Physics Circular Motion For Class11

circular motion

Subject: physics
Topic: circular motion
Class: 11


When a particle moves in a horizontal surface with a constant speed the motion of the particle is called uniform circular motion.


Uniform Circular Motion

Anim'n of object undergoing UCMUniform circular motion can be described as the motion of an object in a circle at a constant speed. As an object moves in a circle, it is constantly changing its direction. At all instances, the object is moving tangent to the circle. Since the direction of the velocity vector is the same as the direction of the object's motion, the velocity vector is directed tangent to the circle as well. The animation at the right depicts this by means of a vector arrow.
An object moving in a circle is accelerating. Accelerating objects are objects which are changing their velocity - either the speed (i.e., magnitude of the velocity vector) or the direction. An object undergoing uniform circular motion is moving with a constant speed. Nonetheless, it is accelerating due to its change in direction. The direction of the acceleration is inwards. The animation at the right depicts this by means of a vector arrow.
The final motion characteristic for an object undergoing uniform circular motion is the net force. The net force acting upon such an object is directed towards the center of the circle. The net force is said to be an inward or centripetal force. Without such an inward force, an object would continue in a straight line, never deviating from its direction. Yet, with the inward net force directed perpendicular to the velocity vector, the object is always changing its direction and undergoing an inward acceleration.


It is in Constant speed because the distance between the ground and moving body remain same. So it is called constant speed.


Wen cicular motion take place the angle changes so this is given a name called angular motion. When a partical move in straight line it is called linear motion.


The angle made between the two radius of a circular is called angular displacement and angular displacement is denoted by (θ) and the distance between the two point of radius that make a straight line is called linear displacement
The relation between the angular displacement and linear displacement of circular motion is as the angular displacement increase the linear displacement also increase.
we know that θ=arc /radian
θ=  angular displacement
X = linear displacement
Angular displacement unit = radian


angular displacement# ANGULAR DISPLACEMENT (Δθ)

Angular displacement of a particle in a circular motion is defined as the angle trussed out by the radius vector at the center of circle in the given time it is denoted by (Δθ)
Therefore QθPΔθ = Angular displacement

Angular velocity of a particle  making circular motion 
As we know that velocity = displacement upon time 
ω= angular displacement angular /time taken
ω = θ/T
angular velocity unit is =  radian per second



Angular velocity of particle in a circular motion is defined as the time rate of change of its angular displacement.
Therefore Angular velocity (ω) = Δθ/ΔT
If  ΔT= 0 then angular velocity becomes equal to instantaneous velocity. 

If there is less time and the angle is very small its called instantaneous velocity .



LT =Δθ/ΔT =dθ/dT -----------(1)
IF ΔT = 0

#Frequency of circular motion
Frequency of circular motion = number of cycle in 1 second. For an example a particle moves 10 cycle in one second it frequency will be 10.

#  1 Radian Displacement  
When a point is making a circular motion  hence the length of arc is equal to radius between two position 
that angle subtended at centre is 1 radian.


circular motion

Consider a particle moving in a horizontal circle with linear velocity (V) and angular velocity (ω) At anytime (T) then particle is at point P where the radius vector OP = R
After time T+ΔT the particle reached at point Q the angle made at the center is θ
OP = R
ω = Δθ/ΔT
Δθ ωAT -------- (1) 
If the linear displacement of particle is PQ =ΔL in  time  (ΔT) then
Linear displacement
The angle ΔθΔL/R
Now putting the value
ω.ΔT = VΔT/R
V= ω.R ---------(3)


Angular acceleration of a particle in a circular motion is defined
 as the time rate of change of its angular velocity.

Δ ω The change in angular velocity of the particle in the time ΔT then

Angular acceleration = Δω / ΔT
If ΔT tends to be 0 then angular acceleration becomes equal to Instantaneous angular acceleration.

∴ Instantaneous acceleration (α) = LT Δω / ΔT 
= dω / dT


(a)          Scalar Form
If ω1 and ω2 be the angular velocity of particle with time t1 and t2 then
Angular acceleration = change in velocity / change in time
Angular acceleration = ω1 - ω2 / t2 - t1
[V= rω , V1=rω1 ,V2=rω2 ]
α = (V2/r - V1/r)/(T2 – T1)
α =1/r (V2 - V1)/(T2 – T1)
α = a/r
r. α = a    ------------------------------------(1)

(b) Vector Form  

We know that V is linear velocity
V= ω * r     ----------------------------------(1)
Different the above equation (1) with respect to time T
d/dt * v = d/dt (ω * r)
a= dω/dt * r + ω *dr/dt
a= α * r + ω*v     -----------------------(2)
The linear acceleration is composed of two components.
(1)     Tangential acceleration (at) = α * r  according to the rule of cross product of this component acts along the tangent there for this component is called Tangential acceleration.
(2)     Radial acceleration or centripetal acceleration (ac)= ω*v According to the law of cross product this component acts along the radius towards the center of circle therefore this acceleration is called Radial acceleration or Centripetal acceleration.

Net acceleration (a) can be calculated from the prallogram law
(a)= ac2+at2  -------------------(3)