Posts Tagged ‘inertia’

What Came First? The Wheel or the Flywheel?

Monday, September 25th, 2017

   What came first?   The wheel or the flywheel?  Archeologists have been debating this question for decades.   One thing is certain, they both date back to prehistoric times.

What Came First? The Wheel or the Flywheel?

What Came First? The Wheel or the Flywheel?


   One of the oldest flywheel discoveries was a potter’s wheel, used to make pottery.   It’s a turntable made of stone or heavy wood that’s connected to a massive wheel by a spinning shaft.   Once the potter got the flywheel spinning with his hand or foot, the wheel’s heavy weight kept it in virtual perpetual motion, allowing the potter to concentrate on forming the clay he shaped with his hands.

   A potter’s wheel, or any other flywheel for that matter, takes a lot of initial effort to put into motion.   In other words, the potter must put a lot of his own muscles’ mechanical energy into the flywheel to get it moving.   That’s because its sheer weight binds it to the Law of Inertia and makes it want to stay at rest.

   But once the flywheel is in motion, the potter’s mechanical energy input is transformed into kinetic energy, the energy of motion.   The kinetic energy the potter produces by his efforts results in surplus energy stored within the flywheel.   Hence, the flywheel serves as a kinetic energy storage device, similar to a battery which stores electrical energy.   As long as the flywheel remains in motion, this stored energy will be used to keep the turntable spinning, which results in no additional mechanical energy needing to be exerted by the potter while forming pots.

   The flywheel’s stored energy also makes it hard to stop once it’s in motion.   But eventually the frictional force between the potter’s hands and the clay he works drains off all stored kinetic energy.

   Since the Industrial Revolution flywheels have been used to store kinetic energy to satisfy energy demands and provide a continuous output of power, which increases mechanical efficiency.

   Next time we’ll begin our exploration into the science behind flywheels and see how they’re used in diverse engineering applications.


Copyright 2017 – Philip J. O’Keefe, PE

Engineering Expert Witness Blog



Overcoming Inertia

Monday, February 3rd, 2014

      Inertia.   It’s the force that keeps us in bed after the alarm has rung.   It seems to have a life of its own, and today we’ll see how it comes into play in keeping other stationary objects at rest.

      Last time we identified a specific point of contact between spur gear teeth in a gear train and introduced the opposing forces, F1 and F 2, generated there.   Today we’ll see what these forces represent, identifying one of them as inertia.

gear engineering expert witness

      So where do these forces come from?   They’re forces generated by different means that converge at the same point of contact, the point at which gear teeth mesh.   They follow a very specific geometric path to meet there, an imaginary straight line referred to as the line of action.

      F1 is always generated by a source of mechanical energy.   In our locomotive example introduced earlier in this blog series that source is an electric traction motor, upon which a driving gear is mounted.   When the motor is energized, a driving force F1 is generated, which causes gear teeth on the driving gear to push against gear teeth of the driven gear.

      Force F2 is not as straightforward to understand, because it’s not generated by a motor.   Instead, it’s the resisting force that the weight of a stationary object poses against its being moved from an at-rest position, known as inertia.   The heavier the object, the more inertia it presents with.   Trains, of course, are extremely heavy, and to get them to move a great deal of inertia must be overcome.   Inertia is also a factor in attempting to stop objects already in motion.

      To get a stationary locomotive to move, mechanical energy must be transmitted from the driving gear that’s attached to its traction motor, then on to the driven gear attached to its axle.    At their point of contact, the driving force of the motor, F1, is met by the resisting force of inertia, F2. In order for the train to move, F1 must be greater than F2.   If F1 is less than or equal to F2, then the train won’t leave the station.

      Next week we’ll animate our static image and watch the interplay between gear teeth, taking note of the line of action during their movement.