Some interesting facts about the motion of bodies, which we ordinarily find out only as the result of long experience, can readily be understood by a knowledge of the laws of motion and momentum. A body set in motion by a force, such as steam or electricity, starts slowly and its speed increases in proportion to the strength of the force and the resistance of the body. To illustrate: When an electric car moves we experience a heavy jarring; this is due to the seat starting before our body and pulling us along.
The natural state of inorganic or lifeless bodies is one of rest, called inertia. Every body continues in a state of rest, or when set in motion continues to move in a straight line, unless acted upon by some external force. This is the first law of motion.
When an object is moving, its speed may be increased by applying more force. If the force is applied in a different direction from that in which the body is moving, the body will either stop or change its direction of motion. This principle may be expressed by stating that every change of motion is in the direction of the new force applied to the body, and is proportionate to it. This is the second law of motion.
A force never appears singly. That is, there are always two or more contending forces in every mechanical operation and in all mechanical work. To illustrate: When a mechanic attempts to unscrew a nut, the pull or force he applies to the nut is called action, and the resistance is called reaction. The reaction consists of friction and of the tendency of the nut to remain stationary. The relation between action and reaction is such that every action is resisted by an opposite and equal reaction. This is the third law of motion.