How does drill variable speed work?

[rshayes]

The final speed does not depend on the weight of the disks at all. It will be determined by the motor characteristics and the drag in the system. There will be some drag due to the bearings, and this might be slightly different if the supported weights are different. The other source of drag will be the aerodynamic drag of the moving disk. If the disks are the same shape, this will probably be equal in both cases. Thus, the final speeds will be close to equal.

The rate of accelleration will depend on the angular momentum of the motor armature combined with the angular momentum of the load (ie, the disk). The angular momentum of an object depends both on the mass of the object and where that mass is located relative to the axis of rotation. The effect of the mass is proportional to the square of its distance from the axis of rotation. A small diameter, thick disk of a heavy metal can have a lower angular momentum than a large diameter, thin disk of a light metal. The disk with the higher moment of inertia will take longer to accellerate to the final speed.

There is a slight delay in starting current flow through the armature due to the inductance of the armature windings. This is usually ignored, since it is usually much shorter than the mechanical time constants due to the moments of inertia of the motor and its load.

<small>[ February 19, 2006, 11:31 PM: Message edited by: stephen ]</small>

[labview1958]

Hi,

V=I*R, if V and R is constant, how can I change?

[Chris Smith]

You cant have any of the above equations and dismiss the weight or mass of an object. That’s like having ohms law, leaving out amps, ohms, or volts. The equation is a whole, not a partial.

Weight and mass directly affect the speed both during acceleration and maintenance of that speed. If not, then you have the first free lunch in physics ever.

If Volts and Resistance in the windings remain fairly constant [for argument sake], and you change the mass of the object, the current draw will increase as defined by horse power.

Current draw is defined by How many amps are used and how much time to accomplish the task.

If the disk spends two more seconds of time to reach full speed using a even the same current as the lighter object, then more amps over time were consumed, even though the resistance and voltage remain constant, thus more amps were consumed than the lighter disk.

What will change most with heavier objects is the speed [acceleration] to accelerate, and the final speed over all.

If the final speed of the motor could remain unaffected as suggested, then the conclusion of that formulae or argument would be that a million pound object could be spun buy a tiny motor unaffected by the increase in mass. And we know this is not how things work. Any increase in mass directly affect the force being used to move it.

<small>[ February 20, 2006, 09:06 AM: Message edited by: Chris Smith ]</small>

[MrAl]

Hello again,


The reason why the top speeds of a rotating heavy disk and a light weight disk
are the same is because once the inertia is overcome the only force
required is for the friction (bearings and air drag of disk and armature).
Theoretically, the two disks spin at the same speed once they reach top
speed. This is not like two trucks going up hill because the light truck
will no doubt go faster over the top while the heavy truck goes slower, because
of the heavier weight taking more power to move up hill, and so in the limit
of that equation you get two different answers: one for the heavy truck and
another totally different for the light truck. A better example would be
two trucks starting out on level ground, one with more weight than the other,
and what is their top speed(?)...Again the *same*, unless the friction in the
axles increase noticably in the heavier truck because of the extra weight.
There is probably more air drag resistance than bearing resistance anyway.

There is however the friction in the bearings, which will increase with
a heavier disk. The reason for this is because the more weight the more
pressure in the bearings...and the friction function of a bearing is
going to show more friction with more 'across' force (more pressure against
the bearing balls and the balls rub against the bearing sleeve).
This could in fact be the main reason why the
heavier disk doesnt start...the rest friction cant be overcome by the
motor...whereas with the light disk the rest friction is less so it can
start. Since the rest friction is greater with the heavy disk
the friction during motion is probably greater too, which will reduce top
speed but most likely only very slightly in the practical case. Also,
it would depend on position: there could be much less difference when the
disks are spinning horizontally than when spinning vertically because
the friction function is different for different angles of pressure in the
bearings.
Remember also that in most drills there is some sort of cooling device...
such as a set of fan blades attached to the axle, which will most likely
dominate the friction component once the disks are turning fast. This
could very well overshadow the effect of the bearings and so the two
disks still turn at the same top speed because the moving friction of
the bearings is much less than the friction caused by the spinning fan
blades.

