# Why doesn't an object that collides with one that is at rest just do a 180?

Newton's third law says "for every action, there's an equal and opposite reaction." So why is it, say, that when a pool/billiards ball hits the wall of the table, the ball doesn't just turn around?

• The actions and reactions Newton refers to are forces, not velocities. – dfan Dec 15 '14 at 23:55

Note that in your suggested motion after the collision, momentum would not be conserved. The Law of Conservation of Momentum is kind of a big deal in Physics, and especially useful when analyzing collisions. It states that for a closed system, momentum is conserved. It's also sometimes restated that for a closed system experiencing a collision, the momentum before the collision is equal to the momentum after. In your example, object $A$ has some mass, $m_A$ and is traveling with some velocity $v$ (If we choose a coordinate system such that $A$ is moving in the positive x direction)and a second object, $B$ is at rest. So before the collision the total momentum $p$ is given by $p=m_Av$ However, if $A$ just "turned around" after colliding with $B$ it would now be moving in the negative x direction and so would have a velocity of $-v$. As a result the momentum after the collision would be given by $-m_Av$ which is decidedly not equal to $m_Av$ anymore than -5 equals 5.