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Homework Help: Physics multiple choice

  1. Mar 8, 2007 #1
    1. The problem statement, all variables and given/known data
    A rocket moves through empty space in a straight line with constant speed. It is far from the gravitational effect of any star or planet. Under these conditions, the force that must be applied to the rocket in order to sustain its motion is ?

    3. The attempt at a solution
    I would say zero, since in the absence of an external force, a moving object will move with constant velocity...

    A ball falls from the top of a building, through the air (air resistance is present), to the
    ground below. How does the kinetic energy (KE) just before striking the ground
    compare to the potential energy (PE) at the top of the building?
    A) KE is equal to PE.
    B) KE is greater than PE.
    C) KE is less than PE.
    D) It is impossible to tell.

    The ball is falling, the potential energy decreases and the kinetic energy increases. When the ball hits the ground, I would say that KE is equal to PE. Would this be correct?

  2. jcsd
  3. Mar 8, 2007 #2
    The final kinetic energy is actually less than than the initial PE because of the energy loss to air resistance. PE=KE+Wf
  4. Mar 8, 2007 #3
    Thanks! Is the first one correct?
  5. Mar 8, 2007 #4
    yes, Newton's first law.
  6. Mar 8, 2007 #5
    Thank you!
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