It will help to avoid confusion if we reserve the simple terms "gram" and "pound" to denote exclusively an amount of matter, that is, a mass, and to use the full expression "gram of force" or "pound of force" whenever we have in mind the pull of the earth upon these masses. Or, one may speak of a _pound-weight_ meaning the amount of attraction exerted by the earth upon the object. The same is true of _gram-mass_ and _gram-weight_. The mass of a body does not change when the body is transferred to another place. The weight, however, may vary, for on moving a body from the equator toward the poles of the earth the weight is known to increase. Important Topics 1. Definition of force. 2. Classification of forces. (a) Duration: constant, impulsive, variable. (b) Direction: attractive, repulsive. 3. Methods of measuring force. (a) By distortion. (b) By change of motion. 4. Graphic representation of forces: component, resultant, equilibrant. 5. Three cases of combining forces. (1) Two forces acting on the same line. (2) Two forces acting in parallel lines. (3) Two forces acting at the same point at an angle. 6. Units for measuring force, pound, gram. Exercises 1. Name five natural forces. Which produce a tension? Which a pressure? 2. How much can you lift? Express in pounds and kilograms. 3. Show graphically the resultant of two forces at right angles, one of 12 lbs., the other of 16 lbs. What is the magnitude of this resultant? Then determine the answer, first by measurement and then by computation. Which answer is more accurate? Why? 4. Represent by a parallelogram the two forces that support a person sitting in a hammock and draw the line representing the resultant. 5. Find graphically the resultant of the pull of two forces, one of 500 lbs. east and one of 600 lbs. northwest. 6. Determine the equilibrant of two forces, one of 800 lbs. south and one of 600 lbs. west. 7. Would the fact that weight varies on going from the equator to either pole be shown by a spring balance or a beam balance? Explain. (2) MOTION. NEWTON'S LAWS OF MOTION =75. Motion a Change of Position.=--Motion is defined as a continuous _change in the position_ of a body. The _position_ of a body is usually described as its _distance_ and _direction_ from some fixed point. Thus a man on a boat may be at rest with respect to the boat and moving with respect to the earth. Or, if he walks toward the stern as fast as the boat moves forward, he may keep directly over a rock on the bottom of the lake and hence not be moving with reference to the rock and yet be in motion with respect to the boat. Motion and rest, therefore, are _relative_ terms. The earth itself is in motion in turning on its axis, in moving along its orbit, and in following the sun in its motion through space. Motions are classified in several ways: (A) MODES OF MOTION 1. _Translation._--A body is said to have motion of _translation_ when every line in it keeps the same direction.