Sabtu, 23 Oktober 2010

Fisika untuk Universitas

Fisika untuk Universitas

Ditujukan untuk meningkatkan kualitas proses dan hasil perkuliahan Fisika di tingkat Universitas

7: Weight, Perceived Gravity, and Weightlessnes

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So far in these lectures we've talked about mass, about acceleration and about forces, but we never used the word "weight," and weight is a very nonintuitive and a very tricky thing which is the entire subject of today's lecture.

What is weight? Here you stand on a bathroom scale.

Gravity is acting upon you, the force is mg, your mass is m.

The bathroom scale is pushing on you with a force F scale and that F scale--

which in this case if the system is not being accelerated is the same as mg--

that force from the bathroom scale on you we define as weight.

When I stand on the bathroom scale I could see my weight is about 165 pounds.

Now, it may be calibrated in newtons but that's, of course, very unusual.

If I weigh myself on the moon where the gravitational acceleration is six times less then I would weigh six times less--

so far, so good.

Now I'm going to put you in an elevator and I'm going to accelerate you upwards and you're standing on your bathroom scale.

Acceleration is in this direction and I will call this "plus" and I will call this "minus." Gravity is acting upon you, mg and the bathroom scale is pushing on you with a force F.

That force, by definition, is weight.

Before I write down some equations, I want you to realize that whenever, whenever you see in any of my equations "g" g is always plus 9.8.

And my signs, my minus signs take care of the directions but g is always plus 9.8 or plus 10, if you prefer that.

Okay, it's clear that if this is accelerated upwards that F of s must be larger than mg; otherwise I cannot be accelerated.

And so we get Newton's Second Law: F of s is in plus direction...

minus mg--

it's in this direction--

equals m times a and so the bathroom scale indicates m times a plus g.

And I have gained weight.

If this acceleration is five meters per second squared in this direction I am one and a half times my normal weight.

If I look on the bathroom scale, that's what I see.

Seeing is believing--

Ucapan Terima Kasih Kepada:

1. Para Dosen MIT di Departemen Fisika

a. Prof. Walter Lewin, Ph.D.

b. Prof. Bernd Surrow, Ph.D.

2. Para Dosen Pendidikan Fisika, FPMIPA, Universitas Pendidikan Indonesia.

Terima Kasih Semoga Bermanfaat dan mohon Maaf apabila ada kesalahan.

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