Requirement of uniform circular motion

The uniform circular motion represents the basic form of rotational motion in the same manner as uniform linear motion represents the basic form of translational motion. They, however, are different with respect to the requirement of force to maintain motion.

Uniform linear motion is the reflection of the inherent natural tendency of all natural bodies. This motion by itself is the statement of Newton’s first law of motion : an object keeps moving with its velocity unless there is net external force. Thus, uniform linear motion indicates “absence” of force.

On the other hand, uniform circular motion involves continuous change in the direction of velocity without any change in its magnitude (v). A change in the direction of velocity is a change in velocity (v). It means that an uniform circular motion is associated with an acceleration and hence force. Thus, uniform circular motion indicates “presence” of force.

Let us now investigate the nature of force required to maintain uniform circular motion. We know that a force acting in the direction of motion changes only the magnitude of velocity. A change in the direction of motion, therefore, requires that velocity of the particle and force acting on it should be at an angle. However, such a force, at an angle with the direction of motion, would have a component along the direction of velocity as well and that would change the magnitude of the motion.

Figure 1: A change in the direction of motion requires that velocity of the particle and force should be at an angle.

Change of direction

In order that there is no change in the magnitude of velocity, the force should have zero component along the direction of velocity. It is possible only if the force be perpendicular to the direction of velocity such that its component in the direction of velocity is zero (Fcos90° = 0). Precisely, this is the requirement for a motion to be uniform circular motion.

Figure 2: Force is perpendicular to the direction of velocity.

Uniform circular motion

In plain words, uniform circular motion (UCM) needs a force, which is always perpendicular to the direction of velocity. Since the direction of velocity is continuously changing, the direction of force, being perpendicular to velocity, should also change continously.

The direction of velocity along the circular trajectory is tangential. The perpendicular direction to the circular trajectory is, therefore, radial direction. It implies that force (and hence acceleration) in uniform direction motion is radial. For this reason, acceleration in UCM is recognized to seek center i.e. centripetal (seeking center).

This fact is also validated by the fact that the difference of velocity vectors, whose time rate gives acceleration, at two instants (Δv) is radial.

Figure 3: Force is centripetal.

Uniform circular motion

The yet another important aspect of the UCM is that the centripetal force is radial and hence does not constitute a torque as the force is passing through the axis of rotation. A torque is force multiplied by the perpendicular distance of the line of action of the force from the axis of rotation. It must be clearly understood that the requirement of centripetal force is essentially additional or different to the force, which is required in non-uniform circular motion to accelerate the particle tangentially. The centripetal force is required to accelerate particle in radial direction and is different to one required to accelerate particle tangentially.

Irrespective of whether circular motion is uniform (constant speed) or non-uniform (varying speed), the circular motion inherently associates a radial acceleration to ensure that the direction of motion is continuously changed – at all instants. We shall learn about the magnitude of radial acceleration soon, but let us be emphatic to differentiate radial acceleration (accounting change in direction that arises from radial force) with tangential acceleration (accounting change in the speed that arises from tangential force or equivalently a torque).

We have different force systems present at different levels of universe. Motion of bodies and sub-atomic particles are always under certain force system. Absence of force in the observable neighborhood is rare. Thus, uniform linear motion is rare, while uniform circular motion abounds in nature as there is availability of external force that continuously changes direction of the motion of the bodies or particles. Consider the electrostatic force between nucleus and an electron in an atom. The force keeps changing direction as electron moves. So is the case of gravitational force (indicated by red arrow in the figure), which keeps changing its direction as the planet moves around sun.

Figure 4: Gravitational force is radial, whereas velocity is tangential.

Approximated circular motion of Earth around Sun

It may sometimes be perceived that a force like centripetal force should have caused the particle or body to move towards center ultimately. The important point to understand here is that a force determines initial direction of motion only when the particle is stationary. However, if the particle is already in motion, then force modifies the direction in accordance with initial inclination between velocity and force such that the resulting acceleration (change in vector velocity) is in the direction of force. We shall soon see that this is exactly the case in uniform circular motion.

Let us summarize the discussion of uniform circular motion so far :

Analysis of uniform circular motion

It has been pointed that any motion, that changes directions, requires more than one dimension for representation. Circular motion by the geometry of the trajectory is two dimensional motion.

In the case of circular motion, it is a matter of convenience to locate origin of the two dimensional coordinate system at the center of circle. This choice ensures the symmetry of the circular motion about the origin of the reference system.

Velocity of the particle

The magnitude of velocity of the particle (v) is constant by the definition of uniform circular motion. In component form, the velocity (refer to the figure) is :

v = v x i + v y j v = v x i + v y j (3)

From the geometry as shown in the figure,

Figure 5: Component velocities in UCM.

Uniform circular motion

v x = - v sin θ v y = v cos θ ⇒ v = - v sin θ i + v cos θ j v x = - v sin θ v y = v cos θ ⇒ v = - v sin θ i + v cos θ j (4)

We may emphasize here that it is always easy to find the sign of the component of a vector like velocity. Decide the sign by the direction of component (projection) with respect to positive direction of reference axis. Note from the figure that component along x-direction is opposite to the positive reference direction and hence negative.

