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Momentum physics using vectors.

  1. Mar 22, 2013 #1
    1. The problem statement, all variables and given/known data
    Block 1 (0.5 kg) travels with an initial velocity of [ 60i ] + [ 40j ] m/s and then collides with block 2 (0.5 kg) traveling with an initial velocity of [ -60i ] + [ -60j ] m/s.
    After the collision, block 1 has a final momentum of [ 0i ] + [ 5j ] kg*m/s. Assume that no external forces are present and therefore the momentum for the system of blocks is conserved.

    What is the total initial momentum of the system of blocks?
    i + j kg*m/s


    What is the change in momentum for block 1?
    i + j kg*m/s


    What is the change in momentum for block 2?
    i + j kg*m/s


    What is the final momentum of block 2?
    i + j kg*m/s
    --------------------------------------------------------------------------------
    What are the initial kinetic energies of block 1, block 2, and the system of blocks?
    Block 1: J
    Block 2: J
    System of blocks: J


    What are the final kinetic energies of block 1, block 2, and the system of blocks?
    Block 1: J
    Block 2: J
    System of blocks: J


    What is the change in kinetic energy of the system of blocks due to the collision?
    J



    2. Relevant equations

    p=mv

    3. The attempt at a solution

    I tried P1x= -10-40*.5
    p1y=0
    p2x=2.5-40*.5
    where i got the final velocity from m1v1 +m2v2 + etc. =m1v1 +m2v2 + etc.
     
  2. jcsd
  3. Mar 22, 2013 #2

    Simon Bridge

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    It helps to be clearer about your notation:
    If the initial momentum is ##\vec{p}## and the final is ##\vec{q}## then
    ##\vec{p}=\vec{p}_1+\vec{p}_2## right?

    If ##\vec{u}## and ##\vec{v}## are the initial and final velocities respectively, then:
    ##\vec{p}_1=m_1\vec{u}_1## etc. but in your case ##m_1=m_2=0.5\text{kg}=m## so ##\vec{p}=m(\vec{u}_1+\vec{u}_2)##

    Vector addition using unit vectors works like this:

    if ##\vec{u}=u_x\vec{\imath}+u_y\vec{\jmath}## and ##\vec{v}=v_x\vec{\imath}+v_y\vec{\jmath}##;
    then ##\vec{u}+\vec{v}=(u_x+v_x)\vec{\imath}+(u_y+v_y)\vec{\jmath}##

    Rewrite your calculations like this and they'll probably make more sense.
     
    Last edited: Mar 22, 2013
  4. Mar 22, 2013 #3

    HallsofIvy

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    Are these your answers or just a "template"? Do you mean for each that the answer is supposed to be of the form __ i+ __ j kg*m/s where you are to fill in the blanks? If so, why have you not made any attempt to solve them yourself? Do you know what "momentum" is?

    --------------------------------------------------------------------------------
    What are the initial kinetic energies of block 1, block 2, and the system of blocks?
    Block 1: J
    Block 2: J
    System of blocks: J


    What are the final kinetic energies of block 1, block 2, and the system of blocks?
    Block 1: J
    Block 2: J
    System of blocks: J


    What is the change in kinetic energy of the system of blocks due to the collision?
    J



    2. Relevant equations

    p=mv

    3. The attempt at a solution

    I tried P1x= -10-40*.5
    p1y=0
    p2x=2.5-40*.5
    where i got the final velocity from m1v1 +m2v2 + etc. =m1v1 +m2v2 + etc.[/QUOTE]
    So what did you get? Why are you not showing us what you did and what result you got?
     
  5. Mar 24, 2013 #4
    Yes this is a "template" for my class. I am to solve for each of the following. I have tried and simply do not understand. I missed this particular class due to illness and havent managed to understand the material. Least not this part.
    My answers werent submitted because i figured they'd be irrelevant. The computer counted them as wrong. I showed how i got the answers.
     
  6. Mar 24, 2013 #5

    Simon Bridge

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    We don't understand your working as you have written it - we need to see how you are thinking about the problem more clearly in order to best help you. Did you follow the suggestions in post #2?
     
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