Angular Acceleration How to Calculate It and Examples

The angular acceleration is the variation that affects the angular velocity taking into consideration a unit of time. It is represented by the Greek letter alpha, α. Angular acceleration is a vector quantity; therefore, it consists of module, direction and sense.

The unit of measurement for angular acceleration in the International System is the radian per second squared. In this way, the angular acceleration makes it possible to determine how the angular velocity varies over time. Angular acceleration linked to uniformly accelerated circular motions is often studied.

Thus, in a uniformly accelerated circular motion the value of the angular acceleration is constant. On the contrary, in a uniform circular motion the value of the angular acceleration is zero. Angular acceleration is the equivalent in circular motion to tangential or linear acceleration in rectilinear motion..

In fact, its value is directly proportional to the value of the tangential acceleration. Thus, the greater the angular acceleration of the wheels of a bicycle, the greater the acceleration it experiences..

Therefore, angular acceleration is present both in the wheels of a bicycle and in the wheels of any other vehicle, as long as there is a variation in the speed of rotation of the wheel..

In the same way, angular acceleration is also present in a Ferris wheel, since it experiences a uniformly accelerated circular motion when it begins its movement. Of course, the angular acceleration can also be found on a merry-go-round.

Article index

• 1 How to calculate angular acceleration?
  • 1.1 Uniformly accelerated circular motion
  • 1.2 Torque and angular acceleration
• 2 Examples
  • 2.1 First example
  • 2.2 Second example
  • 2.3 Third example
• 3 References

How to calculate angular acceleration?

In general, the instantaneous angular acceleration is defined from the following expression:

α = dω / dt

In this formula ω is the angular velocity vector, and t is the time.

The mean angular acceleration can also be calculated from the following expression:

α = ∆ω / ∆t

For the particular case of a plane motion, it happens that both the angular velocity and the angular acceleration are vectors with a direction perpendicular to the plane of motion..

On the other hand, the modulus of the angular acceleration can be calculated from the linear acceleration by means of the following expression:

α = a / R

In this formula a is the tangential or linear acceleration; and R is the radius of gyration of the circular motion.

Uniformly accelerated circular motion

As already mentioned above, angular acceleration is present in uniformly accelerated circular motion. For this reason, it is interesting to know the equations that govern this movement:

ω = ω₀ + α ∙ t

θ = θ₀ + ω₀ ∙ t + 0.5 ∙ α ∙ t^two

ω^two = ω₀^two + 2 ∙ α ∙ (θ - θ₀)

In these expressions θ is the angle traveled in the circular motion, θ₀ is the initial angle, ω₀ is the initial angular velocity, and ω is the angular velocity.

Torque and angular acceleration

In the case of linear motion, according to Newton's second law, a force is required for a body to acquire a certain acceleration. This force is the result of multiplying the mass of the body and the acceleration it has experienced..

However, in the case of a circular motion, the force required to impart angular acceleration is called torque. Ultimately, torque can be understood as an angular force. It is denoted by the Greek letter τ (pronounced “tau”).

Similarly, it must be taken into account that in a rotational movement, the moment of inertia I of the body plays the role of mass in linear movement. In this way, the torque of a circular motion is calculated with the following expression:

τ = I α

In this expression I is the moment of inertia of the body with respect to the axis of rotation.

Examples

First example

Determine the instantaneous angular acceleration of a body moving in a rotational motion, given an expression of its position in the rotation Θ (t) = 4 t³ i. (I being the unit vector in the direction of the x axis).

Likewise, determine the value of the instantaneous angular acceleration when 10 seconds have elapsed from the beginning of the movement.

Solution

From the expression of the position, the expression of the angular velocity can be obtained:

ω (t) = d Θ / dt = 12 t^twoi (rad / s)

Once the instantaneous angular velocity has been calculated, the instantaneous angular acceleration can be calculated as a function of time.

α (t) = dω / dt = 24 t i (rad / s^two)

To calculate the value of the instantaneous angular acceleration after 10 seconds, it is only necessary to substitute the value of time in the previous result.

α (10) = = 240 i (rad / s^two)

Second example

Determine the mean angular acceleration of a body that experiences a circular motion, knowing that its initial angular velocity was 40 rad / s and that after 20 seconds it has reached the angular velocity of 120 rad / s.

Solution

From the following expression, the mean angular acceleration can be calculated:

α = ∆ω / ∆t

α = (ω_F  - ω₀) / (t_F - t₀ ) = (120 - 40) / 20 = 4 rad / s

Third example

What will be the angular acceleration of a Ferris wheel that begins to move in a uniformly accelerated circular motion until, after 10 seconds, it reaches the angular velocity of 3 revolutions per minute? What will be the tangential acceleration of the circular motion in that period of time? The radius of the Ferris wheel is 20 meters.

Solution

First, it is necessary to transform the angular velocity from revolutions per minute to radians per second. For this, the following transformation is carried out:

ω_F = 3 rpm = 3 ∙ (2 ∙ ∏) / 60 = ∏ / 10 rad / s

Once this transformation has been carried out, it is possible to calculate the angular acceleration since:

ω = ω₀ + α ∙ t

∏ / 10 = 0 + α ∙ 10

α = ∏ / 100 rad / s^two

And the tangential acceleration results from operating the following expression:

α = a / R

a = α ∙ R = 20 ∙ ∏ / 100 = ∏ / 5 m / s^two

References

1. Resnik, Halliday & Krane (2002). Physics Volume 1. Cecsa.
2. Thomas Wallace Wright (1896). Elements of Mechanics Including Kinematics, Kinetics and Statics. E and FN Spon.
3. P. P. Teodorescu (2007). "Kinematics". Mechanical Systems, Classical Models: Particle Mechanics. Springer.
4. Kinematics of the rigid body. (n.d.). In Wikipedia. Retrieved on April 30, 2018, from es.wikipedia.org.
5. Angular acceleration. (n.d.). In Wikipedia. Retrieved on April 30, 2018, from es.wikipedia.org.
6. Resnick, Robert & Halliday, David (2004). Physics 4th. CECSA, Mexico
7. Serway, Raymond A .; Jewett, John W. (2004). Physics for Scientists and Engineers (6th edition). Brooks / Cole.