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Suppose that you want to move a heavy file cabinet which is standing in the middle of your office into a corner.  You push on it, but nothing happens.  What is going on? 
  • You exert a force, but there is no acceleration.  The net force must be zero. 

Which force of equal magnitude points in a direction opposite to the direction of the force you are applying?

  • The force of static friction (fs) cancels the applied force when the cabinet is at rest while you are pushing on it.

You push harder.  Eventually the cabinet breaks away and starts accelerating.  But you have to keep on pushing just to keep it moving with a constant velocity.  When you stop pushing, it quickly slows down and comes to rest.  Why?

  • While the cabinet is moving the force of kinetic friction (fk) opposes the applied force.  When it is moving with constant velocity, the two forces exactly cancel.

Where do these frictional forces come from?