Moment of Inertia and practical application

Both great examples

Just a little light reading for you, and mind the slugs.

While we're on the subject, what's the difference between lb ft and ft-lbs?

While we're on the subject, what's the difference between lb ft and ft-lbs?

Here's some stuff I found on the Web:

"At one time foot pound was used when talking about torque. Later the term was changed to pound feet. I assume it was to differentiate between the movement of a mass through a distance, and the force applied to a lever arm.

As an example:

If you move a pound mass one foot you get a foot pound.

If you apply a force of 1 pound on a lever arm of one foot length you get a pound foot.

So if you see hear someone talking about pound feet, they are usually talking about torque."

Columbia Encyclopedia definitions:

foot-pound, abbr. ft-lb, unit of work or energy in the customary English gravitational system; it is the work done or energy expended by a force of 1 pound acting through a distance of 1 foot. It is equal to 1.356 joules.

The term foot-pound is also used to designate a unit of torque that is sometimes called the pound-foot to distinguish it from the energy unit. A force of 1 pound applied 1 foot from and perpendicular to the direction to an axis of rotation produces a 1 foot-pound (or pound-foot) torque at the axis.

So apparently motor guys (like me) who have been referring to torque in ft/lbs are only semi-correct? Guess I never got the memo.

Ray

Ok, so a pound foot applied to a one-pound mass on a foot long lever will equalize (keep it from moving if it's stopped or prevent acceleration/deceleration)?

So a foot pound is work (force over distance) and a pound foot is torque (force)?

Yeah, I never got the memo either, but from a layman's perspective, they're interchangeable?

Ok, so a pound foot applied to a one-pound mass on a foot long lever will equalize (keep it from moving if it's stopped or prevent acceleration/deceleration)?

Yes, that sounds right.

So a foot pound is work (force over distance) and a pound foot is torque (force)?

I'd maybe put it a little differently; a foot-pound is force applied over a linear distance, while torque (using either term) is force applied around a rotational axis.

Yeah, I never got the memo either, but from a layman's perspective, they're interchangeable?

I guess so! It seems to me like an incorrect usage becoming 'correct' over time, after enough people do it; just one of the pitfalls of a living language, I guess.

Ray

Newton =.225 pounds on earth =(Kg)(m/s^2)

Joule= Newton X meters= .738 ft pounds, or .738 pounds ft

Joules/sec= Watt =(1/746) HP

Joule, is the unit for WORK as well as Torque

True, but to differentiate, you use N-m for torque and Joules for work. At least, that's what I do.

Joules is also a unit of energy, in some countries, the nutrition labels list Joules per serving, where we use Calories. But no one understands that 1 Calorie is equal to 1,000 calories.

IIRC, 1 calorie = 18 lb * 1 foot, because calories, while a unit of energy directly translates into a measurement of work.

This stuff does not make my head hurt. Slugs do.

Ray, I bet if you ask a common man, what the difference is between an engine that produces 100 ft-lbs of torque and one that produces 200, he'll say "the second one is more powerful", when power is work over time and has no bearing on torque. A 100 ft-lb engine spinning at 1,000 RPM produces a lot more power than a 200 ft-lb engine spinning at 100 rpm.

Pound feet foot pound same thing. Torque is force times distance where one end of that distance is a rotational axis.

differentiate what? I believe the derivative of power is torque, is that what your talking about.

He means differentiate in the conventional English language sense, not in the mathematical sense; if I'm not mistaken.

Although torque and work/energy (coincidentally?) have the same units, they are not the same quantities by any means. For one thing, torque is a vector quantity while work is a scalar (yeah, I dug out my old physics textbook). Calling one newton-meters and the other Joules is just a convention to distinguish the two (a newton-meter actually is a Joule).

Torque is the derivative of angular momentum with respect to time. Not sure what the derivative of power is.

Oh, and people who do physics in English units have got to be crazy.

