Question regarding "equal and opposite" reactions

Question

If every action produces and equal and opposite reactions, how does anything ever get moved? If I apply 100 Newtons to a mass and the mass pushes 100 Newtons back, wouldn't the two forces cancel each other out, leaving the mass stationary, because of this principal? Thanks.

Answer 1

It's really just a different way of thinking about it, but that way helps you think in a way that leads to solving physics problems.  No, the forces don't cancel.  Force is applied to the mass by your hand.  The mass could just sit there and ignore it.  This is a way of saying the mass feels that force, it applies a force in the opposite direction, 'pushing' back against your hand.  If the mass stays stationary, that force is also transmitted to the ground it sits on, which also pushes back, holding the mass in place.  If that doesn't happen, then the mass pushing off of your hand causes the mass to accelerate away from your hand.  It's a way of thinking of each object as more self-contained than if you simply say the push of your hand caused the ball to sail toward home plate.  Most of the time, you won't think about it, but sometimes it can be useful to help sort out complicated situations.