Newton’s 3rd law

But first, REVIEW Newton’s 1st Law: An object at rest will stay at rest, an object in motion will stay in motion, unless acted upon by a net force Newton’s 2nd Law: An object’s acceleration is proportional to the force applied to it More force, more acceleration Less force, less acceleration An object’s acceleration is inversely proportional to its mass More mass = less acceleration Less mass = more acceleration

Forces and Interactions Newton’s 3rd Law of Motion: Whenever one object exerts a force on a second object, the second object exerts an equal and opposite force on the first object A force is a push or pull Forces are not individual things but always occur in PAIRS Action Force Reaction Force An action force cannot exist without a reaction force…and vice versa

Action – Reaction Pairs Forces act in action-reaction pairs Equal in magnitude Opposite in direction Example: A man pushes on a wall and the wall pushes on him back. Why doesn’t he move? ACTION ACTION REACTION

What happens when a ball is dropped? When a basketball is dropped, why does it bounce back up? The ball exerts a force (weight) onto the ground The ground exerts a force back on the ball

What happens when a ball is dropped? If the action-reaction pairs are EQUAL and OPPOSITE, don’t they just cancel each other out? If the forces cancel out, then the net force is equal to zero. If the net force is zero, there is NO movement. So why does the ball move?? The ball has less mass than the ground Less mass = more acceleration