Subject:
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Re: "real" LEGO Hovercraft ? (with/without batteries/RCX "onboard")
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Newsgroups:
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lugnet.robotics
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Date:
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Sat, 30 Nov 2002 02:02:32 GMT
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Original-From:
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Jim Choate <RAVAGE@EINSTEIN.SSZ.antispamCOM>
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Viewed:
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1098 times
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On Sat, 30 Nov 2002, Nick Tarleton wrote:
> Jim Choate wrote:
>
> > On Fri, 29 Nov 2002, Steve Baker wrote:
> >
> > > So long as we are down here on the surface of the earth talking about
> > > the air pressure under the skirt of a hovercraft and whether it'll
> > > lift or not, weight and mass are equivelent concepts.
> >
> > That's a pretty basic physics mistake <shrug>.
>
> How? It seems to me that as long as acceleration due to gravity is constant
> (i.e. same altitude, same planet; in this case, 9.8 m/s/s) then weight and
> mass have a simple proportional relationship.
Ok, remember you asked...
Let's take example 1-
You're in an elevator. The elevator goes down. Your weight decreases but
your mass does not.
Q: Where did the weight go? Did you mass change?
Example 2-
Take a 1lb weight and a scale.
Scenario a:
Place the weight on a surface with zero friction (you can use ice, or a
Teflon surface for a real world sandbox)
Take the scale and lift the weight 1ft. how much does the scale measure?
Answer, 1lb.
Scenario b:
Now, take the same scale and the same weight and pull the weight
horizontally. What does the scale measure? A number -considerably- less
than 1lb.
Q: Where did that 'weight' disappear to?
Example 3-
Take a refrigerator and place it on a roller-bed or flat dolly of some
sort.
Scenario a:
Take a scale and attach to the refrigerator side, pull the scale until the
regrigerator moves. Note the amount of force.
Scenario b:
Take the scale and attach to the top of the regrigerator, pull the scale
until the regrigerator moves. Note the amount of force.
Example 4-
Note that you can demonsrate this with your car as well. You can push your
car on a flat surface, but you can't lift it.
Q: Why?
---------
What's going on here?
Weight is a vector (because g is a vector force), mass is a scalar (mass
isn't a function of a force nor is it a vector).
Q: What is the purpose of the -g in your basic mechanics texts?
Q: What is the sin(theta) in the description of a pendulum?
Weight is -only- equal to mass when the applied force is -parallel- to the
gravity force. In -all- other cases there is a sin(theta) factor.
When you act on a body vertically (with respect to the ground) then weight
and mass are equivalent. When a force acts perpendicular to gravity only
the mass is a factor (assuming friction can be ignored, which -is- a
function of gravity since it works perpendicular to the applied force).
Further, if you negate gravity (eg jump off a tall building) then you have
-zero- weight but your mass is -always- constant.
Bottom line, mass and weight are -not always- equivalent even on Earth.
As I said, a very basic physics mistake.
--
____________________________________________________________________
We don't see things as they are, ravage@ssz.com
we see them as we are. www.ssz.com
jchoate@open-forge.org
Anais Nin www.open-forge.org
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