Motion

Motion is change in position of an object with time.Motion of object along a straight line is called rectilinear motion. Examples include flying kite, moving train, earth’s rotation etc.

 

Frame of Reference

In order to know the change in position of an object, a reference point is required. Point O in the figure is the reference point or Origin and together with three axes, this system is called the coordinate system. A coordinate system with time frame is called frame of reference.

  • Objects changing positions with time with respect to the frame of reference are in motion while those which do not change position are at rest.
  • For a moving car, for the frame of reference outside the car, it appears moving. While for the frame of reference inside the car, the car appears stationary.



Motion along a straight line

Motion along a straight line is described using only X-axis of the coordinate system.

 

Path Length (Distance) Vs. Displacement

Path Length: It is the distance between two points along a straight line. It is scalar quantity.

Displacement: It is the change in position in a particular time interval. It is vector quantity. Change is position is usually denoted by Δx (x2-x1) and change in time is denoted by Δt (t2-t1).

For the above example, if a person goes from home (O) to school (x2) and comes back from school to Park (x1), then

Path length(Home to School and School to Park) = Ox2 + x2x1 = (+80) + (+60) = +140m. This is always positive.

Displacement(Home to Park) = Ox2 - x2x1 = +80 – (+60) = +20m. This can be positive as well as negative. The negative sign indicates the direction.

  • Magnitude of Displacement may or may not be equal to the path length.
  • For a non-zero path length, displacement can be 0 (case where an object returns to origin).