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birdy
01-17-2005, 05:54 PM
I hate to be the one to bring this subject up again,but I'm hav'n trouble get'n my inadequate little mind round the theory of inerta on fly'n objects. :mad:

To keep it simple [for me :rolleyes: ]I'll take out all interfearing complicateing things like airodynamics,gravity and pilots.

A cue ball is cruising through space at x mph.
It has stored inertia.
How do you measure the quantity of inertia ?,and wot is it's inertia "relitive" to?.[it has to be relative to something, or it can't be measured]

banaari
01-17-2005, 06:13 PM
I'll have a crack at this one...

"Inertia" is a measure of how much force has to be exerted on an object to achieve a certain acceleration. (Disregarding friction and drag, etc.) It is purely a function of mass. (More complex than this if you want to talk about rotating objects).

"Kinetic energy" (which is what you mean) is what an object has when it is moving... measured from, and relative to, a given frame of reference.

So a cue ball moving across the surface of the earth at X knots has Y energy, relative to the earth.
If it is moving because it's in the pocket of someone flying their gyro at X knots, then relative to the gyro it has no kinetic energy, because relative to the gyro it isn't moving.

We won't get into the prevailing conditions inside the gyro pilot's pockets...

You can COMPUTE kinetic energy by measuring speed...
k = 1/2 * mass * velocity * velocity
but you can't measure it directly.

cheers,
John

Udi
01-17-2005, 08:34 PM
Newton's Laws of Inertia state that force is required to change the motion of matter. Momentum is an extension of inertia and energy is a result of matter in motion. :rolleyes:

p (momentum) = m (mass) * v (velocity)

and, as John has said:

k (kinetic energy) = 1/2 m * v(squared)

Udi

Victor Duarte
01-18-2005, 01:22 AM
David , in addition to Udi.

K , the kinetic energy, is the energy that would be liberated if you decide to stop the ball. Imagine, a heavy Basketball, you want to stop that ball with your hand, then you will need to apply K energy with your muscles to stop it (and eat a lot of Spinach).

For example, to push your car that ran out of gas in the outback, you will need a big effort to make it move , the force that is resisting is the inertia, but once moving you will need a small effort to maintain it rolling > you have transmitted to your car : K energy.... you will need this K energy to stop it again...or this energy can be absorbed by the next tree :D
cheers

Doug Riley
01-18-2005, 06:17 AM
The key concept is that inertia is an object's reluctance to CHANGE its velocity. A CHANGE in velocity is the same, no matter where you happen to be sitting when you measure it.* Therefore, a body of given mass has definite inertia. The actual velocity of the body will, OTOH, be different depending on where you sit when measure it. Since kinetic energy is a function of velocity, the body's kinetic energy will also be different, depending on where you sit.

*If you're driving north in your car at 60 mph and the guy in the lane next to you is travelling north at the same speed, his velocity relative to you is zero. His velocity relative to the road surface is, of course, 60 mph north . If he then speeds up to 80, his velocity relative to you is now 20 mph north, and his CHANGE of velocity relative to you is also 20 mph north. Relative to the road, obviously his velocity is now 80 north, but his CHANGE of velocity relative to the road is, again, 20 north.

You can measure a change in velocity from any vantage point and you'll get the same number.

birdy
01-18-2005, 04:21 PM
Righto,everything is equal to my present understanding of the subject,and the thread has stayed on tack.
Give me a little time to compose the following question in a way that you'll understand,coz the thread could easly get off tack.

birdy
01-20-2005, 03:25 AM
"The key concept is that inertia is an object's reluctance to CHANGE its velocity"
So a mass that is NOT moving has inertia??

Chopper Reid
01-20-2005, 03:30 AM
Inertia is that thing that hurts you when it comes to a sudden stop.
Sorry Birdy :)

Jazzenjohn
01-20-2005, 05:18 AM
If a car is stopped on the road, you need to overcome inertia to get it moving. Inertia is the cars tendancy to prefer to stay stopped if it's stopped or to continue moving if it is already moving.

Dean_Dolph
01-20-2005, 05:24 AM
"The key concept is that inertia is an object's reluctance to CHANGE its velocity"
So a mass that is NOT moving has inertia??Yes! If an object is moving it wants to continue moving. If it is stopped it doesn't want to move. In both cases this reluctance to change velocity from zero (when it is stopped) or speed up or slow down from what ever speed it is traveling is expressed as inertia.

Doug Riley
01-20-2005, 06:11 AM
Yes, an object that isn't moving (from your particular viewpoint!) has inertia. Inertia is resistance to acceleration (=change in velocity) -- whether the change is a speedup, a slowdown, or only a change in direction with no speed change.

Inertia is NOT a quality that expressed in units or measured in itself. We simply speak of "mass" (NOT the same thing as weight!) and measure IT.

The unit of mass in the English system is the "slug." A "slug" of mass is the amount of mass that will accelerate at 1 foot/second per second when pushed by a force of one lb. An object having a mass of one slug weighs about 32 lb. on earth. One in awhile you hear the slug referred to as the "geepound."

The metric kilogram is technically a measure of mass. If you have X kg of mass on earth, you still have the same X kg on the moon (while your weight in pounds will be different on the moon). It's common to use kg as a unit of force, though, in the same way that the pound is a unit of force in the English system.

MichaelBurton
01-20-2005, 07:45 AM
In classical physics Force=Mass * Acceleration. So from any relative position we can measure the acceleration of the object to a given force. This measurement will alow us to calculate the objects inertia or resistance to change in motion.

Doug Riley
01-20-2005, 08:02 AM
Yes, measuring an object's acceleration in response to a known force is one way to come up with the object's mass (if you aren't able just to put the thing on a scale and weigh it).

It would theoretically be possible to count the number of protons, neutrons and electrons in the object and figure its mass that way... but you'd have to write an awful lot of zeroes.

Jazzenjohn
01-21-2005, 11:26 AM
<"The key concept is that inertia is an object's reluctance to CHANGE its velocity"
So a mass that is NOT moving has inertia??>

Inertia is more like a property of mass. If an object has mass then inertia will be a property that applies to that mass, the same as the property of gravity applies to the mass of the earth.

Doug Riley
01-21-2005, 11:40 AM
Yup, exactly.