2. _Rotation._--A body has motion of _rotation_ when it turns upon a fixed axis within the body, as a wheel upon its axle or the earth upon its axis. 3. _Vibration_ or _Oscillation_.--A body is said to have _vibratory_ or _oscillatory_ motion when it returns to the same point at regular intervals by reversals of motion along a given path, _e.g._, a pendulum. (B) DIRECTION OF MOTION 1. _Rectilinear._--A body has rectilinear motion when its path is a straight line. Absolute rectilinear motion does not exist, although the motion of a train on a straight stretch of track is nearly rectilinear. 2. _Curvilinear._--A body has _curvilinear_ motion when its path is a curved line, _e.g._, the path of a thrown ball. (C) UNIFORMITY OF MOTION 1. _Uniform._--A body has uniform motion when its speed and direction of motion do not change. Uniform motion for extended periods is rarely observed. A train may cover, on an average, 40 miles per hour but during each hour its speed may rise and fall. 2. _Variable._--A body has variable motion when its speed or direction of motion is continually changing. Most bodies have variable motion. 3. _Accelerated._--A body has accelerated motion when its speed or direction of motion continually changes. If the speed changes by the same amount each second, _and the direction of motion does not change_ the motion is said to be _uniformly_ accelerated, _e.g._, a falling body. Uniformly accelerated motion will be studied further under the topic of falling bodies. _Velocity_ is _the rate of motion_ of a body in a given direction. For example, a bullet may have a velocity of 1300 ft. a second upwards. _Acceleration_ is _the rate of change of velocity_ in a given direction, or the change of velocity in a unit of time. A train starting from a station gradually increases its speed. The gain in velocity during one second is its acceleration. When the velocity is decreasing, as when a train is slowing down, the acceleration is opposite in direction to the velocity. A falling body falls faster and faster. It has _downward acceleration_. A ball thrown upward goes more and more slowly. It also has _downward acceleration_. =76. Momentum.=--It is a matter of common observation that a heavy body is set in motion with more difficulty than a light one, or if the same force is used for the same length of time upon a light and a heavy body,[E] the light body will be given a greater velocity. This observation has led to the _calculation_ of what is called the "quantity of motion" of a body, or its _momentum_. It is computed by multiplying the mass by the velocity. If the C.G.S. system is used we shall have as the momentum of a 12 g. body moving 25 cm. a second a momentum of 12 × 25 or 300 C.G.S. units of momentum. This unit has no name and is therefore expressed as indicated above. The formula for computing momentum is: _M = mv_. [E] By a light body is meant one of small mass, a heavy body possessing much greater mass.