The conclusion, as it seems to me then, is:
Without considering friction in the bearings (assuming air drag is same)
the two disks reach the same top speed (can easily be proved with two
equations that show in the limit the two have the same solution).
When considering friction in the bearings, the top speed of the heavier
disk will be slightly less, but only VERY slightly less, unless there is
clearly something wrong with the bearings. Most likely though there will be
something else dominating the friction component (air drag) which will
make the two disks spin at the same speed even though there is a slight
difference in bearing friction between the two.
The reason the heavy disk doesnt start is probably because the rest friction
of the bearing is greater with more weight...but possibly if the drill is
held so that the heavy disk is horizontal it might in fact then be able to
start successfully without the aid of a push.

Chris:
Try thinking about the two trucks starting out on level ground with
different weights and see what top speed they both reach. Even though
one has more weight they both reach the same top speed, unless you
consider friction in the wheel bearings which will then decrease the
heavy trucks top speed...but only very, very, slightly since the trucks
would have to be built with bearings that can take a full load and if
there was too much more friction due to the heavier weight the bearings
would fail too soon. Make sense? There's also going to be more
friction from the tires on the ground, but how much more...if there was
too much more the tires would wear out very soon.


Take care,
Al

[Chris Smith]

Al.....

In the presence of our gravity, even in a vacuum and with frictionless bearings, any free spinning object comes to a halt sooner or later.

Entropy exists even in the planets.

Mass, Kinetic Energy, and Entropy form three sides of this triangle.

It never goes on for ever because of the influence of gravity over a mass. To reverse this slowing down requires that you inject energy back into the equation and maintain it, and that energy will be greater for the larger mass,.. than with the lesser one.

I have never seen two identical vehicles with the same power, one lighter than the other, going the same top speed even on the level.

The heavier one always goes slower, hence the weight restrictions of every moving vehicle including air planes.

A fully loaded 747 can not reach the same top speed as a empty one.

A Indy car that has a full tank of gas goes slightly slower than the one with a empty tank.

Gravity always influences mass at any angle.

Slight as it may be, world speed records are based on these slight variations.

For them to be exact, no matter what the difference in mass is to dismiss the influence of gravity on all objects. And this is not even including air friction, bearing friction, etc, all of which resist for the same exact reason.

Even air has to be displaced at a cost of weight of the air [gravity], resistance to move against another molecule [weight caused by gravity], and all because of gravity.


However, a greater mass always requires a greater force to maintain the same speed as a lighter object of similar design because even as the inertia drops, the mass is never spinning for free and requires a continual force to over come even gravity [its effects] , and that force is always greater for the greater mass.

A one ounce object requires less energy to maintain at speed than a two ounce object at the same speed and conditions. Always.

If not then there would be a free gain, at no cost. The two ounce mass required more energy to get up to speed, had more kinetic energy stored and at no additional loss other than inertia.... would be a free gain.

But You cant get more from less.

Inertia drops as the speed comes up to full or near full speed, but the mass is never free from out side forces not even with frictionless bearings, and because its never free, it weighs down the equation based on the difference in mass.

Slight or great is the factor of the mass. If the two disks are the same mass at rest, but even the diameter is greater on one, the two will not react the same as it accelerates or is near its top speed. The reason is like ohms law, if you change any single factor, the others follow. There is never a free lunch with physics, and to change any corner of the triangle of equations changes the position of the other two. They may seem to spin at the same, but the difference will be equal to the difference in mass always. Also IF the motor has sufficient power to over come the increase in mass, while still peaking out its maximum RPM, then the only limit to the speed is the motor design. If the increase in mass is over that limit, then the motor must slow down.

<small>[ February 20, 2006, 01:28 PM: Message edited by: Chris Smith ]</small>

[dyarker]

An airplane doesn't compare to a spinning disk. In level flight lift equals mass. To have level flight with a heavier load requires more lift. More lift is created by increasing the angle of attack of the wing (a little nose up), or increasing velocity. Either way drag is increased. Drag is only partly friction, work is done keeping the larger mass up against gravity.

In a vertical spinning disk, the effect of gravity on the half going up is balanced by the effect of gravity on the half going down.

In a horizontal spinning disk, gravity is perpendicular to the rotation and has no effect, except bearing load.

Disks of the same material implies that the more massive disk is larger; thickness, diameter or both. This means more air friction. But air friction is zero till it starts spinning. Back to bearing stiction and friction if it won't start.

