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Homework Help: Kinetic Energy and Velocity

  1. Apr 13, 2017 #1
    1. The problem statement, all variables and given/known data
    Hi Guys, I dont have a particular problem that I need to solve but I need help with understanding a concept. So we agree that irrespective of mass of an object they will both fall at a same rate if there is no air resistance. However, why is it then when we increase the mass of an object, the potential energy increases, which as a result increases the kinetic energy and which would also increase the velocity of that object.

    Isn't this contradictory? Or am I misunderstanding something? Please help!

    2. Relevant equations
    Eg = mgh
    Ek = 1/2mv^2

    3. The attempt at a solution
  2. jcsd
  3. Apr 13, 2017 #2


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    Staff: Mentor

    That is false. Kinetic energy is not velocity. It depends on both mass and velocity. Different mass, different KE for the same velocity.
  4. Apr 13, 2017 #3
    For an object dropped from rest from a specific height, y = h, the total energy at h, is TE = PE + KE. At y = h, PE = mgh and KE = 0 (rest). So TE = mgh.

    The value of TE calculated does not change as the object falls. At any y value along the path, the total energy is the same, this includes the point at y=0. So, at y = 0, TE = PE + KE --> mg0 + 1/2 mv2 = TE = mgh. The mass cancels out which makes the velocity independent of the mass.
  5. Apr 13, 2017 #4
    in a simple way...try imagining a bowling ball falling from 1m on your hand vs a tennis ball falling they're pulled at the same speed 9.8m/s just the mass differs the impact on your hand) that velocity is the same just if maybe your friend pushes it down while dropping it it increases the speed..
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