When a pair of balanced forces acts on an object, the motion is

  A group ... of two or more forces ... may be balanced or unbalanced,
but there's no such thing as a force that's balanced or unbalanced.

If there are two forces acting on an object, and the pair is balanced,
then they have the same effect on the object as if there were no force
acting on it at all. 

Then, the object's acceleration is zero, and it moves in a straight line
at a constant speed (which may be zero, but doesn't have to be).

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