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Dynamics of Circular Motion


We know that linear acceleration of a particle in circular motion has two components, tangential and radial (or centripetal). So, normally we resolve the forces acting on the particle in two directions:

(i) Tangential

(ii) Radial

Circular MotionCircular Motion

In tangential direction, net force on the particle is maand in radial direction net force is mar. In uniform circular motion, tangential acceleration is zero. Hence, net force in tangential direction is zero and in radial direction

Dynamics of Circular Motion | Physics Class 11 - NEET

This net force (towards centre) is also called centripetal force.

In most of the cases plane of our uniform circular motion will be horizontal and one of the tangent is in vertical direction also. So, in this case we resolve the forces in:

(i) horizontal radial direction

(ii) vertical tangential direction

In vertical tangential direction net force is zero (at =0) and in horizontal radial direction (towards centre) net force is mv2/r or mrω2

Note:  Centripetal force mv2/r or mrω2 (towards centre) does not act on the particle but this much force is required to the particle for rotating in a circle (as it is accelerated due to change in direction of velocity). The real forces acting on the particle provide this centripetal force or we can say that vector sum of all the forces acting on the particle is equal to mv2/r  or mrω2 (in case of uniform circular motion). The real forces acting on the particle may be, friction force, weight, normal reaction, tension etc.


Conical Pendulum

If a small particle of mass m tied to a string is whirled in a horizontal circle, as shown in Fig. The arrangement is called the ‘conical pendulum’. In case of conical pendulum, the vertical component of tension balances the weight in tangential direction, while its horizontal component provides the necessary centripetal force in radial direction

(towards centre). Thus,

Circular Motion in a PendulumCircular Motion in a Pendulum

Dynamics of Circular Motion | Physics Class 11 - NEET

From these two equations, we can find

∴ Angular speed   Dynamics of Circular Motion | Physics Class 11 - NEET

So, the time period of pendulum is

Dynamics of Circular Motion | Physics Class 11 - NEET         (as r = L cos θ)

Motion of a Particle Inside a Smooth Cone

A particle of mass ‘m’ is rotating inside a smooth cone in horizontal circle of radius ‘r’ as shown in figure. constant speed of the particle is suppose ‘v’. Only two forces are acting on the particle in the shown directions:

(i) Normal reaction N

(ii) Weight mg

We have resolved these two forces in vertical tangential direction and horizontal radial direction. In vertical tangential direction, net force is zero.

Motion of Particle Inside a ConeMotion of Particle Inside a Cone

∴ N cosθ = mg

In horizontal radial direction (towards centre), net force is mv2/r

∴ Dynamics of Circular Motion | Physics Class 11 - NEET


Circular Turning on Roads

When vehicles go through turnings, they travel along a nearly circular arc. There must be some force which will produce the required centripetal acceleration. If the vehicles travel in a horizontal circular path, this resultant force is also horizontal. 

The necessary centripetal force is being provided to the vehicles by following three ways:

1. By Friction only

2. By Banking of Roads only

3. By Friction and Banking of Roads both.

In real life the necessary centripetal force is provided by friction and banking of roads both. Now let us write equations of motion in each of the three cases separately and see what are the constant in each case.

➢ By Friction Only 

Suppose a car of mass m is moving at a speed v in a horizontal circular arc of radius r. In this case, the necessary centripetal force to the car will be provided by force of friction f acting towards centre

Thus, f = mv2/r

Further, limiting value of f is or μN or fL = μN = μmg (N = mg)

Therefore, for a safe turn without sliding  mv2/r ≤ fL

or Dynamics of Circular Motion | Physics Class 11 - NEET

Here, two situations may arise. If m and r are known to us, the speed of the vehicle should not exceed Dynamics of Circular Motion | Physics Class 11 - NEET and if v and r are known to us, the coefficient of friction should be greater than Dynamics of Circular Motion | Physics Class 11 - NEET.

 By Banking of Roads Only 

Friction is not always reliable at circular turns if high speeds and sharp turns are involved to avoid dependence on friction, the roads are banked at the turn so that the outer part of the road is somewhat lifted compared to the inner part.

