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Homework Help: Change in Impulse and Momentum

  1. Nov 1, 2007 #1
    1. The problem statement, all variables and given/known data

    An object of mass 3.0 kg is projected into the air at a 45° angle. It hits the ground 3.7 s later. What is its change in momentum while it is in the air? Ignore air resistance.
    _____ kg·m/s downward

    2. Relevant equations

    F[tex]\Delta[/tex]T = [tex]\Delta[/tex]P = m[tex]\vec{v}[/tex][tex]_{f}[/tex]-m[tex]\vec{v}[/tex][tex]_{i}[/tex]

    3. The attempt at a solution

    This is a really basic question i am sure but i am getting thrown off by the 45 degree angle what do I do?
  2. jcsd
  3. Nov 1, 2007 #2
    The object moves in projectile motion, so you need to use the equations for constant acceleration, and the two projectile equations relating each component of the initial velocity to the initial velocity vector. This will allow you to find the initial and final velocities of the mass. Does this make sense?
  4. Nov 4, 2007 #3
    No this doesnt make any sense to me at all. I know this should be a very simple problem but i dont know wat to do
  5. Nov 4, 2007 #4
    Is this question worded exactly as you say? I don't know if we have enough info to solve the problem.
  6. Nov 4, 2007 #5
    it is worded exactly how it was posted. i copied and pasted it
  7. Nov 4, 2007 #6

    Doc Al

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    Staff: Mentor

    You have all the information needed to figure out the initial and final velocity of the mass. (It's just a change in momentum question, not really an impulse question.)
  8. Nov 4, 2007 #7
    I still do not understand how to figure it out. I knwo it is a change in momentum question but i dont know how to do it.
  9. Nov 4, 2007 #8
    can anybody help on this one?
  10. Nov 4, 2007 #9
    Start by considering the object's movement in the vertical direction. It moves with constant acceleration, so we can use a kinematic equation. We'd have:

    [tex] y = y_0 + v_{0y}t - .5gt^2 [/tex]

    And we know that [tex] v_{0y} = v_0sin\theta [/tex]

    Now you can solve for the initial velocity, right?
  11. Nov 4, 2007 #10


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    Homework Helper

    At the point of projection the vertical component of the velocity is vsin(theta). By using angle of projection and time of flight you can find this velocity. Just before the object hits the ground this velocity is the same but the direction is changed. So take one as positive and other as negative and find the change in velocity and hence momentum.
  12. Nov 4, 2007 #11
    if i took one as postive and one as negative wouldnt they just cancel each other out if they were the same? or is it a negative minus a negative?
  13. Nov 4, 2007 #12


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    Homework Helper

    Yes. It is a negative minus a negative = positive.
  14. Nov 4, 2007 #13
    negative minus a negative = a bigger negative
  15. Nov 4, 2007 #14
    You can work it out on paper and see if it's true on not.
  16. Nov 4, 2007 #15
    I have been trying to do that. I just dont know how to do it thats why im having problems with this problem. It is an extremely easy problem, for some reason I cannot figure out how to incorporate the angle into the F(deltaT) = mvf-mvi equation.
  17. Nov 4, 2007 #16
    You have the mass, all you need are the initial and final velocities. Lets start by using the equation:

    [tex] y = y_0 + v_{0y}t - .5gt^2 [/tex]

    where [tex] v_{0y} = v_0sin\theta [/tex].

    You assume that y is zero, because the object moves up some distance, and comes back down covering that same distance, right?
  18. Nov 4, 2007 #17


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    Homework Helper

    F(deltaT) = mvf-(-mvi ) = ?
  19. Nov 4, 2007 #18
    ok then the Vi = 25.6393 roughly .. then what?
  20. Nov 4, 2007 #19
    Yes, correct. Convert that into the initial velocities in the x and y directions using :

    [tex] v_{0x} = v_0cos\theta, v_{0y} = v_0sin\theta [/tex]

    and find the final velocity components using [tex] v_f = v_0 + at [/tex]

    So far so good?
  21. Nov 4, 2007 #20
    ok whats next?
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