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Name: Nicole S.
Status: educator
Age: 30s
Location: N/A
Country: N/A
Date: 6/12/2004


Question:
I am trying to explain to my 6th graders the idea of the difference between mass and weight and how gravity pulls on all objects about the same at 9.8N. They are completely confused as to the relationship of newtons to lbs. Do you have a clear way of explaining to them the idea that the gravitational pull is the same on all of us, but that our mass is different therefore, our weight is different? They think the pull of gravity should change if our mass changes. I am stuck.


Replies:
Could you say that the "pull" of gravity DOES change if our mass changes -- pull is another way of saying "force", and the force goes up as the mass goes up. But the pull per unit mass, that is, weight for each unit mass, or in units, newtons per kilogram, THAT is constant all over the earth. g = F/m, g is constant.

And of course if you go to another planet, then g will change. This can be explained by saying that g is what comes of one mass attracting another, and if you change the first mass (earth, to say the moon) then you change the acceleration, the g.

Do not mix pounds with newtons. The English unit of mass is a pound-mass, or something called a slug, and dates back to medieval times, and is seldom used. In metric system it seems to be just the opposite, the colloquial speak is to talk of kilos, but you do not really "weigh" 50 kg, and few people on the street use the word "newton". Either way its sloppy physics and one reason this gets confusing.

Steve Ross



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