RE: List Reborn

From: <Steve>
Date: Thu Oct 20 2005 - 03:54:00 EDT

One last try with reference to your last sentence. Consider a mass whirling at the end of a string, horiz plane. The kinetic energy is 1/2mV^2, as you say. Because momentum is a vector quantity is is dealt with by resolving into a reference frame. In cartesians x axis , at say 6 oclock the momentum is mV and at 9 oclock it is 0. So the momentum in the x direction has changed as the mass has moved through 90 degrees but the energy has stayed the same. As I said mixing vector quantities with scalars is not a good thing to do. If you wish to redefine momentum as the product of speed and mass, then fine; the ballistics guys do it all the time.

But it aint physics.

Cheers

Steve

-----Original Message-----
From: <a href="/group/PipingDesign/post?postID=BkzNDPFt0zOshlM_SMw2ziyqNu8RweQD3qJjjYTd-XF4UzCkoo50d1eXNYwWSCPZg9UASc2BeyLdcyhhQoJ_V211S9qW2wk">PipingDesign@yahoogroups.com</a> [mailto:<a href="/group/PipingDesign/post?postID=BkzNDPFt0zOshlM_SMw2ziyqNu8RweQD3qJjjYTd-XF4UzCkoo50d1eXNYwWSCPZg9UASc2BeyLdcyhhQoJ_V211S9qW2wk">PipingDesign@yahoogroups.com</a>] On Behalf Of Christopher Wright
Sent: Thursday, October 20, 2005 5:09 PM To: <a href="/group/PipingDesign/post?postID=BkzNDPFt0zOshlM_SMw2ziyqNu8RweQD3qJjjYTd-XF4UzCkoo50d1eXNYwWSCPZg9UASc2BeyLdcyhhQoJ_V211S9qW2wk">PipingDesign@yahoogroups.com</a> Subject: Re: [PipingDesign] List Reborn

On Oct 19, 2005, at 10:14 PM, <Mechproj@xtra.co.nz> wrote:

> your comments treat speed the same as velocity. Velocity has a
> magnitude and a direction. If you change the direction, you change the

> velocity, but not necessarily the speed.
Presumably you're referring to speed as the magnitude of the velocity vector. The relationship and the underlying physics still holds. The change in momentum doesn't equal the change in energy, as we agreed, because momentum can't equal energy. But the change in speed (or mass) which causes a change in momentum also causes the kinetic energy to change. That's because kinetic energy equals half the momentum times the speed. If you change the momentum the kinetic energy inescapably changes.
Christopher Wright P.E. |"They couldn't hit an elephant at <a href="/group/PipingDesign/post?postID=PEp-ZvnDFctWkhgLzaukMd7ZxLL9TzJxL2e1eU4kgf0Y9lMEXKd8ddF3kl0rrHbOSTuPZdlABobXu8Ih">chrisw@skypoint.com</a> | this distance" (last words of Gen.

.......................................| John Sedgwick, Spotsylvania

1864)
<a href="http://www.skypoint.com/~chrisw/">http://www.skypoint.com/~chrisw/</a>

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Yahoo! Groups Links Received on Thu Oct 20 03:54:00 2005