Chapter 3 Motion
Motion is the action of changing position of an object over time. It is a relative property. It depends upon the relative position of the observer and the object. A person sitting in a spaceship is at zero velocity relative to the spaceship, but is moving at the same velocity as the spaceship with respect to the ground. The earth is at rest with respect to us, but it is moving about 30 km/s around the sun. Two bicycle riders cycling on a straight road with the same velocity are at rest with respect to each other but are in motion with respect to the observer on the road.
The study of motion without knowing its cause (force and energy) is called kinematics. It is the simplest branch of mechanics. Mechanics deals with the effects of force, energy, and motion on an object. It has three sub-divisions, kinematics, statics, and dynamics. Dynamics deals with action of forces on motion, and statics deals with the mechanical equilibrium and its relation to forces. Motion can be of translational, rotational, oscillatory, and random type. If an object position is changing with time it is termed as a translational motion. (The transalational motion along a straight line is called a rectilinear motion). The bullet fired from a gun horizontally has a translational motion, it is not a rectilinear though because it may take a parabolic path to reach from one location to another. A billiard ball moving on the table before it strikes with another ball or edge of the table has a rectilinear motion. If an object is retracing its original path over and over about a mean position then the motion is said to be oscillatory. A restoring force is responsible for oscillatory motion. A ball displaced a little from its rest position in a bowl performs an oscillatory motion. Motion that occurs when an object rotates about an axis is said to be rotational. Instead of changing position as in translational motion the object changes its angle in a rotational motion. Ice skaters spin around on the axis from their head to their feet, the earth spins around on the axis that runs from the North Pole to the South Pole, creating day and night, and the revolution of the earth around the sun are all in rotational motion. The motion of a pendulum is both oscillatory and periodic but the motion of the wheels of a car is only periodic because the wheels rotate in a circular motion. Circular motion is periodic but not oscillatory. The wheels do not move to and fro about a mean position. Motion which can not be predictable in practice is called a random motion. The motion in which a body do not have fixed pattern of following specific direction.
