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Homework Help: Help please! velocity

  1. Oct 11, 2007 #1

    klm

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    i thought i figured it out, but i didnt

    A rocket-powered hockey puck starts from the origin and moves on a horizontal frictionless table. In which direction is the puck moving at t=2s? (B) how far from the origin is the puck at t=5


    for the first part i just thought i needed to look at t=2 s on both graphs, i guessed that vx=15 and vy=30 then i just found the inverse tan to find the direction to be 63.4 degrees. but that is incorrect
     
    Last edited: Oct 11, 2007
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  3. Oct 11, 2007 #2

    PhanthomJay

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    If you look at the slope of the first graph, by determining the vx/t ratio at 5 seconds, you can get exactly what vx is at 2 seconds.
     
  4. Oct 11, 2007 #3

    klm

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    i'm sorry i do not understand what you mean? because i think that the slope is equal to (40-30)/(5-4) = 10 , but vx does not equal 10 at t=2s
     
  5. Oct 11, 2007 #4

    klm

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    bump please. i am online so if anyone is willing to help and talk me through it that would be great
     
  6. Oct 11, 2007 #5

    learningphysics

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    slope is (40-0)/(5-0) = 8.

    So what is vy at t = 2?
     
  7. Oct 11, 2007 #6

    klm

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    vy=30 ?
     
  8. Oct 11, 2007 #7

    klm

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    so does that mean vx=16 and vy=30. so you just do inverse tan (30/16)= 61.9 and that is the direction?
     
  9. Oct 11, 2007 #8

    klm

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    bump. please can someone tell me if i am thinking about this correctly?
     
  10. Oct 11, 2007 #9

    learningphysics

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    Yes, that's correct.
     
  11. Oct 11, 2007 #10

    klm

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    thank you very much. can you please help me out with the 2nd part as well? how far from the origin is the puck at t=5
     
  12. Oct 11, 2007 #11

    learningphysics

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    area under the v-t graph gives displacement... area under the vx-t graph gives horizontal displacement. area under the vy-t graph gives vertical displacement.
     
  13. Oct 11, 2007 #12

    klm

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    okay so do you do for vx .5(5x40)=100 and vy (5x30)=150
    then do you take the magnitude?
     
  14. Oct 11, 2007 #13

    learningphysics

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    yes, exactly.
     
  15. Oct 11, 2007 #14

    klm

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    so square root ( 100^2 + 150^2) = 180.28 m ?
     
  16. Oct 11, 2007 #15

    learningphysics

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    yes.
     
  17. Oct 11, 2007 #16

    klm

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    ^when i try to put that in, it says it is incorrect.
     
  18. Oct 11, 2007 #17

    learningphysics

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    oops sorry... it's 180.28cm = 1.8028m
     
  19. Oct 11, 2007 #18

    klm

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    oohhh thank you so much! i wouldn't have caught that! thank you for your help!
     
  20. Oct 11, 2007 #19

    learningphysics

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    no prob.
     
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