Object in Motion

An object in motion or rest depends on its change in position with time and with respect to its surroundings. In kinematics, an object has two states, it can either be at rest or motion.

Rest

An object is said to be at rest if it does not change its position with time, with respect to its surroundings. A book lying on a table, and a person sitting on a chair are examples of an object at rest.

Motion

An object is said to be in motion if it changes its position with respect to its surroundings with the passage of time.

Example: An airplane or a bird flying through the air, a ball thrown by a boy, a ship sailing on a sea are some examples of the objects in motion. Sometimes, the objects are in motion but the motion may not be visible by us. For example, the gas molecules are always in ceaseless random motion is not to visible to the naked eye.

Let’s consider a car at rest with respect to a tree at the origin on the co-ordinate axis at time t = 0.

After a while the car start moving away from the tree and at t = 4 the car is at another position with respect to the tree. During this interval of time the car was in motion and before moving the car was at rest.

Rest and motion are relative

It means an object in one situation can be addressed but in another situation the same object can be in motion. For illustration, a person sitting in a moving train is at rest with respect to his fellow passengers but is in motion with respect to the objects outside the train. A person sitting in his house is at rest with respect to earth but is in motion with respect to other planets.

Types of motion of a body

1. Rectilinear or Translatory motion.

Rectilinear motion is that motion in which a particle or point mass body is moving along a straight line.

Translatory motion is that motion in which a body which is not appoint mass body is moving such that all its constituent particles move simultaneously along parallel straight lines and shift through equal distance in a given interval of time. Example a body sleeping along the inclined plane has translatory motion.

Rectilinear or translatory motion can be uniform or non uniform.

2. Circular or Rotatory motion

A circular motion is that motion in which a particle or a point mass body is moving on a circle.

A rotatory motion is that motion in which a body which is not appoint mass body, is moving such that all its constituent particle moves simultaneously along concentric circle whose centers lie on a line, called axis of rotation and shift through equal angle in a given time.

Circular aur rotatory motion can be two dimensional or three dimensional motion and can be uniform or non uniform motion. If the circular or rotatory motion is uniform it is periodic also.

3. Oscillatory or Vibratory motion

Oscillatory motion is that motion in which a body move to and fro or back and forth repeatedly about a fixed point (called mean position) in a definite interval of time. In such a motion the body is confined within well defined limit (called extreme positions) on either side of mean position. It means a periodic and bounded motion of a body about a fixed point is called an oscillatory motion.

Example: The motion of the pendulum of wall clock is oscillatory motion.

If in the oscillatory motion the amplitude is very small that is microscopic, motion of body is said to be a vibratory motion. In the oscillatory or vibratory motion if the restoring force (or torque) is linear, the motion of the body is said to be simple harmonic motion.

The concept of a point mass object

While studying the motion of an object; sometimes it’s dimensions are of no importance. For example if one travels from one place to another distance place by a bus, the length of the bus may be ignored as compared to the distance travelled. in other words, although the bus has a finite size, yet for the study of the motion of the bus along the road; is motion may be considered as the motion of a point or a particle.

In mechanics, a particle is a geometrical mass point or a material body of negligible dimension.

It is only a mathematical idealisation. In practice, the nearest, approach to a particle is a body, size is much smaller than the distance or the length measurements involved.