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# What is Rotational Energy?

Rotational energy is the component of kinetic energy that comes from a body’s rotation. It results when any form of matter revolves around a center of rotation. It can be converted into other forms of energy, most typically translational and heat energy. Many analogies exist between rotational kinetic energy and linear kinetic energy. There are some practical applications for rotational energy, such as the storage of energy in a spinning flywheel.

The law of conservation of energy holds that the total amount of energy in an isolated system must remain constant over time. Energy losses of one type must result in energy gains of another type. Transfer of energy between types often occurs through the exchange of momentum between the atomic particles of matter. Examples of different forms of energy include chemical, potential, and thermal, in addition to rotational. Rotational energy, therefore, is one of many possible ways matter can hold energy.

There are many analogies between rotational energy and linear kinetic energy. Instead of mass, rotational systems have a moment of inertia. The moment of inertia can be thought of as the resistance to angular acceleration—it's similar to how mass is the resistance to linear acceleration. Moments of inertia increase when matter is further from the center of rotation. This is because it is more difficult to get a system spinning if its matter is located far from the center.

Similarly, rotational systems have an angular velocity instead of a linear velocity. Angular velocity is measured in radians per second, which is equal to about 57.3 degrees per second. Both high moment of inertia and high angular velocity correspond to high rotational energy. According to the law of conservation of energy, the same amount of rotational energy can be obtained by reducing a system’s moment of inertia while increasing the angular velocity.

One practical application of rotational energy is the use of flywheel batteries. Just as a standard battery stores electrical energy, a flywheel battery stores rotational energy. In a train with a flywheel battery, the linear kinetic energy of the moving train can be transferred to the rotational energy of the onboard flywheel. The effect of this transfer will be a reduction in the train’s velocity. If no energy is lost to heat, all the energy of the train’s motion can be stored in the flywheel and later used to accelerate the train up to speed again.

## Discussion Comments

@hamje32 - Yes, I think the flywheel needs to be constantly spinning. It gets its energy from the object in motion just as the car battery is constantly being recharged while the car is moving. Without the jolt of electricity to get it spinning, the flywheel cannot overcome the moment of rotational inertia and therefore would have to remain stationary.

@allenJo - Okay, I understand using it for temporary storage of electricity, but can it replace a car battery? After all, a car battery can deliver power when the car is stationary. It appears that the flywheel needs to be constantly spinning to store its energy.

@nony - I am afraid to confess that I don’t watch too many sports, including the Olympic variety. I do know what you’re talking about however.

But I am more interested in the commercial applications of rotation energy, such as that demonstrated by a flywheel. I saw someone do a demonstration of a flywheel battery.

The kinetic energy gets stored in the battery as it spins around, and when you need to tap the electricity, the battery releases its energy. One application of this is in backup generators for homes.

Since backup generators don’t always kick in right away, there is sometimes a lag between when the power is cut off and when the generator kicks in. A flywheel battery can immediately kick in during this lag time, for a few seconds, long enough for the actual generator to kick in. I think it’s an ideal solution.

If you’ve ever watched the Olympics, you know all about rotational motion. I am referring of course to the shot put games. This is where the athlete holds a metal ball to his chin, and then spins around as fast and as hard as he can and releases the ball into the air, trying to throw it as far as possible.

As you can see, the guys who do this kind of thing need to have strong builds because the balls are heavy and take a significant rotational force to get thrown as far as possible. It’s spinning kinetic energy, basically.

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