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¿Ã¸° ³¯Â¥ 2008/09/21 ÀÏ¿äÀÏ ¿ÀÈÄ 11:47:17
¿Ã¸° »ç¶÷ "Theresa" <therlam@yahoo.com>
"I am new to this list and hope this is not too naive a question. I am curious to hear a few comments on how mass remains constant according to the equations:
If W=mg; then m=W/g.
I believe that some students may have a common misconception regarding "zero gravity" in relation to weightlessness. What happens to mass if this is at all possible?
T. Lam

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I do this with a paper clip or clothespin suspending a rubber band perpendicular to its "rest length", with a clothespin for extra mass beneath it. I tell the kids that we're weighing the lower clothespin by the rubber band, and they can see how even the rubber band alone sags under its own weight.
Then, when I drop it, the rubber band returns to its rest length (it looks horizontal) and the lower clothespin is pulled up to the rubber band.
This is to show that everyday objects instead of special Physics scales behave the same way.
 Steve >>>>
At Sunday 9/21/2008 03:42 PM, you wrote:
>Simple demo. Hold a medium size spring vertically with a small mass on the bottom. Choose a spring/mass combinations that will stretch visibly when held vertically. Ignoring the mass of the spring for discussion, the extent of the stretching of the spring is directly proportional to the weight of the mass. To convince yourself of this, try different masses. Of course, this is how we teach Hooke's law.
Now watch what happens when you release the top of the spring. On the way down, the spring and mass is in free fall. Your "measurement of weight" now records a value of zero because the spring immediately unstretches back to its horizontal length. In free fall, the weight of the mass is not opposed by the force applied by your hand, so we record no weight. Nevertheless, the force of gravity on the suspended mass never changed.
Dr. Daniel Kaplan
Physics Teacher
Matawan Regional High School, Aberdeen, NJ
________________________________
From: physicalsciencerequest@list.nsta.org on behalf of diana soehl
Sent: Sun 9/21/2008 6:27 PM
To: physicalscience@list.nsta.org; Scott Orshan
Subject: Re: zero gravity
Just to infuse some humor I got so excited when I saw the thread
>on my email I thought it was about the ZeroG flights that have
>been occurring in any case I can attest to weight changing as I
>felt my chest get heavy when we came out of the parabolas on the
>flight and when we were floating in air! What a crazy
>experience If you find a state offering it I suggest you apply
>and then drive there to experience it!
>
>Diana Soehl >
>
>"Don't ask yourself what the world needs; ask yourself what makes
>you come alive. And then go and do that. Because what the world
>needs is people who have come alive."
>harold whitman
>
> On Sun, 9/21/08, Scott Orshan <sdorshan@aol.com> wrote:
> >
> From: Scott Orshan <sdorshan@aol.com>
> Subject: Re: zero gravity
> To: physicalscience@list.nsta.org
> Date: Sunday, September 21, 2008, 5:28 PM
> >
> I see the issue here. It's the misconception that 'g' means
> gravity. Gravity simply exists. It obeys the inverse square law
> when it comes to distance.
>
> Every particle in one object attracts every particle in
> another object with some force that is dependent on the inverse square of the distance. Suppose that object A has 5 particles, and object B has 3 particles. Let's look at all of the attractive forces:
>
> A1B1
> A1B2
> A1B3
>
> A2B1
> A2B2
> A2B3
>
> A3B1
> A3B2
> A3B3
>
> A4B1
> A4B2
> A4B3
>
> A5B1
> A5B2
> A5B3
>
> The force between these two objects is 15 times as big as
> the force between any two of its particles.
>
> The total attractive force between A and B is proportional
> to the product of the number of particles in each, divided by the distance squared. Since we don't really like to count the number of absolute particles in an object, we aggregate them into units known as kilograms.
>
> Given that we have units that we already use  kilograms,
> meters and Newtons, we have to establish the proportionality constant so that the measurement systems relate properly. Through careful measurements, it was discovered that this proportionality constant was 6.67 x 10^11. Two 1 kg objects whose centers are 1 meter apart attract each other with a force of 6.67x10^11 Newtons.
>
> This makes the total computation work as follows:
>
> Attractive force = 6.67x10^11 * Number of particles in A
> expressed in kilograms * Number of particles in B expressed in kilograms divided by the distance in meters squared. This is the law of universal gravitation. The proportionality constant is called 'G'.
>
> Let's say that you are determining the force between the
> Earth, and an object on its surface. You could use this formula each time, but you would soon realize that you are repeatedly entering three of the same numbers into your calculator  G, the mass of the Earth (5.97x10^24 kg), and the distance from the center of the Earth to its surface (the radius, or 6371 km). So you precompute 6.67x10^11 * 5.97 x 10^24 / (6371)^2. Go ahead, try it.
>
> If you entered the numbers correctly, you will get 9.81, or
> the number we call 'g'. The only thing left in the computation of the attractive force is to multiply by the mass of the object. So you get Force = g * Mass of Object.
>
> The answer to your question is that 'g' can never be zero,
> not because gravity can never be zero, but because 'g' is a
> precomputed product of numbers that themselves will never be zero.
> Any particular 'g' (e.g., on the moon or Jupiter) only exists as a function of the body's mass and radius.
>
> Scott
>
> Theresa wrote:
>
>I understand everyone's response. I just needed confirmation that there is no such thing as "zero gravity". It's the rearrangement of the equation to m=w/g that I was not sure about. Caryn's explanation makes sense; take a step back and see if the question/answer is reasonable. It just makes for an interesting "What if" question...what if there was no gravity, thus no weight? What happens to the mass?
>
>T. Lam
>
> On Sun, 9/21/08, Steve Schoenbaechler <steveschoen@yahoo.com>
><mailto:steveschoen@yahoo.com> wrote:
> >
> From: Steve Schoenbaechler <steveschoen@yahoo.com>
> <mailto:steveschoen@yahoo.com>
> Subject: RE: zero gravity
> To: physicalscience@list.nsta.org
> Date: Sunday, September 21, 2008, 11:43 AM
> >
>Gravity is an attraction between two bodies. If there is no other body for attraction, there is no gravity. Thus, the "g" in W = m*g is zero.
>
>Till Then,
>
>Steve >
>
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