The circular motion is a motion in which the distance of a particle (radius) moving in a plane from a fixed point (centre) remains constant then its motion. The Circular motion is a 2D motion in a plane. It can be classified into two types: uniform with constant speed and constant angular rotation rate or non-uniform with changing rotation rate.
Variables of Motion:
Angular Position: The angle subtended by the position vector with the reference line is called the angular position of a particle.
Angular Displacement (θ): It is the angle by which the position vector of the moving particle is rotated with respect to the reference line. It is the dimensionless quantity and its SI unit is radian. The angular displacement is also measured in degrees or revolutions.
Angular Velocity (ω): If θ1 and θ2 are angular positions of a particle at time t1
respectively, then, the average angular velocity is given by:
In a circular motion, Newton’s law is applied in two perpendicular directions. The first one is along the tangent and the other one is perpendicular to it (towards the centre). The component of force acting along the tangent is called the tangential force (Ft) and the component of force acting along the centre is called the centripetal force (Fc).
Tangential Force Ft=Mat=Mdtdv=Mαr
Centripetal Force Fc=Mat=mω2r=rmv2
Note: In absence of the centripetal force the object will move in a straight line with constant speed.
The radius of curvature of the instantaneous circle (R):
Motion in a Vertical Circle:
Consider the motion of a stone tied to a string and whirled in a vertical circle. If at any time (t) the body is at the angular position θ. Now, the forces acting are: tension (T) in the string along the radius (r) towards the centre and the weight (w) of the body i.e mg acting vertically downwards.
Now, Applying Newton’s law towards centre we get,
Now, if Tmin > 0, the stone will move on the circular path.
And, if Tmin ≤ 0, the string will slack and the body will fall down instead of moving on the circle.
Therefore, for looping the loop:
Now applying the conservation of mechanical energy between the lowest point (L) and the highest point (H):
Therefore, for looping the loop the velocity at the lowest point must be greater than or equal to 5gr.