Newton's Third Law of Motion

Newton's Third Law of Motion states: To every action there is an equal and opposite reaction.

By action, the law means force or impulse These are very closely related, since It means basically that if you give something a shove, it shoves back.

In a collision there is always an exchange of momentum. If two bodies collide, then each exerts an impulse on the other. The impulses are equal in magnitude and opposite in direction. If a body experiences an impulse, it's momentum changes by an amount equal to the impulse: The actions must be equal and opposite not only in magnitude but also in nature: If one force is gravitational, so is the other, if one force is electrical, so is the other, if one force is a contact force so is the other etc. Notice that the bodies do not actually have to be in contact. Nor do the accelerations have to be equal in magnitude – only the forces do. For the two masses above the smaller body will have a larger acceleration than the larger, since it's mass is sammler.

The forces must act along the same line.  