From the ΔOAP,

sin θ = y r cos θ = x r sin θ = y r cos θ = x r

Putting these values in the expression for velocity, we have :

v = - v y r i + v x r j v = - v y r i + v x r j (5)

Centripetal acceleration

We are now sufficiently equipped and familiarized with the nature of uniform circular motion and the associated centripetal (center seeking) acceleration. Now, we seek to determine this acceleration.

Knowing that speed, "v" and radius of circle, "r" are constants, we differentiate the expression of velocity with respect to time to obtain expression for centripetal acceleration as :

a = - v r ( ⅆ y ⅆ t i - ⅆ x ⅆ t j ) a = - v r ( ⅆ y ⅆ t i - ⅆ x ⅆ t j )

⇒ a = - v r ( v y i - v x j ) ⇒ a = - v r ( v y i - v x j )

Putting values of component velocities in terms of angle,

⇒ a = - v r ( v cos θ i + v sin θ j ) = a x i + a y j ⇒ a = - v r ( v cos θ i + v sin θ j ) = a x i + a y j (6)

Figure 6: Components of radial accelerations

Uniform circular motion

where

a x = - v 2 r cos θ a y = - v 2 r sin θ a x = - v 2 r cos θ a y = - v 2 r sin θ

It is evident from the equation of acceleration that it varies as the angle with horizontal, "θ" changes. The magnitude of acceleration is :

a = | a | = ( a x 2 + a y 2 ) ⇒ a = | a | = v r { v 2 ( cos 2 θ + cos 2 θ ) } ⇒ a = v 2 r a = | a | = ( a x 2 + a y 2 ) ⇒ a = | a | = v r { v 2 ( cos 2 θ + cos 2 θ ) } ⇒ a = v 2 r (7)

The radius of the circle is constant. The magnitude of velocity i.e. speed, "v" is constant for UCM. It, then, follows that though velocity changes with the motion (angle from reference direction), but the speed of the particle in UCM is a constant.

Example 1

Problem : A cyclist negotiates the curvature of 20 m with a speed of 20 m/s. What is the magnitude of his acceleration?

Solution : The speed of the cyclist is constant. The acceleration of cyclist, therefore, is the centripetal acceleration required to move the cyclist along a circular path i.e. the acceleration resulting from the change in the direction of motion along the circular path.

Here, v = 20 m/s and r = 20 m

⇒ a = v 2 r = 20 2 20 = 20 m / s 2 ⇒ a = v 2 r = 20 2 20 = 20 m / s 2

This example points to an interesting aspect of circular motion. The centripetal acceleration of the cyclist is actually two (2) times that of acceleration due to gravity (g = 10 m / s 2 m / s 2 ). This fact is used to create large acceleration in small space with appropriate values of “v” and “r” as per the requirement in hand.

The large acceleration so produced finds application in particle physics and for equipments designed to segregate material on the basis of difference in density. This is also used to simulate large acceleration in a centrifuge for astronauts, who experience large acceleration at the time of take off or during entry on the return.

Generation of high magnitude of acceleration during uniform circular motion also points to a potential danger to pilots, maneuvering circular trajectory at high speed. Since a pilot is inclined with the head leaning towards the center of motion, the blood circulation in the brain is low. If his body part, including brain, is subjected to high acceleration (multiple of acceleration due to gravity), then it is likely that the pilot experiences dizziness or sometimes even looses consciousness.

Circular motion has many interesting applications real world and provides explanations of many natural events. In this module, however, we restrict ourselves till we study the dynamics of the circular motion also in subsequent modules.

Direction of centripetal acceleration

Here, we set out to evaluate the angle “α” as shown in the figure. Clearly, if this angle “α” is equal to “θ”, then we can conclude that acceleration is directed in radial direction.

Figure 7: Radial acceleration

Uniform circular motion

Now,

tan α = a y a x = - v 2 r sin θ - v 2 r cos θ = tan θ α = θ tan α = a y a x = - v 2 r sin θ - v 2 r cos θ = tan θ α = θ

This proves that centripetal acceleration is indeed radial (i.e acting along radial direction).

Uniform circular motion and projectile motion

Both these motions are two dimensional motions. They are alike in the sense that motion in each case is subject to continuous change of the direction of motion. Otherwise, they are different in other details. The most important difference is that projectile motion has a constant acceleration, whereas uniform circular motion has a constant magnitude of acceleration.

Significantly, a projectile motion completely resembles uniform circular motion at one particular instant. The projectile has only horizontal component of velocity, when it is at the maximum height. At that instant, the force of gravity, which is always directed downward, is perpendicular to the direction of velocity. Thus, projectile at that moment executes an uniform circular motion.

Figure 8: Projectile meets the condition of UCM at maximum height

Projectile motion

Let radius of curvature of the projectile trajectory at maximum height be “r”, then

a = g = v x 2 r = v 2 cos 2 θ r ⇒ r = v 2 cos 2 θ g a = g = v x 2 r = v 2 cos 2 θ r ⇒ r = v 2 cos 2 θ g