The ratio of the rotational kinetic energy to the translational kinetic energy of a sphere is a fundamental concept in classical mechanics. This ratio is an important parameter in understanding the dynamics of rolling motion and has various applications in fields such as engineering, sports, and robotics.
Understanding the Rotational and Translational Kinetic Energy of a Sphere
The rotational kinetic energy of a sphere is given by the formula:
$K_{rot} = \frac{1}{2}I\omega^2$
where $I$ is the moment of inertia and $\omega$ is the angular velocity of the sphere.
The moment of inertia of a sphere about its central axis is:
$I = \frac{2}{5}MR^2$
where $M$ is the mass of the sphere and $R$ is its radius.
The translational kinetic energy of the sphere is given by the formula:
$K_{trans} = \frac{1}{2}Mv^2$
where $v$ is the linear velocity of the center of mass of the sphere.
Calculating the Ratio of Rotational and Translational Kinetic Energy
To find the ratio of the rotational kinetic energy to the translational kinetic energy, we take the ratio of the two energies:
$\frac{K_{rot}}{K_{trans}} = \frac{\frac{1}{2}I\omega^2}{\frac{1}{2}Mv^2}$
Substituting the expressions for $I$ and $v$ (where $v = R\omega$), we get:
$\frac{K_{rot}}{K_{trans}} = \frac{\frac{1}{2}\left(\frac{2}{5}MR^2\right)\omega^2}{\frac{1}{2}M(R\omega)^2}$
Simplifying, we arrive at the final ratio:
$\frac{K_{rot}}{K_{trans}} = \frac{1}{5}$
This means that the translational kinetic energy of a sphere is five times greater than its rotational kinetic energy.
Technical Specifications and Importance
The moment of inertia of a sphere is a measure of its resistance to rotational motion. It has units of $kg\cdot m^2$ and is a crucial parameter in describing the rotational dynamics of a sphere.
The angular velocity of a sphere, $\omega$, is a measure of how fast it is rotating and has units of $rad/s$. It is related to the linear velocity $v$ through the equation $v = R\omega$.
Understanding the ratio of rotational and translational kinetic energy is important in various applications, such as:

Rolling Motion: The ratio of rotational to translational kinetic energy is crucial in understanding the dynamics of rolling motion, which is relevant in sports (e.g., bowling, golf) and industrial applications (e.g., wheel design, robotic locomotion).

Energy Efficiency: The ratio can be used to optimize the energy efficiency of systems involving rolling spheres, such as in the design of energyefficient vehicles or machinery.

Stability and Control: The ratio of rotational to translational kinetic energy affects the stability and control of rolling objects, which is important in fields like robotics and aerospace engineering.

Collision Dynamics: The ratio of rotational to translational kinetic energy plays a role in the analysis of collisions involving rolling spheres, which is relevant in areas like sports, industrial processes, and particle physics.
Examples and Numerical Problems
 Example 1: A solid sphere with a mass of 2 kg and a radius of 0.1 m is rolling on a horizontal surface with a linear velocity of 5 m/s. Calculate the ratio of its rotational kinetic energy to its translational kinetic energy.
Given:
– Mass of the sphere, $M = 2 kg$
– Radius of the sphere, $R = 0.1 m$
– Linear velocity of the sphere, $v = 5 m/s$
Step 1: Calculate the angular velocity of the sphere.
$\omega = \frac{v}{R} = \frac{5 m/s}{0.1 m} = 50 rad/s$
Step 2: Calculate the rotational kinetic energy of the sphere.
$K_{rot} = \frac{1}{2}I\omega^2 = \frac{1}{2}\left(\frac{2}{5}MR^2\right)(50 rad/s)^2 = 2.5 J$
Step 3: Calculate the translational kinetic energy of the sphere.
$K_{trans} = \frac{1}{2}Mv^2 = \frac{1}{2}(2 kg)(5 m/s)^2 = 25 J$
Step 4: Calculate the ratio of rotational to translational kinetic energy.
$\frac{K_{rot}}{K_{trans}} = \frac{2.5 J}{25 J} = \frac{1}{10}$
 Example 2: A hollow sphere with a mass of 3 kg and an outer radius of 0.2 m is rolling on a horizontal surface with an angular velocity of 20 rad/s. Calculate the ratio of its rotational kinetic energy to its translational kinetic energy.
Given:
– Mass of the sphere, $M = 3 kg$
– Outer radius of the sphere, $R = 0.2 m$
– Angular velocity of the sphere, $\omega = 20 rad/s$
Step 1: Calculate the linear velocity of the sphere.
$v = R\omega = (0.2 m)(20 rad/s) = 4 m/s$
Step 2: Calculate the rotational kinetic energy of the sphere.
$K_{rot} = \frac{1}{2}I\omega^2 = \frac{1}{2}\left(\frac{2}{3}MR^2\right)(20 rad/s)^2 = 4 J$
Step 3: Calculate the translational kinetic energy of the sphere.
$K_{trans} = \frac{1}{2}Mv^2 = \frac{1}{2}(3 kg)(4 m/s)^2 = 24 J$
Step 4: Calculate the ratio of rotational to translational kinetic energy.
$\frac{K_{rot}}{K_{trans}} = \frac{4 J}{24 J} = \frac{1}{6}$
These examples demonstrate the application of the formulas and the calculation of the ratio of rotational to translational kinetic energy for both solid and hollow spheres.
Conclusion
The ratio of the rotational kinetic energy to the translational kinetic energy of a sphere is a fundamental concept in classical mechanics. This ratio is 1/5 for a solid sphere, meaning that the translational kinetic energy is five times greater than the rotational kinetic energy. Understanding this ratio is crucial in various applications, such as rolling motion, energy efficiency, stability and control, and collision dynamics. The moment of inertia and angular velocity are important technical specifications that contribute to the understanding of the rotational dynamics of a sphere.
References
 Lumen Learning. “Moment of Inertia and Rotational Kinetic Energy.” Lumen Learning, https://courses.lumenlearning.com/sunyosuniversityphysics/chapter/104momentofinertiaandrotationalkineticenergy/.
 YouTube. “If a sphere is rolling, the ratio of its rotational energy to the total kinetic energy is given by (1 …).” YouTube, https://www.youtube.com/watch?v=6HLPwSoCq_A.
 YouTube. “If a sphere is rolling the ratio of the translational energy to … – YouTube.” YouTube, https://www.youtube.com/watch?v=1Ud5TyoNczM.
 BYJU’S. “The ration of rotational to kinetic energy for a hollow sphere in pure rolling motio.” BYJU’S, https://byjus.com/questionanswer/therationofrotationaltokineticenergyforahollowsphereinpurerollingmotio/.
 Reddit. “Why does a sphere experience a larger translational kinetic energy … – Reddit.” Reddit, https://www.reddit.com/r/AskPhysics/comments/reyjlm/why_does_a_sphere_experience_a_larger/.
Hi…I am Ankita Biswas. I have done my B.Sc in physics Honours and my M.Sc in Electronics. Currently, I am working as a Physics teacher in a Higher Secondary School. I am very enthusiastic about the highenergy physics field. I love to write complicated physics concepts in understandable and simple words.