Re: What is Forced (accelerated) Motion?

From: Herman Family (celcaps_at_frontiernets.net/without_any_s/)
Date: 07/21/04 

Date: Wed, 21 Jul 2004 02:41:03 GMT

"Spaceman" <Spaceman@realspaceman.com> wrote in message
news:YejLc.148279$Oq2.88010@attbi_s52...
>
> "Alex Hunsley" <lard@tardis.ed.ac.molar.uk> wrote in message
> news:_4jLc.86783$q8.8405@fe1.news.blueyonder.co.uk...
> } You misunderstand the use of acceleration here. What is being said is
> } that all points on the disk (except the central point) are in
> } acceleration when the disk is spinning, since they are changing their
> } velocity constantly. Any point has constant *speed*, but not velocity
> } (which has a direction). And a change in velocity is acceleration.
>
> That is where you are losing the original "real world" meaning
> of acceleration (speeding up)
> A change in direction does not automaticall infer a change in velocity,
> for if it did, the disk would be speeding up it's speed according to
> the old fashion meaning of acceleration.

No, we aren't confusing the issue. We are disentangling it very precisely.
Your "old fashioned meaning" is very imprecise and could mean several
different things, even if you think you mean only one or two. We are
segregating out each of them to allow them to be studied independently
before putting them back together to form a whole picture. In that picture
we have all the different effects of going around a circle. This includes
the angular velocity, the tangential velocity, and the centrifugal and
centripedal forces. To top it off, you can go ahead and measure these
values and discover how correct physics really is. How well does your "old
fashioned" terminology handle that.

>
> The mathematical world definition is very bad,
> If I asked to to accelerate the point on the disk,
> Would you speed up the disk, or tell me it is already accelerating,
> constantly?
>

The disk is traveling at 1000 rpm, that is a rotational velocity. At any
instant, a point 1 foot from the center of the disk would be traveling at
6280 feet per second in the direction of its tangent. If the point is to
stay on the disk, it must change direction. A change of direction requires
acceleration in that direction.

It gets a little easier if it is broken down into vectors. If the disk is
horizontal and spinning clockwise, then at some moment the point is
traveling north. Its velocity could be considered to be 6280 feet/second N,
0 feet/second East. After an eigth of a turn, it is now traveling 4440
feet/second N, 4440 feet/second East.

At the moment that it was heading due north, it was accelerating eastward
towards the center of the disk. An instant later, it was accelerating both
eastward and southward (or negative northward). The cumulative effect in an
eigth of a turn was to reduce the North component and increase the East
component.

As it was heading northeast at a speed of 6280 feet/second, it was being
pulled in the negative north and postiive east directions, again with the
same magnitude of force as before. This further diminished the North
component, and increased the East component. By the time the point was at
the north of the disk, it would be heading east, with a velocity of 0 f/s
North, 6280 f/s East. If it fell off the disk at this point, it would
travel at 6280 f/sec in an easterly direction.

Michael

Relevant Pages

  • Re: What is Forced (accelerated) Motion?
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  • Re: What is Forced (accelerated) Motion?
    ... >} You misunderstand the use of acceleration here. ... >} acceleration when the disk is spinning, ... > A change in direction does not automaticall infer a change in velocity, ... Its velocity could be considered to be 6280 feet/second N, ...
    (sci.math)
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  • Re: What is Forced (accelerated) Motion?
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