The constant relation, between the acting force and the change of momentum it produces in a body, has led to the adoption of a convenient C.G.S. unit of force called the _dyne_. _The dyne is that force which can impart to a mass of one gram a change of velocity at the rate of one centimeter per second every second._ This definition assumes that the body acted upon is free to move without hindrance of any kind, so that the acting force has to overcome only the _inertia_ of the body. _However_, the _law_ applies in every case of application of force, so that each force produces its full effect independently of other forces that may be acting at the same time upon the body. =80. Newton's Third Law.=--This law has been experienced by everyone who has jumped from a rowboat near the shore. The muscular action that pushes the body forward from the boat also pushes the boat backward, often with awkward results. The law is stated: _To every action, there is always an opposite and equal reaction, or the mutual actions of any two bodies are always equal and opposite in direction_. Many illustrations of this law are in every one's mind: a stretched rope pulls with the same force in one direction as it does in the opposite direction. If a bat hits a ball, the ball hits the bat with an equal and opposite force. The third law is therefore sometimes called the law of _reaction_. When a weight is hung upon a spring balance the action of the weight pulls down the spring until it has stretched sufficiently (Hooke's Law) to produce an elastic _reaction_ that equals and hence supports the weight. When a man stands at the center of a plank supported at its ends, the action of the man's weight bends the plank until the elastic force developed in the plank equals the weight applied. Further, when a train or a wagon is on a bridge the bridge yields until it has developed an elastic reaction equal to the weight applied. If a person stands in the center of a room, the floor beams yield until the third law is satisfied. In fact, whenever a force acts, a contrary equal force always acts. =81. Stress and Strain.=--A pair of forces that constitute an action and a reaction is called a _stress_. The two forces are two parts of one _stress_. If the two forces act away from each other, as in the breaking of a string, the stress is called a _tension_, but if they act toward each other as in crushing anything, the stress is called a _pressure_. In order for a body to exert force it must meet with resistance. The force exerted is never greater than the resistance encountered. Thus one can exert but little force upon a feather floating in the air or upon other light objects. A fast moving shot exerts no force unless it encounters some resistance.