Dynamics of Circular Motion | Physics Class 11 - NEET

Applying Newton's second law along the radius and the first law in the vertical direction.

Dynamics of Circular Motion | Physics Class 11 - NEET

from these two equations, we get

Dynamics of Circular Motion | Physics Class 11 - NEET

 By Friction and Banking of Road Both

If a vehicle is moving on a circular road which is rough and banked also, then three forces may act on the vehicle, of these force, the first force, i.e., weight (mg) is fixed both in magnitude and direction.

Dynamics of Circular Motion | Physics Class 11 - NEET

The direction of second force i.e., normal reaction N is also fixed (perpendicular or road) while the direction of the third i.e., friction f can be either inwards or outwards while its magnitude can be varied upto a maximum limit (fL = mN). So the magnitude of normal reaction N and directions plus magnitude of friction f are so adjusted that the resultant of the three forces mentioned above is mv2/r towards the center. Of these m and r are also constant. Therefore, magnitude of N and directions plus magnitude of friction mainly depends on the speed of the vehicle v. Thus, situation varies from problem to problem. Even though we can see that :

(i) Friction f will be outwards if the vehicle is at rest v = 0. Because in that case the component weight mg sinθ is balanced by f.

(ii) Friction f will be inwards if

v > Dynamics of Circular Motion | Physics Class 11 - NEET

(iii) Friction f will be outwards if

v < Dynamics of Circular Motion | Physics Class 11 - NEET and

(iv) Friction f will be zero if

v = Dynamics of Circular Motion | Physics Class 11 - NEET

(v) For maximum safe speed 

Dynamics of Circular Motion | Physics Class 11 - NEET

As maximum value of friction f = μN
Dynamics of Circular Motion | Physics Class 11 - NEET

Dynamics of Circular Motion | Physics Class 11 - NEET

similarly; Dynamics of Circular Motion | Physics Class 11 - NEET

  • The expression tanθ = Dynamics of Circular Motion | Physics Class 11 - NEET also gives the angle of banking for an aircraft, i.e., the angle through which it should tilt while negotiating a curve, to avoid deviation from the circular path.  
  • The expression tanθ = Dynamics of Circular Motion | Physics Class 11 - NEET also gives the angle at which a cyclist should lean inward when rounding a corner. In this case, θ is the angle which the cyclist must make with the vertical to negotiate a safe turn.

Death well

A motor cyclist is driving in a horizontal circle on the inner surface of vertical cyclinder of radius R. Friction coefficient between tyres of motorcyclist and surface of cylinder is m. Find out the minimum velocity for which the motorcyclist can do this. v is the speed of motor cyclist and m is his mass.

Dynamics of Circular Motion | Physics Class 11 - NEET

Dynamics of Circular Motion | Physics Class 11 - NEET

Cyclist does not drop down when

Dynamics of Circular Motion | Physics Class 11 - NEET


Motion of a Cyclist on a Circular Path

Suppose a cyclist is going at a speed v on a circular horizontal road of radius r which is not banked. Consider the cycle and the rider together as the system. The centre of mass C (figure shown) of the system is going in a circle with the centre at O and radius r.

Dynamics of Circular Motion | Physics Class 11 - NEET

Let us choose O as the origin, OC as the X-axis and vertically upward as the Z-axis. This frame is rotating at an angular speed w = v/r about the Z-axis. In this frame the system is at rest. Since we are working from a rotating frame of reference, we will have to apply a centrifugal force on each particle. The net centrifugal force on particle will be Mω2r = Mv2/r, where M is the total mass of the system. This force will act through the centre of mass. Since the system is at rest in this frame, no other pseudo force is needed.

Figure in shows the forces. The cycle is bent at an angle θ with the vertical. The forces are

(i) weight Mg,

(ii) normal force N

(iii) friction f

(iv) Centrifugal force 

In the frame considered, the system is at rest. Thus, the total external force and the total external torque must be zero. Let us consider the torques of all the forces about the point A. The torques of N and f about A are zero because these forces pass through A. The torque of Mg about A is Mg(AD) in the clockwise direction and that of Mv2/r is Mv2/r (CD) in the anticlockwise direction. For rotational equilibrium,

Mg(AD) = Mv2/r (CD)

Dynamics of Circular Motion | Physics Class 11 - NEET

Dynamics of Circular Motion | Physics Class 11 - NEET

Thus, the cyclist bends at an angleDynamics of Circular Motion | Physics Class 11 - NEETwith the vertical.

