| Description in Words | Basic Formula |
| (1) Newton's First Law | When no force acts on a body, or when the vector sum of all forces acting on it (the net force) is zero, the body is in equilibrium. If the body is initially at rest, it remains at rest; if it is initially in motion, it continues to move with constant velocity. This law is valid only in inertial frames of reference. A body acted on by no net force moves with constant velocity (which may be zero) and zero acceleration.——A body acted on by no net force moves with constant velocity (which may be zero) and zero acceleration.——If the resultant force acting on a particle is zero, the particle will remain at rest (if originally at rest) or will move with constant speed in a straight line (if originally in motion). | ΣF = 0 |
| (2) Newton's Second Law | If a net external force acts on a body, the body accelerates. The direction of acceleration is the same as the direction of the net force. The net force vector is equal to the mass of the body times the acceleration of the body.——If a net external force acts on a body, the body accelerates. The direction of acceleration is the same as the direction of the net force. The net force vector is equal to the mass of the body times the acceleration of the body.——If the resultant force acting on a particle is not zero, the particle will have an acceleration proportional to the magnitude of the resultant and in the direction of this resultant force.——The force acting on a body is proportional to the product of the mass and the acceleration in the direction of the force. | ΣF = ma |
| (3) Newton's Third Law | If a body A exerts a force on body B (an "action"), then body B exerts a force on body A ("reaction"). These two forces have the same magnitude but are opposite in direction. These two forces act on different bodies.——If body A exerts a force on body B (an "action"), then body B exerts a force on body A (a "reaction"). These two forces have the same magnitude but are opposite in direction. Every force represented on a body is an external force acting on that body. Only the reaction forces acting on a body are represented.——The forces of action and reaction between bodies in contact have the same magnitude, same line of action, and opposite sense. | FA on B = –FB on A |