Ditujukan untuk meningkatkan kualitas proses dan hasil perkuliahan Fisika di tingkat Universitas
7: Weight, Perceived Gravity, and Weightlessnes
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...
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--
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