Re: How can I tell if F is a string or if it is a number?
- From: "Paul E. Black" <p.black@xxxxxxx>
- Date: Thu, 22 May 2008 14:00:35 -0400
On Wednesday 21 May 2008 16:37, rpost wrote:
Pioneer1 wrote:
[...] I see
Newtonian mechanics as a consistent system of units made of physical
quantities. It is a computational device, or an algorithm, that takes
given values of physical quantities as its arguments and transforms
them according to some rule.
This is a big mistake. An algorithm is directed: it transforms input
into output. Newton's laws are undirected: they relate quantities
without saying that one of them results from the other. ... [PEB]
Expressed another way, Newtonian mechanics is a method for building
mathematical models for physical events. (In fact, much of basic
college Physics is not calculating - the equations are pretty simple -
but learning how to use which equation.)
Suppose I want to know how high my model rocket will go if I shoot it
straight up. I could try launching it and measure the height somehow.
Or I could do some physics ...
The rocket weighs 23 grams.
http://www.estesrockets.com/products.php?number=1225
I'll use a C6-5 engine, which produces an average force of 6 newtons.
I'll simplify by leaving out air friction and using a gravitational
force of 9.8 m/s^2.
The (average) initial acceleration of the rocket by the engine is
6 N / 23 g = 260 m/s^2
subtracting gravity, the net upward acceleration is
260 - 9.8 = 250 m/s^2
The engine fires for 1 2/3 seconds. At burn-out the rocket is going
250 m/s^2 * 1.666 s = 420 m/s at a height of
1/2 * 250 m/s^2 * (1.666... s)^2 = 350 m.
Ignoring air friction, which is actually substantial in this case,
the rocket coasts up for
420 m/s / 9.8 m/s^2 = 43 s
At 43 seconds, the rocket reaches a height of
350 m + 43 s * 420 m/s - 1/2 * (43 s)^2 * 9.8 m/s^2 = 9350 m
(The force of air friction is so great that the engine is only has a 5
second delay. The simplifying assumption of no air friction is far
from realistic. http://my.execpc.com/~culp/rockets/rckt_sim.html has
equations that account for drag.)
Classical mechanics is what leads me to multiple, divide, or add this
and that.
-paul-
.
- References:
- Re: How can I tell if F is a string or if it is a number?
- From: Pioneer1
- Re: How can I tell if F is a string or if it is a number?
- From: Patricia Shanahan
- Re: How can I tell if F is a string or if it is a number?
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- Re: How can I tell if F is a string or if it is a number?
- From: rpost
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