Re: Hydraulic Presses and Force



Timo A. Nieminen wrote:
On Sat, 1 Jul 2006, Dennis B wrote:

Timo A. Nieminen wrote:
On Fri, 30 Jun 2006, Dennis B wrote:

Force is defined as mass times acceleration.

Close enough. (The original definition was rate of change of momentum, and
this is still often a better definition, when one of the interacting
things has no mass or acceleration.)

In other words, force is
essentially a measure of the total energy it takes to move a certain
mass a certain distance within a certain amount of time.

No, that's not F=ma in "other words", that's completely different. And
wrong.

You make the claim that I am wrong, yet you provide no evidence to
support your argument, which makes your argument weak (dare I say
invalid). My definition is proven correct by the very simple fact that
force is measured in Newtons or dynes, which is a measure of distance
traversed (noting the incremental time units, in addition to the total
time involved, so as to determine how the energy or force is
distributed over the total time measured).

Energy, as a quantity in physics, has units of Nm, while force has units
of N.

Yet, force is a measure of the distance a mass traverses as a result of
the energy transferred to or from the mass. One cannot measure energy
without ultimately measuring the energy over time, whether one measures
the time or not. Work is therefore a measure of total energy over an
unspecified interval. Force is also a measure of total energy, albeit
over a specified time interval.

Clearly not the same thing. Force is not energy, and energy is not
force.

I disagree. I see no difference.

Neither N or dynes are a measure of distance.

Force, whether measured in Newtons or dynes, is a measure of the energy
required to move a certain mass a certain distance. For example, a
Newton is the energy required to accelerate a mass of 1 kg to 1
meter/second^2 (for a distance of .5 meters).


Keep in mind that "force", "energy", "momentum" etc are all technical
terms with specific meanings. If you start writing "force is energy", how
will the reader know whether you're writing about force, energy, or
something else?

I understand what you are saying. A mass put into motion was put into
motion by a the energy of a force, such as by the transfer of momentum
with a moving mass. Yet, although a mass in motion may not be
accelerating, it has the potential to exert a force or transfer it's
momentum. One could regard the momentum as potential energy. One might
argue that momentum is kinetic energy. Yet, one could also argue that
the mass stores or carries the energy and therefore is a potential
force.


Beyond that, your assertion without proof that "force is essentially a
measure of the total energy it takes to move a certain mass a certain
distance within a certain amount of time" is incompatible with F=ma. At
the very least, you have left out necessary conditions.

Consider a 1kg mass, which moves a distance of 1m in 1s. A certain mass,
distance and time, which is _all_ you specified above.

Case 1: mass is initially moving at 1m/s. a=0, so F=0.


If you are starting your measurement after the mass was put into motion
at a constant velocity, yes, the force is zero. Yet, that is not the
complete story. The mass was subject to a force in order for it to be
put into motion. The mass in motion then becomes a "potential force"
(or potential energy), which is easily calclated. This potential force
is a measure of the energy required to put the mass into motion as well
as the energy (or potential force) it carries.

Case 2: mass initially at rest, F=constant. Need a=2m/s^2, so F=2N. Final
KE = 2J, work done by force = 2*1 = 2J.

Both cases: same mass, same distance, same time. Different force.

Now try Case 3: mass initially at rest, F=At, where A is a constant.

Go read some Newton. It's all there right at the start of Principia.
Newton wrote in Latin, and our terminology has changed. Newton's "vis
insita" (innate force) and "vis inertiae" (inert force, or force of
inactivity) are what we now call "momentum", "impressed force" is what we
now call "force". Newton goes to some effort to explain the difference
between "innate force" and "impressed force", a difference sufficient for
us to use two completely different names today.

If your definition of force as a change of momentum is to be applied,
then force is the transfer of momentum.

Not my definition, it's Newton's definition.

I have used this definition
often myself.

So why make up a new definition?

It's not a new definition. Force is often written as mass times
acceleration. this is a standard definition. I simply translated the
mathematical definition of force as "ma" into the simplest english
equivalent. Admittedly, I should have explained what "mass times
acceleration" actually means.


Although I appreciate your input, you have not falsified
the facts I have presented.

Since you insisted, I have now falsified the non-facts you presented. Why
would I falsify the facts you presented?

--
T

Although we disgree, I appreciate your input. I am very interested to
hear what your response will be to my definition of momentum as
"potential force" or potential energy.

-Dennis B

.

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