"In the presence of our gravity, even in a vacuum and with frictionless bearings, any free spinning object comes to a halt sooner or later."

Go read Sir Isaac Newton's work again. The problem is there is no such thing as frictionless bearings. Very, very good bearings, but not absolutely perfect. Gravity itself has zero effect on spin acceleration, maximum rate of spin, or slowing the spin.

The Indy car's gas tank example doesn't "cut it" either. Acceleration will definitely be less with a full tank. If the car can't go as fast, it is because the additional mass causes increased running resistance of the tires on the road (side wall flexing and "foot print"), and bearings.

"A one ounce object requires less energy to maintain at speed than a two ounce object at the same speed and conditions. Always."

Never. To stop an object takes the same amount of energy as it took to start it. No energy transfer, no change. On Earth, you should visit sometime, speed maintenance requires energy input due to friction; a side effect of gravity, not gravity itself. With the same friction, a two once object needs the same energy to maintain speed as a one once object. It is just that the more massive object almost always has more friction.

[Chris Smith]

Dale,...Close, but no cigar....Planet earth and the cosmos disagree with you as do all physicists.

“In a horizontal spinning disk, gravity is perpendicular to the rotation and has no effect, except bearing load”?

Tell that to attrition. Even the spinning planets are slowing down with no friction from any bearings or air, just the gravity of other masses.

As I recall, the moon doesn’t spin on its axis at all. A product of earths gravity, time, and ZERO bearing friction. AKA attrition.

“more massive” ............is self descriptive, more mass, thus the load changes when you accelerate, or try to maintain a speed with a greater mass.

And the mass of any object increases with speed, also affecting attrition.

No free lunches or even free partial lunches in physics

Frictionless bearings pertains to hypothetical space, which in hypothetical space do exist. Its called math. And the results are the goal, not the reality that we cant make them on earth. Things slow down because of Gravity,.. and its influence on all objects of mass. Perpendicular or otherwise.

The Indy car's gas tank example doesn't "cut it" either. Acceleration will definitely be less with a full tank. If the car can't go as fast, it is because the additional mass causes increased running resistance of the tires on the road (side wall flexing and "foot print"), and bearings.

Tell that one to Einstein, as a object accelerates and gains speed, its mass also increases. As its mass increases towards the speed of light it can never achieve the speed of light because of the increase in resistance from its own mass. Catch 22.

What resistance are we talking about here? No tires involved, no air to push, just the forces of nature which includes the greater force, gravitational force and the lesser force, etc. If a mass makes no change to the equation, then any speed, and any mass would require the exact same amount of energy to maintain?

Lets see you manintain a million pound spinning object in space, with a one tenth HP motor where friction is not part of the equation but gravity is.

Good luck

All mass requires a force to over come it to make or maintain a velocity with gravity in the room.

A small mass requires a small effort of force to maintain a velocity in the presence of gravity, while a larger mass requires more force than the lighter mass. And then we have friction.

Two masses of different sizes require two different energy out puts to maintain any velocity in the presence of gravity before the common friction equation is even entered.

After this fact is equated, then we have simple and common values to add to this “law” of physics called with such simple names as bearing friction, air resistance, etc.

<small>[ February 20, 2006, 10:27 PM: Message edited by: Chris Smith ]</small>

[dyarker]

What a curtious person, puts own foot in mouth (up to knee), and doesn't even ask for ketchup!

"As I recall, the moon doesn’t spin on its axis at all. A product of earths gravity, time, and ZERO bearing friction. AKA attrition."

The Moon does spin on it's axis. The rate of rotation equals the rate of revolution around the Earth. The rotation originally was different. Where did the energy go? Heat caused by flexing the moon. AKA Tidal friction. An effect caused by gravity. Gravity keeps the Moon in orbit. The Moon is receeding. More gravity would pull it closer and speed it up. Therefore, gravity cannot be slowing it down. (a larger orbit means slower speed). Where did the energy go? Heat of tidal flexing again.

The Earth's rotation is also slowing. The crust flexes and warns a little, and the sea water is moved a little and is a bit warmer do to tides. Gravity caused the rotation and orbit around the sun in the first place. If gravity caused slowing there would be no rotation or orbit. Loss of energy as heat from tidal friction which is also caused by gravity, causes slowing. A perfectly rigid body (no such thing) would rotate and orbit forever.

