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Need help by tonight!

  1. Sep 22, 2003 #1
    For number 11(please see the attachment), I keep messing up and I don't know what I'm doing wrong.

    To find the answer I know I have to do this:
    Speed = sqrt(Vxsquared + Vysquared)

    I know that velocity of x is gonna stay constant, so its v is 25.5

    And for the velocity of y I did this(Vy=Vo*sin(ang)-gt):
    Vy= 25.5*sin(29.1)-9.8(4.41) (time is right)
    Vy= -30.82

    so after plugging these values into the speed equation, I got this answer: 40.

    What's wrong with this?
     
  2. jcsd
  3. Sep 22, 2003 #2
    For some reason, I can't attach the file. Here is the question.

    Given: g = 9.8 m/s2
    A stone is thrown from the top of a building upward at an angle of 29.1(degrees) to the horizontal and with an initial speed of 25.5 m/s. The height of the building is 40.7 m.

    What is the speed of the stone just before it strikes the ground?
     
  4. Sep 22, 2003 #3

    enigma

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    EDIT: ACK hold on... misread...
     
    Last edited: Sep 22, 2003
  5. Sep 22, 2003 #4
    anyway I tried that too got 12.4 for Vy and 28.3 for speed which is still wrong
     
    Last edited: Sep 22, 2003
  6. Sep 22, 2003 #5

    enigma

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    25.5m/s is the magnitude of the total velocity, therefore, it cannot be the same as the x component.
     
  7. Sep 22, 2003 #6
    Damn it got it wrong again, I calculated Vx by 25.5*cos29.1 and still got the answer wrong. I only have 2 chances left to getting it right
     
  8. Sep 22, 2003 #7

    enigma

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    What was the answer you got, NIN?
     
  9. Sep 22, 2003 #8
    37.3 with my last calculation
     
  10. Sep 22, 2003 #9

    enigma

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    That's a bit off my answer. Are you rounding off anywhere?

    Vx = 25.5*cos(29.1) = 22.3
    Vy = -30.8

    sqrt(Vx^2+Vy^2)
     
  11. Sep 22, 2003 #10
    THANX A LOT.
     
  12. Sep 22, 2003 #11

    enigma

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    round off error?

    plugged wrong number in?
     
  13. Sep 22, 2003 #12
    yep damn system accepts only 1% error wrong so 37.3 was wrong. 38 was right
     
  14. Sep 22, 2003 #13

    enigma

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    Is your prof doing anything to make sure that people aren't copying each others' work?

    Submitting the HW to a website just seems like it's a quick easy way for unscrupulous students to cheat...
     
  15. Sep 22, 2003 #14
    LOL, last year the university was giving out hw by handouts, everybody cheated then. That's why they have HW on the web now. Everyone's numbers are different.
     
  16. Sep 23, 2003 #15

    krab

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    Don't worry about x and y components: Use conservation of energy.

    (1/2)mv_f^2=(1/2)mv_i^2+mgh

    or

    v_f=sqrt(v_i^2+2gh)=sqrt(25.5^2+2*9.8*40.7)=38.05
     
  17. Sep 23, 2003 #16

    enigma

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    krab,

    I have a feeling that NIN hasn't gotten to that topic in his classes yet. It is still early in the semester.

    You're right, though. Conservation is the way to go.
     
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