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Homework Help: Conservation of energy

  1. Apr 1, 2008 #1
    1. The problem statement, all variables and given/known data
    use an imaginary scenario of a falling rock in a tall vacuum chamber. suppose the rock has a mass of 0.5 kg, and it falls from a height of 78.4, the falling time from the top to the bottom of the chamber is 4.0 sec. the kinetic energy of the rock is zero (Ek = 0) at the top of the chamber and the gravitational potential energy of the rock (Eg = 0) at the bottom of the chamber.

    h = 78.4 cm
    mass = 0.5kg
    time= 4 sec

    PROVE that Etotal = Eg + Ek during falling time. use 1.0 sec interval to calculate total energy of the rock at different height

    2. Relevant equations

    Ek = 1/2 mv^2
    Eg = mgh

    3. The attempt at a solution

    i am not sure how to find velocity.... because it changes when it falls down (Accelerates) :S
  2. jcsd
  3. Apr 1, 2008 #2
    Use kinematics. You have v_initial, the height, the acceleration, and the time it takes to fall. You've got more than enough information to calculate the final velocity when it reaches the ground and the velocity at any point during its fall.
  4. Apr 1, 2008 #3
    ahh :D
    i see i see =p
    thanks Snazzy =]
  5. Apr 1, 2008 #4
    they want you to use the formulas for movement with constant acceleration

    x = x_0 + v_0 t + (1/2) a t^2
    v = v_0 + a t

    to find the height and the velocity at time t, and then use these to calculate the total energy at time t and find out that it is constant.
  6. Apr 1, 2008 #5
    Usually with those you can use the kinematic equations to help solve conservation of energy problems.
  7. Apr 1, 2008 #6
    yup i got it :P
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