"Tell that one to Einstein, as a object accelerates and gains speed, its mass also increases. As its mass increases towards the speed of light it can never achieve the speed of light because of the increase in resistance from its own mass. Catch 22."

The change in mass due to relativity effects speed on a race car are at least 16 digits to the right of the decimal point, but real. However, pick a reference point in space, if the race car is traveling on the Earth's surface in the opposite direction from Earth's motion; then the faster the car goes across the surface, the slower it's overall motion. Therefore, as the car goes faster the mass decreases!

"All mass requires a force to over come it to make or maintain a velocity with gravity in the room."

The "over come" means acceleration and is true. But that is not gravity, it is mass and inertia plus friction. Energy for maintaining velocity is only to make up for friction losses.

[Chris Smith]

Dale,....Actually you did the swallowing.

“Therefore, as the car goes faster the mass decreases!”

What a joke!

Lets examine a common bullet. Its mass starts off at 50 grains at rest. At 2000 fps its mass has increased by several thousand fold, hitting the object with a force of several hundred pounds. Thus as the bullet goes faster, the mass increases like all solid objects of mass.

“Tidal friction”, fancy word for gravity, the same thing that slowed down the moon.

Contrary to your statement ..."In a vertical spinning disk, the effect of gravity on the half going up is balanced by the effect of gravity on the half going down"
or
"In a horizontal spinning disk, gravity is perpendicular to the rotation and has no effect, except bearing load"

Cant have it both ways. Gravity affects all mass in the cosmos including the spin rates of bodies. Just ask the moon.

And the moon doesn’t spin like other planetary bodies on its own, it spins around the earths axis based solely on earths gravitational effect.

The moon did spin on its own once but earths gravity put a stop to that long ago. The only real point here.

The face of the moon is at all times in the same place facing earth representing earths effects over a otherwise spinning body.

Any spin as in one per day is not the moons spin per se, but rather earth taking it for a gravitational ride. But then this too is irrelevent, as were talking about where all the spin went, and why? [gravity]

With out the effect of earth on the moon, the moon would shoot off into space with no spin at all or a slow and dying spin that would either pick up speed from other bodies, or simple die off from the influence of gravity and plain attrition.

And so far, ...all the spin it once had is gone directly related to the effects of gravity on a spinning body [earths]. Attrition.

But, then you still ignored the fact of gravity slowing down the spin trying to use semantics to hide the fact that gravity stopped its original spin, a direct contradiction to your own words.


“In a horizontal spinning disk, gravity is perpendicular to the rotation and has no effect, except bearing load.” Tell that to the moon.

And then .....

“The Earth's rotation is also slowing”.....

is another fact of gravitational influences and attrition, again contrary to your statement of gravity playing no role in slowing down a spinning object.

The moon equally slows down the earth as do all the other bodies including the mass of the sun.

“effects speed on a race car are at least 16 digits to the right” ..........are irrelevant.

If we were splitting hairs, then we would split .0009 hairs, also irrelevant to the facts that I stated.

How big or how much is not part of the argument.

Again, a semantic smoke screen that doesn’t work.

Keep your eye on the ball.

The "over come" means acceleration and is true. But that is not gravity, it is mass and inertia plus friction. Energy for maintaining velocity is only to make up for friction losses.


FALSE, same reason the moon slowed down, [no bearings involved] same reason the earth is slowing down.

Gravity effects all objects, spinning perpendicular or otherwise, and that force is real.

If the object is spinning, it will influence its spin and even stop the spin eventually.

If the object is just “passing by” the orbit or path will also be affected into a small parabola out of its normal path as gravity does it thing.

There are NO BODIES or MASSES in the cosmos that are not affected by gravity, linear or spinning.

For every action there is a equal and opposite reaction. And there are no bearings in space to be influenced by gravity, just gravity it self.

Just Ask the moon.

[rshayes]

"Lets examine a common bullet. Its mass starts off at 50 grains at rest. At 2000 fps its mass has increased by several thousand fold, hitting the object with a force of several hundred pounds. Thus as the bullet goes faster, the mass increases like all solid objects of mass."

