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System in static equilibrium given a nudge. What happens?

  1. Mar 12, 2015 #1
    We're given a problem: A 2.0 kg mass hangs from a rope attached to an unknown mass m on a frictionless ramp inclined at 20 degrees above the horizontal. This is drawn as a diagram which shows the incline at 20 degrees where the hanging mass is 2.0 kg and the attatched mass on the frictionless ramp is labeled m (found in an earlier problem m=5.8 kg).

    The question i'm having issues with is if the 2.0 kg mass is given a quick nudge downwards, both masses will start to move. After being nudged, will the masses gradually speed up, slow down, or maintain at a constant speed? explain.

    What I was thinking is that since it's a frictionless, there's nothing stopping the objects from maintaining the velocity they were given by whatever was nudging them, so they would have no reason to stop and therefore continue at a constant speed.
    I'm not sure if this is right or not...?
  2. jcsd
  3. Mar 12, 2015 #2


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    I'd like to see the original diagram, just in case anything is missing, but as posed you are right.
    If there are no frictional forces and no change in geometry and the system was in equilibrium to start with, then I can't see anything to change their motion.
  4. Mar 12, 2015 #3


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    The mental picture I have is with the 2 kg mass hanging below the ramp (maybe there is a slot cut in the ramp through which the rope can feed). Even if the ramp is frictionless then there may be other forces acting. What about gravity, for instance?

    Edit: Scratch that. Based on the numbers (5.8 kg, 2.0 kg and 20 degrees), it is clear that the 2 kg weight hangs from a pulley near the top of the ramp. The resulting tension parallel to the ramp is just enough to hold the 5.8 kg weight in place.
    Last edited: Mar 12, 2015
  5. Mar 12, 2015 #4


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    Staff: Mentor

    Good deduction! (If correct, that is. :wink: )

    This illustrates the perils of attempting to solve problems where the poster withholds crucial information, viz., the diagram.
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