Suppose there is a motorcycle riding on a road. It is observed that when the acceleration is more the wheels of the bike spins more and rotates through many revolutions. This only happens when the wheels have angular acceleration for a long time. Let us study more about angular acceleration in detail.

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## Kinematics of Rotational Motion about a Fixed Point

We all know that rotational motion and translational motion are analogous to each other. In rotational motion, the angular velocity is ω which is analogous to the linear velocity** v** in the transitional motion. Let us discuss further the kinematics of rotational motion about a fixed point.

The kinematic quantities in rotational motion like the angular displacement θ, angular velocity **ω** and angular acceleration **α** respectively corresponds to kinematic quantities in linear motion like displacement **x**, velocity **v** and acceleration **a.**

**Browse more Topics under System Of Particles And Rotational Dynamics**

- Introduction to Rotational Dynamics
- Vector Product of Two Vectors
- Centre of Mass
- Motion of Centre of Mass
- Moment of Inertia
- Theorems of Parallel and Perpendicular Axis
- Rolling Motion
- Angular Velocity and Angular Acceleration
- Linear Momentum of System of Particles
- Torque and Angular Momentum
- Equilibrium of a Rigid Body
- Angular Momentum in Case of Rotation About a Fixed Axis
- Dynamics of Rotational Motion About a Fixed Axis

### Angular Acceleration

The rate of change of angular velocity is the angular acceleration.

α = \( \frac{dω}{ dt } \) ( rad/sec²)

Now let us consider a particle P on a rotating object. The object undergoes a rotation motion at the fixed point. The angular displacement of a particle P is θ. Hence in time t = 0, the angular displacement of the particle P is 0. So we can say that in time t, its angular displacement will be equal to θ.

### Angular Velocity

Angular velocity is the time rate of change of angular displacement. We can write it as,

ω = \( \frac{dθ}{ dt } \)

As we know that the rotational motion here is fixed, so there is no need to change the angular velocity. We know angular acceleration is α = \( \frac{dω}{ dt } \). So the kinematics equations of linear motion with uniform acceleration is,

v = v_{0}+ at

x = x_{0 }+ v_{0}t + \( \frac{1}{2} \) at²

v² = v_{0}^{2}+ 2ax

Where x_{0 }is the initial displacement and v_{0 }is the initial velocity of the particle. Here initial means t = 0. Now, this equation corresponds to the kinematics equation of the rotational motion.

## Kinematics Equations for Rotational Motion with Uniform Angular Acceleration

ω = ω_{0}+ αt

θ = θ_{0 }+ ω_{0}t + \( \frac{1}{2} \) αt²

ω² = _{ }ω_{0}² + 2α (θ – θ_{0})

Where θ_{0 }is the initial angular displacement of the rotating body and ω_{0 }is the initial angular velocity of the particle of the body.

## Solved Examples For You

Q1. A wheel rotating with uniform angular acceleration covers 50 rev in the first five seconds after the start. If the angular acceleration at the end of five seconds is x π rad/s² find the value of x.

- 4
- 8
- 6
- 10

Solution: B

θ = \( \frac{1}{2} \) αt²

α = \( \frac{2θ}{t²} \) = \( \frac{2(50) (2π)}{5²} \) = 8π rad/s² = 25.14 rad/s²

comparing with α, x = 8 rad/s²

Q2. Starting from rest, a fan takes five seconds to attain the maximum speed of 400 rpm. Assume constant acceleration, find the time taken by the fan attaining half the maximum speed.

- 11 s
- 2.0 s
- 2.5 s
- 2.0 s

Solution: D. The maximum angular velocity is given by,

w_{m }= 400rpm = 400 × \( \frac{2π}{60} \) = \( \frac{40}{3} \) rad/sec

Initial angular velocity is w_{m }= 0

So angular accleration α = \( \frac{w_ m – ω – 0}{t} \) = \( \frac{40/3 – 0}{5} \) = \( \frac{8π}{3} \) rad/sec²

Now ω = w_{0 }+ at we get ω_{m/2 }= 0 + at = \( \frac{40π/3}{2(8π/3)} \) = 2.5s

Q3. Identify the direction of the angular velocity vector for the second hand of a clock going from 0 to 60 seconds?

- outward from clock face
- inward towards the clock face
- upward
- dwonward

Solution : B

Angular velocity = ω × r. The second hand of the clock rotates in clockwise direction. From the above figure, the direction of angular velocity is into the plane of the page that is inward towards the clock face.

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