Posts Tagged ‘acceleration’

Horsepower Required to Accelerate a Flywheel

Friday, November 24th, 2017

    Last time we discussed how torque is created as a flywheel spins.   This torque is a factor of the flywheel’s moment of inertia, which is dependent on how far the masses of the flywheel’s parts are located from its center of rotation.   Today we’ll present a formula to compute how much horsepower is required to accelerate a flywheel.   And here it is,

Horsepower Required to Accelerate a Flywheel

Horsepower Required to Accelerate a Flywheel

where, T is the torque created on the flywheel’s shaft in units of inch-pounds.   The term n is the flywheel shaft’s speed of rotation in revolutions per minute, RPM.   Horsepower, HP, is engineering shorthand for a unit of power equal to 6600 inch-pounds per second, and the number 63,025 is a constant needed to convert torque, T, and the spinning shaft’s rotations per minute, RPM, into horsepower units.

    Torque is present whether the flywheel’s spin accelerates or decelerates.   During acceleration torque is created, which contributes to the production of kinetic energy that’s stored inside the flywheel.  When a flywheel’s spin decelerates, its mass experiences the effects of negative acceleration, and stored kinetic energy is released.

    As we learned awhile back, horsepower is a function of torque in any moving machinery, including engines and flywheels.  An engine must produce horsepower to accelerate a flywheel connected to its shaft.   By the same token, when the engine’s horsepower output diminishes or stops, the flywheel begins to decelerate.   This deceleration causes kinetic energy stored within the flywheel to be released, providing horsepower necessary to keep the engine and flywheel spinning.   That is, until the power output of the engine returns or the stored kinetic energy of the flywheel is ultimately exhausted.

    We’ll see how that works next time when we take a look inside a reciprocating engine.

opyright 2017 – Philip J. O’Keefe, PE

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What Determines Rate of Fall?

Thursday, September 4th, 2014

      Picture yourself holding a feather in one hand, a hammer in the other.   Your buddy has bet you that if you simultaneously drop them, the hammer will hit the ground first, and he’s got a beer riding on it.

      This exact experiment was performed in 1971 by Apollo 15 Astronauts David Scott and Jim Irwin when they landed on the moon.    We’ll tell you how it turned out later, but first let’s review the history behind the study of falling objects.

      Aristotle, the ancient Greek philosopher, would have bet with your buddy.   Back in the 4th Century BC he developed a theory of gravity to explain the physics behind falling objects.   He asserted that the heavier the object, the faster it will fall.   His theory seems intuitively obvious on its face, but although Aristotle was a great philosopher, he was a lousy scientist.   He never ran tests to actually prove his theory.   Nevertheless, it was accepted by academics of his time, and it remained the theory of choice until Galileo came along in the 16th Century.

      Galileo was a scientist, and he came up with his own theory concerning falling objects.   He believed that all falling objects continue to accelerate, picking up speed as they fall, and that this rate of acceleration is the same for all objects, regardless of their weight or density.

      The story goes that in 1589, at the age of 25, young Galileo attempted to prove his theory by climbing to the top of the Leaning Tower of Pisa with two balls in hand, one large and heavy, the other small and light.   He dropped them at the same time, and guess what happened?

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      Both balls hit the ground at almost the same time.   They would have hit the ground at precisely the same moment had there been no air resistance, a subject which will be discussed at length later in this blog series.

      Because Galileo’s experiment was subject to the dense atmosphere of Earth, the influence of air resistance prevented him from proving his theory correct, however he did manage to prove Aristotle’s theory wrong because the balls did not strike the ground at significantly different times.

      We’ll see how the astronauts’ experiment turned out next time.