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Energy conservation with a projectile - please help!

  1. Oct 10, 2008 #1
    Energy conservation with a projectile -- please help!

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

    A 2.0 kg projectile is fired with initial velocity components Vox= 30 m/n and Voy = 40 m/s from a point on the earth's surface. Neglect all effects due to air resistance.

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

    2. Relevant equations
    KE= 1/2mv^2
    PE= mgh

    3. The attempt at a solution

    To find the initial velocity, I took the components and did the math as thought it is a right triangle and got 50 m/s. What I don't understand about part A is that when the projectile is at the maximum height, it's only there for an instant and the velocity id zero, which should make the KE zero also correct?

    but the answer is apparently 900J

    I don't know how to find the highest point of the projectile's path, nor part B

    help please?
    Last edited: Oct 10, 2008
  2. jcsd
  3. Oct 10, 2008 #2
    Re: Energy conservation with a projectile -- please help!

    Are you sure the velocity is zero? Or could that only be the y component of the velocity?
    If there is no acceleration velocity does not change. In this example there is no x acceleration.

    Using this information gives the correct answer.

    Part B, consider that it was fired from rest. The instant it's fired is when it's kinetic energy is maximum. At this point all of its energy is kinetic, use:

    [tex] E_k = W [/tex]
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