    Last edited: Nov 10, 2008
  2. jcsd
  3. Nov 10, 2008 #2
    The velocity of the second skater's not inputted correctly; reassess the system.
     
  4. Nov 10, 2008 #3
    Keep in mind the skaters are skating "head on". What does that say about their directions relative to each other? So what does that say about the signs of the velocities?

    Casey

    Edit: Asleight is quick! Last three posts we have been typing at the same time! :smile:
     
  5. Nov 10, 2008 #4
    the velocity of the 2nd skater is in wrong? i dont see where its wrong. m1=35, v1=12, m2=65 and v2=4. m1+m2=100. thats what i plugged in isnt it?

    would the skaters going head on make any difference? theres no difference in the distance they traveled since they are together.
     
  6. Nov 10, 2008 #5
    i got it! haha im kind of slow. the velocity for the 2nd skater was supposed to be negative. i dont know why i guess because shes going backwards to the 1st guy. but i get it now.

    thank you!
    sweedeljoseph
     
  7. Nov 10, 2008 #6
    Yes! Velocity is a vector and thus has a direction associated with it. Clearly is skater 1 is moving to the right and you consider his velocity to be positive then a skater moving in the OPPOSITE direction must have a velocity that is negative!!

    You could have chosen the opposite signs for each and gotten the same number. Albeit you would have gotten a negative answer, but that only indicates that the skater is moving in the opposite direction of the one you assumed it was.

    Casey
     
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