If you do the actual calculation, the increase in mass is about 1 ten-billionth of a grain. A calculator will probably show no increase at all, due to rounding errors.

"Lets see you manintain a million pound spinning object in space, with a one tenth HP motor where friction is not part of the equation but gravity is."

Consider galaxies, stars, the sun, and assorted planets. Not a single 1/10 horse motor in sight, all under the influence of gravity, and, after something over 4 billion years, still spinning.

I am unaware of any evidence that the rotation rate of the moon was ever substantially different from its presint rate. It may not be a good example.

The present rate of decelleration in the earth's rotation is about 1 second per year. Some of this may be tidal drag, but other factors may also be involved, such as accretion of matter from dust.

The friction of normal bearings or aerodynamic drag will normally stop a rotating disk in a matter of seconds to possibly minutes.

Bearing friction and aerodynamic drag can be reduced by rotating a disk with magnetic suspension in a vacuum. It might be possible for the disk to rotate for several days without additional drive.

If the disk is conductive, eddy currents due to the earth's magnetic field might contribute a little drag, possibly enough to slow the disk down in hours or days.

"Tidal forces" due to the earth's gravity might create enough drag to slow the disk down in a few million years or possibly longer.

"Tidal forces" can obviously be ignored as being insignificantly small.

[Chris Smith]

Steven

I can see the google wanna bee professor entered the room.

Not even worth the ink to reply.

And try looking up the speer hand guide for reloading. The mass exceeds 400 pounds.

Speer Reloading hand guide #9, page 126,

222 Remington
50 grain bullet [3.2399455 grams]
Muzzle velocity at 200 yards is 2449 fps,
665 pounds of mass still remaining at 200 yards.

454 grams per pound times 665# = 310,910 grams divided by 50 grains [3.24 grams]

A increase in mass of 93,182 times the original.

As to the rest, you state it best with the phrase "I am unaware of any evidence" which is much like your past posts.

Ask professor google next time. At least you learn something along the way.

<small>[ February 21, 2006, 06:27 PM: Message edited by: Chris Smith ]</small>

[rshayes]

Another load of bullshit from Chris.

If you calculate the kinetic energy of a 3.24 gram object traveling at 2449 feet per second the result is 665 foot-pounds.

It is not the mass of the bullet.

Incidently, "muzzle velocity" is the velocity of a bullet measured at the muzzle of the gun, not at a distance of 200 yards.

This makes it quite apparent that Chris is unable to read a simple table in his reloading handbook.

This may also explain why he has obviously never read a Physics textbook, as shown by his previous posts.

<small>[ February 21, 2006, 11:27 PM: Message edited by: stephen ]</small>

[dyarker]

If the moon didn't rotate we would NOT always see the same side. Unless it revolves around Chris of course http://ilrs.gsfc.nasa.gov/docs/williams_lw13.pdf, Lunar Geophysics, Geodesy, and Dynamics
"The Moon's rotation is synchronous and slow (27.3 days)." (Page 2)
"Lunar tidal dissipation has a significant influence on this rate and the total rate should
be the sum of Earth (1.3x10-11 /yr) and Moon (0.6x10-11 /yr) effects." (Page 4, but don't skip page 3 getting here.)

The force of gravity causes acceleration.
If a system is loosing energy while gravity is constant (mass and distance), then the energy comes from a source other than gravity (like slower rotation), and is disappated in a form other than gravity (usually heat). The difference between direct effects and indirect effects means objects in space and spinning disks are not a meaningful comparison with airplanes and cars.

To you it is all the same "Uh, duh ... gravity dun it."

The decreasing mass of the car is no joke. It depends on which way the car is going on the track, and which way the track itself is going (unless you think the Earth is at absolute rest).

