
Introduction
Curvilinear stands for along a curve. This is the most general specification of motion. It can either be twodimensional or threedimensional.
Examples of 2D motions are motion of a projectile and circular motion. These involve motions along planar curves.
Examples of 3D motions are a roller coaster, satellite launching etc. They follow a space curve. These are slightly complicated to analyze as 3 space variables are involved.

Description
We defined the quantities distance , displacement , position vector , average speed , instantaneous speed , average velocity , instantaneous velocity and instantaneous acceleration and jerk ” for rectilinear motion. Their definitions are valid for curvilinear motion as well. We will use them extensively.

Specifying the Quantities – Coordinate Systems
The quantities are specified by considering certain reference (the frame). There are 3 general systems of specifications:

Rectangular Coordinate System
It involves 3 mutually perpendicular axes, , and . The axes intersect at a point, known as origin. These axes are fixed. Thus, the unit vectors and are fixed. (THIS IS IMPORTANT)
Any vector can be resolved into its components by projecting it on respective axes. Thus,
When motion is 2dimensional, component is absent.

Specifying Radial and Transverse Components (Polar Coordinates)
The radial direction is that direction to which the radius vector (or position vector) is directed. The unit vector along radial direction is denoted by . The transverse direction is obtained, when is rotated through . Thus,
is the unit vector in transverse direction. Similarly,
denotes the angular position of the particle. Hence, is the rate of rotation and is the rate of change of rate of rotation.
Thus, both velocity and accelerations have radial and transverse components.
Note: Radial component of velocity describes the rate of movement of an object from an observer. In astronomy, the radial velocity describes how quick is the star receding the earth. Transverse component of velocity describes the rate of movement of an object perpendicular to the observer. For an observer observing a star, this is perpendicular to his line of sight.
These 2 apparent components tell us the actual relative velocity of star w.r.t earth.

Specifying Normal and Tangential Components
In this type, a vector is resolved along 2 components, one along the tangent to the path and the other normal to the path. The unit vector along tangent is and along normal is . Unlike and , the unit vectors may change their directions. (THIS IS IMPORTANT).
The velocity vector is always tangential to the path. Hence,
The time derivative of is not equal to zero, as it changes its direction. It is equal to . Therefore,
is the radius of curvature of the curve at that point.
Note: Since the velocity is always tangential to the path, it has only 1 component in this representation. Normal component is zero.

Concept of Radius of Curvature
The term radius is well defined for circles. We also say that for a straight line, the radius is infinite. For any other curve, radius of curvature at a point is the radius of that circle, which best fits the curve or, which shares common tangent with the curve.
Mathematically, if is the equation of curve, then radius of curvature is given by