Forces that are equal in magnitude but opposite in direction. For example, if you push against a brick wall and it doesn't move or become deformed, then the brick wall is exerting an equal force...
the action force is the force which acts on the body due to some external effort, the reaction force is what the body acted upon gives back :D
Your bottoms weight pushes against the chair and the chair pushes back against your bottom.
According to the third law of motion action and reaction are equal. You will find that friction and resistance can make it hard to measure, though.
Forces that are equal and anti-parallel.
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