Frame of reference is the coordinate axes (x and y)
Suppose there are no external forces acting on a particle in motion. Due
to inertia of motion the particle always moves in a straight line and
at uniform speed and uniform velocity.
If a body has to move in a
circular arc, there has to be an external force acting perpendicular to
this straight line to turn the particle in an arc. This external force
has to supply acceleration perpendicular to particle's velocity. So
path of the particle deviates from initial path. After a little time,
again the body continues to go in a straight line along the bent path.
So in order to make it move in a circular arc, the external force acts
on it perpendicular to its velocity and bends the particle in a circular
At each point on the path of the particle, an external
force is needed to make the particle continue in the circular path. Such
a force is called centripetal force.
For earth moving in a
circular path around the Sun, the gravitational force of attraction acts
on the Earth at each point on the circle. It is in the direction of
radius towards the Sun. This is the centripetal force.
force is not a real force. it is a fictitious or imaginary force. It is
used to solve our problems in an easier way. When a particle is moving
with an centripetal acceleration and there are multiple forces acting on it, we use the concept of centripetal force.
we use the frame of reference which is stationary or moving with a
uniform velocity. Then Newton's laws apply. Suppose we attach ourselves
to a frame of reference which is moving with an acceleration, then
Newtons laws will not be valid. To be able use Newtons' laws, we add a
fictitious force to each object in the opposite direction to that of
acceleration of frame of reference. That force is equal to mass of
particles and that acceleration.
As the earth is rotating about
itself, we are moving around in a circle. The frame of reference is
moving with an acceleration in reality. So for each object we attach a
centrifugal force acting along radius and away from the center of earth.
There is gravitational force (weight) acting towards the center. So
they cancel and we remain stationary in the frame of reference of earth.