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Homework Help: Conservation of Mechanical Energy

  1. Nov 8, 2011 #1
    1. The problem statement, all variables and given/known data

    A 1.0 kg projectile is fired from a point on the earths surface at an angle θ = 45° with the horizontal and with an initial velocity v = 30 m/s. Neglect any effects due to air resistance and find:

    a) What is the kinetic energy of the projectile when it reaches the highest point on its trajectory?
    b) How much work was done in firing the projectile?

    2. Relevant equations

    total mechanical energy = KE + PE

    KE_i + PE_i + W = KE_f + PE_f

    3. The attempt at a solution

    I'm not sure how to start this problem, or how to tackle it. I'm assuming I have to use the second equation? I let KE = 1/2 mv^2, m=1.0kg, v=30m/s. Then PE=mgh, m=1.0kg, g=9.81m/s/s, h=?. Then W=Fdcosθ, F=?, d=? cosθ=cos45°. Then I'm sort of stuck... There's a lot of values missing and I'm not sure how to solve or where to start.

    Any help would be appreciated, thank you.
  2. jcsd
  3. Nov 8, 2011 #2


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    Homework Helper

    Hi domtar,

    The keyword is "projectile". What do you know about projectile motion?

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