Centrifugal Force

When a body is rotating in a circular path and the centripetal force vanishes, the body would leave the circular path. To an observer A who is not sharing the motion along the circular path, the body appears to fly off tangentially at the point of release. To another observer B, who is sharing the motion along the circular path (i.e., the observer B is also rotating with the body which is released, it appears to B, as if it has been thrown off along the radius away from the centre by some force. This inertial force is called centrifugal force.)

Its magnitude is equal to that of the centripetal force = mv2/r . Centrifugal force is a fictitious force which has to be applied as a concept only in a rotating frame of reference to apply Newton's law of motion in that frame)

FBD of ball w.r.t non-inertial frame rotating with the ball.

Dynamics of Circular Motion | Physics Class 11 - NEET

Suppose we are working from a frame of reference that is rotating at a constant, angular velocity ω with respect to an inertial frame. If we analyse the dynamics of a particle of mass m kept at a distance r from the axis of rotation, we have to assume that a force mrω2 act radially outward on the particle. Only then we can apply Newton's laws of motion in the rotating frame. This radially outward pseudo force is called the centrifugal force.


Simple Pendulum 

A simple pendulum is constructed by attaching a bob of mass m to a string of length L fixed at its upper end. The bob oscillates in a vertical circle. It is found that the speed of the bob is v when the string makes an angle θ with the vertical.

The force acting on the bob are (figure)

Dynamics of Circular Motion | Physics Class 11 - NEET

(i) Tension T

(ii) Weight mg

As the bob moves in a vertical circle with centre at O, the radial acceleration is v2/L towards O. Taking the components along this radius and applying Newton's second law, we get

Dynamics of Circular Motion | Physics Class 11 - NEET


Circular Motion in Horizontal Plane 

A ball of mass m attached to a light and inextensible string rotates in a horizontal circle of radius r with an angular speed w about the vertical. If we draw the force diagram of the ball. 

Dynamics of Circular Motion | Physics Class 11 - NEET

We can easily see that the component of tension force along the centre gives the centripetal force and component of tension along vertical balances the gravitation force. Such a system is called a conical pendulum.

The document Dynamics of Circular Motion | Physics Class 11 - NEET is a part of the NEET Course Physics Class 11.
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FAQs on Dynamics of Circular Motion - Physics Class 11 - NEET

1. What is circular motion?
Ans. Circular motion refers to the movement of an object or particle along a circular path. It occurs when the object continuously changes its direction while maintaining a constant speed. Examples of circular motion include the motion of planets around the sun or a car turning on a curved road.
2. What is the conical pendulum?
Ans. A conical pendulum is a system in which a mass is attached to a string or rod and swings in a horizontal circle. The string or rod is fixed at one end, and the mass moves in a circular path in a vertical plane. The motion of a conical pendulum is a combination of circular motion and vertical motion.
3. What is the centripetal force in circular motion?
Ans. The centripetal force is the force that keeps an object moving in a circular path. It acts towards the center of the circle and is responsible for changing the direction of the object's velocity without changing its speed. In circular motion, the centripetal force is always directed towards the center of the circle.
4. How does the centrifugal force relate to circular motion?
Ans. The centrifugal force is often misunderstood in circular motion. It is not a real force but rather an apparent force that appears to act outward from the center of rotation. It is a result of the inertia of the object, which tends to make it move in a straight line. In reality, the object is only experiencing the centripetal force, which keeps it in a circular path.
5. What is the simple pendulum and its motion?
Ans. A simple pendulum consists of a mass (bob) attached to a string or rod that is fixed at one end. When the mass is displaced from its equilibrium position and released, it oscillates back and forth in a regular and predictable motion. The motion of a simple pendulum is periodic and exhibits characteristics such as a constant period, amplitude, and frequency.
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