To everybody else,

I had a bad day Monday, and Chris is an easy target. Sorry I got involved in this off-topic argument. Maybe some good well come out of it, like a couple minutes of amusing reading. Or, if you catch yourself agreeing with him, you double check to find your mistake

Now let's do some Nuts & Volts

Later,

[MrAl]

Hello again,


Thanks to everyone for the amusing replies...if i can stop chuckling long
enough to finish this reply i'll post it

I'm not one to favor constantly bringing up new so called 'analogies' when
the previous ones have not yet been rigorously proven to truely apply or
not apply (although i think they have been already by recent replies) but
there is one analogy that really makes an inertial system look very clear
because it's a very direct analogy, where the physical system variables
correspond directly to electrical system variables. Thus, once we know
the relationship between the physical system and the electrical system
(or 'transformations' between the two) we can examine the electrical system
using what we already know about electrical circuits to gain a quick
understanding of the physical system with little or no additional work.
Heck, we can even use a circuit analysis program to do all the
calculations so all that we need to do is take these steps:
1. Transform the physical system into an electrical analog.
2. Enter the 'parts' into a circuit analysis program as a schematic.
3. Run the analysis program.
4. Record the results and transform the output variables back into the
physical system variables.


We can start with the two truck example (mechanical translational system)
because this is the same as the two rotating disks problem (which is the
same system except it's a mechanical rotational system rather than
translational, and it's easy to show that these two are the same when
the appropriate variables are chosen to represent the physical quantities
like mass, speed, etc.).

For one truck with some mass M under the influence of some force F (from
the motor), it will reach some speed V after some time.

Step 1 would be to transform this physical system to an electrical system,
and since this is a translational system we choose variables as follows:
mass M=capacitance C
force F=current I
speed V=voltage V
Since we are going to ignore friction for now we dont include any resistance
(R) which would represent the friction (f) of any 'real' system.
We should however, look at the 'sticking' friction later to explain why
the heavier disk doesnt move right away while the lighter disk does when
the drill motor is first started.

Just to note, the above is usually called the "Force-Current Analogy" in
control system analysis.

Now the circuit is simply a capacitor (connected to ground to make this
even simpler) and being charged by a constant current (I) source (not a voltage
source!). We measure the voltage across the cap in order to determine the
velocity V (speed) at any given time. The capacitance C represents the mass
as above.
At zero time (t=0) there is no voltage across the cap, so the speed is also
zero. The constant current source starts to charge the cap so that at some
time t1 later we end up with speed v1. Note already we didnt even have to
say what the current (force) was, because even with the smallest force the
cap (mass) begins to charge (mass begins to move). In the absence of
sticking friction even the smallest force moves the mass!
Now we watch the voltage (speed) increase as the current remains constant
and we see that it ramps up (a straight line) indefinitely because the
voltage continues to increase with a constant current applied to a capacitor.
But rather than let it ramp up forever, we turn off (disconnect) the constant
current source at some time t2. We note the voltage and it's v2. Thus,
in time t2 the cap charges up to v2 so in the physical system in time t2 the
mass is traveling at speed v2.

Ok, we havent proved anything yet. We have to look at the same circuit with
a larger mass (capacitance) and compare the two next.
Taking the same circuit, we increase the capacitance (and thus the mass) to
two times it's original value. Now we look at the circuit again starting at
time t0 when the cap is completely discharged (no physical movement) and
start the *same* constant current again (this force comes from the motor).
Connecting the constant current source again we again note in time t1
the cap charges to some voltage (we'll call v3). Now because the cap value
is twice what it was before the voltage v3 is only half of what it was
with the original cap value. Continuing to monitor the voltage, at time
t2 we note the voltage has again gone up, and we'll call this v4.

Now we compare voltages (speeds) between the two caps (mass of the trucks).
We'll call the mass of the first truck M1 and the mass of the second truck M2.

With M1 at time t0 the speed was zero,
with M2 at time t0 the speed was zero.
Thus, at time t0 both trucks are at rest.

With M1 at time t1 the speed was v1,
with M2 at time t1 the speed was v3 which was half of v1.
Thus, at time t1 truck 2 is going half the speed of truck 1.

With M1 at time t2 the speed was v2,
with M2 at time t2 the speed was v4, which was also half of v2.
Thus, at time t2 truck 2 is going half the speed of truck 1 again.

Now lets say that time t2 is exactly twice that as t1 and that we disconnect
the first circuit at time t1 and we disconnect the second circuit (truck 2)
at time t2.
Here, at time t1 truck 1 is going at speed v1 and since we disconnect
it's constant current it continues to travel at speed v1 (no friction).
Next at time t2 we disconnect circuit 2 (truck 2) when it's speed is
v4.
Noting that v4=v1 (because at twice the time truck 2 catches up to
truck 1 if truck 1 no longer receives any more force after t1) we know that
both trucks get to the same top speed.


