Motion

“An object is said to be in motion if its position changes with time”.

The concept of motion is a re’ live one and a body that may be in motion relative to one reference system may be at rest relative to another.
There are two branches in physics that examine the motion of an object:

1. Kinematics: It describes the motion of objects, without looking at the cause of the motion.
2. Dynamics: It relates the motion of objects to the forces which cause them.

Point Object:

If the lengths covered by the objects are very large in comparison to the size of the objects, the objects are considered point objects.

Reference Systems:

A rectangular coordinate system consisting of three mutually perpendicular axes, labeled X-, Y-, and Z- axes. The point of intersection of these three axes is called origin (O) and serves as the reference point. The coordinates (x, y. z) of an object describe the position of the object with respect to this coordinate system. To measure time, we position a clock in this system. This coordinate system along with a clock constitutes a frame of reference.

If one or more coordinates of an object change with time, we say that the object is in motion. Otherwise, the object is said to be at rest with respect to this frame of reference.

Total Path Length (Distance):

For a particle in the motion, the total length of the actual path traversed between initial and final positions of the particle is known as the ‘total path length’ or distance covered by it.

The magnitude of displacement may or may not be equal to the path length traversed by an object.

Types of Motion

In order to completely describe the motion of an object, we need to specify its position. For this, we need to know the position coordinates. In some cases, three position coordinates are required, while in some cases two or one position coordinate is required.

Based on these, motion can be classified as:

• One dimensional motion. A particle moving along a straight-line or a path is said to undergo one-dimensional motion. For example, the motion of a train along a straight line, freely falling body under gravity etc.
• Two-dimensional motion. A particle moving in a plane is said to undergo two-dimensional motion. For example, the motion of a shell fired by a gun, carrom board coins etc.
• Three-dimensional motion. A particle moving in space is said to undergo three-dimensional motion. For example, the motion of a kite in the sky, the motion of aeroplane etc.

Displacement

Displacement of a particle in a given time is defined as the change in the position of a particle in a particular direction during that time. It is given by a vector drawn from its initial position to its final position.
 Factors Distinguishing Displacement from Distance

• Displacement has direction. Distance does not have direction.
• The magnitude of displacement can be both positive and negative.
• Distance is always positive. It never decreases with time.
• Distance ≥ | Displacement |