Im not sure about power either, i just guest... (N x m)/s

In other words j x s= power

therfore...

dpower/ds = j

Also, looking at a torque curve, the amount of HP seems to be related with the area under the curve, proving my statement.

Also, looking at a torque curve, the amount of HP seems to be related with the area under the curve, proving my statement.

Well there's an illogical fallacy if I've ever seen one! If power were the integral of torque, it would never drop off, but instead level out. Power = torque x rpm.

i was thinking about that as well

To underscore a point made earlier, there is a distinct difference between torque and horsepower because there's a distinct difference between force and work done. Force can exist in the complete absence of power if nothing is accomplished by it. If I twist a locked doorknob, I have applied torque, but regardless of how hard I twist, no work is done unless I move the bolt by so doing.

Let's look for a minute at a reciprocating gasoline engine to understand why the rotating mass should not simply be reduced to the smallest size possible. The gas recip, whether 4-stroke or 2, produces power and accelerates only during the power stroke, and decelerates at every other point in the cycle. The only thing, then, that enables it to spin through to the next power stroke is the rotating mass of the engine. This is obviously most important at low speeds, where the time between power strokes is longer, and rotating inertia is less. Thus, what really determines the minimum rotating mass required by an engine is that needed to keep it running at the lowest speed that we intend to operate it.

Even when the engine will idle steadily, the rotating mass can be reduced to the point where it will not run smoothly or stably under a load in its lower operating range. At low speeds under a load, the engine may feel jerky due to the cyclic acceleration/deceleration it experiences. This is especially true with single cylinder 4-strokes because they decelerate for 3 times as long as they accelerate. The fact is that by reducing the decel phase at low rpm, a heavier rotating mass can actually result in an increase in net power at low engine speeds.

But the engine must accelerate the mass also, and the rotating inertia resists changes in speed either up or down, so heavier mass does work against the engine in that regard, but to what extent? Sometimes the gain at one end exceeds the loss at the other. My two '06 YZ450's serve as an example of this. Mine has a flywheel with an additional inertia mass of 1.1 Kg/cm², and my son's bike is stock, if we roll off at mid rpm on asphalt, the two bikes are as dead even as can be (they're otherwise identical), but from low to mid rpm, mine pulls his significantly.

There have also been documented instances of increased rotating mass increasing the terminal speed of a vehicle running at either relatively high speed, or with a relatively small engine for its size and weight, or both. Here again, it would appear that the addition of the extra inertia increased the power output.

Another factor in the question of changing the flywheel weight is the percentage of the total rotating mass represented by the flywheel, and what the net percentage change to the total mass will be. In a lot of modern engines, the flywheel is so small that it almost doesn't matter how heavy it is. The one in my YZF is smaller than the gas cap, for example, and is nearly insignificant.

What's best for Cr 250 2005 to ad torque ( Like 4T power ) and low all the way to top ?

porting

What's best for Cr 250 2005 to ad torque ( Like 4T power ) and low all the way to top ?

*** dude...wrong everything here. Wrong forum. Wrong thread. This is for the CR forum at the very least.

Sorry but since they all are talking weight, I thought perhaps someone has an idea for it on the CR. Plus pure laziness. Can someone atleast direct me in to the sites and links for this matter ? After all, I'm an engineer and at the right spot here.

Can someone atleast direct me in to the sites and links for this matter ? After all, I'm an engineer and at the right spot here.

Sorry but since they all are talking weight, I thought perhaps someone has an idea for it on the CR. Plus pure laziness. Can someone atleast direct me in to the sites and links for this matter ? After all, I'm an engineer and at the right spot here.

If you are an engineer you should have no problems finding the correct forum yourself.

Thank you grayracer513 and others. There are people with helping hand and being useful and others whom are always negative, sarcastic and useless, Like Aaron-silidker. I'm new here. Even the great Albert Einstein made mistakes. OK, I'm out. Time is the most important fact.