These two circuits simply prove that inertia is not something that needs
to be 'constantly' overcome. There is no force required after a certain
time in order to keep a mass moving at a certain speed (no friction),
and even the smallest force will move a mass up to ANY given speed when
there is no friction involved (any given speed low compared to the speed
of light of course, which is true of the two trucks).
Geeze, Newtons Laws should be reviewed!


Last, we can look at 'sticking' and 'sliding' friction.
Since sticking friction is often much greater than sliding friction and
the sticking friction must increase with increaing weight, the heavy
disk cant start because the sticking friction increases to a value that
is too high for the starting torque of the motor. Once it gets started,
the sliding friction (which is usually much less) allows the disk to
continue to turn.


If there is any more discussion about this i would suggest sticking with
the capacitor analogy, because it only involves three variables and it's
rather easy to set up circuits that are analogous to physical systems
with only one cap and one current source!

Oh yeah, before i forget, there is one more analogy that needs to be
brought up...
In the above we talked about the mechanical translational system but
the disk(s) are more directly rotational systems. Luckily, the equations
are the same for translational/rotational systems with a simple change
of variables...
Let:
rotational mass J=translational mass M=capacitance C
torque T=translational force F=current I
angular velocity W=translational velocity V=voltage V
(and 'time' is always 't')
and we have the same system again with the same results!!

To see these results for other masses or forces (or whatever) simply
enter a cap being charged up by a constant current in a circuit analysis
program and monitor the voltage across the cap (which represents velocity).
To increase mass simply increase the capacitance.
To increase force increase the current.
Note that more mass requires more force to get to the same speed V,
but even a small force will eventually get any mass up to any speed,
even if it takes all day


As others have noted, when we talk about airplanes we have to take other
things into account which we really shouldnt have to do for a disk rotating.
Some examples which attempt to be analogies just arent true analogies because
other things come into play which account for the different behaviour which
have nothing to do with the original discussion.
I also agree that we dont have to consider relativistic effects for this
'slow' kind of problem because we dont need that kind of accuracy.


Chris:
I really wish you would take the time to understand the analogy presented
here. Many of us other members here have taken time to understand your
examples so please take a little time for this.


Take care,
Al

[Chris Smith]

Steven, your foot is almost gone. Try reading a book or two in your life. The first one I suggest is Speer’s number nine reloading hand book. Only a fool would make such a stupid comment like yours with out having the book in his hand. Thanks for the laugh I really do appreciate it. Did you ever find your lost ham hock?
Speer Reloading Manual Number Nine Copy write 1974,... 464 pages

Dale your semantics shows that you lost the argument, so you trying to pick on straws. Remember keep you eye on the ball? The ball is does gravity affect spinning objects, not if the moon is truly spinning or how fast. The moons spin is unlike other celestial bodies. Its called synchronous rotation, and its completely irrelevant to the fact that gravity and its effects on a spinning body are the subject. The moon once spun like other bodies, now its stuck to earth more like a Planetary gear than a rotating body. All because of Gravity slowing it down. But feel free to wander off subject, the peanut gallery get off on such distractions which is why they have such a hard time learning anything. Well, Steven at least has this problem.

Nothing you say will be changing the laws of physics so all you have to remember is that it is a fact that gravity slows down spinning objects, and friction on a bearing are second to that fact. The moon is the proof, its orbit is a parasite orbit of gravity. Its reduced to a single rotation for each spin slowed down by gravity over the eons. That’s a fact.

From the Text....”The steady-state behavior of the Earth-Moon system takes millions of years to reach”

We don’t need distractions or splitting hairs, just the facts. Gravity affects spinning objects, and slows them down and that is a fact.

Al, nice math but you left out two equations which dismiss the simplistic view that no further energy is required. Gravity was in the discussion because were not operating the motor in deep space, and then too,..... all other forms of friction do come in.

IF you remove the argument of “earthly friction only” [bearings, wind] , you still have not dismissed gravity, and that was the heart of the argument.

ON EARTH a larger object of mass requires a greater force to maintain the same speed. [Because of gravity]