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Balanced and Unbalanced Forces

  1. Sep 24, 2012 #1
    Hey guys,

    So today I was in my Physics lab and a small argument arose. We had a pre-test with a question that stated:

    "Jan pushes on a refrigerator just enough to barely get it to move at a constant speed. Is her force on the refrigerator greater than, less than, or equal to the force of the refrigerator on her"

    So I said that her force on the refrigerator is equal to that of the refrigerator on her. As long as the forces are balanced, the object will move in a constant speed...right? I also said that if there is an unbalanced force, the object will accelerate in the direction of that force.

    My classmates argued against me saying that the force of gravity equals the normal force, yet we don't move. Why would the refrigerator move if the forces were constant?

    Any clarification would be appreciated. Thanks
  2. jcsd
  3. Sep 24, 2012 #2
    It doesn't matter if the object moves at a constant velocity or not even. You're pushing on this object with your hand, and the object pushes back. That pushback force must be equal to the force you apply.

    The question of whether the refrigerator accelerates doesn't depend on the force it applies on you. That's just a matter of the force you apply on it versus the force of kinetic friction the ground applies to the refrigerator. In this case, we're saying those forces balance, and so the refrigerator doesn't accelerate. Nevertheless, it still moves. Compare with an object falling at terminal velocity. There is drag trying to slow the object down and gravity trying to speed it up. They balance, but the object still falls.
  4. Sep 24, 2012 #3
    Newtons third law, often abbreviated to N3, tells us that

    Reaction is numerically equal but opposite in direction to action.

    So the force exerted by Jan on the fridge equals the force exerted by the fridge on Jan, except for the direction.

    The fridge moves because other forces on the fridge are less than this push or action by Jan. In other words the force on the fridge are not in balance.

    So why does the fridge not then accelerate?

    Because we are told that Jan pushes with just enough force to keep the fridge going at constant speed. This allows us to assume that Jan adjusts the push to keep the speed constant.

    Now we know that friction is also an adjustable force and furthermore that the static friction force is just larger than the sliding friction force.
    Wo we can imagine Jan's effort being large enough to overcome static friction and then reducing to just exceed the sliding friction opposition.

    This idea of an infinitesimal excess force moving something at constant speed is common in physics.
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