Question

https://www.youtube.com/watch?v=zfTE4J87aQQ#t=1806
Hi what is the force F with sub small s and why does Walter Lwein defined that as Weight ?

Answer 1

Sorry, I can't go over the 50 min video.
Please tell us what time this \(F_S\) appears.

The whole point in the video is to give a proper definition of weight when you are in a moving frame of reference. In this case, weight and gravitation do not coincide anymore and in the case of free fall, weight becomes zero.

Weight can be defined as the opposite of the force that is needed to keep you at rest in your reference frame.

Answer 2

It's on the first Seconds.

Answer 3

That's exactly what I've just written, only in magnitude.

\(\vec W=-\vec F_S\) by definition.

Since \(\vec F_S= -m\vec g\), then \(\vec W=m\vec g\)

Walter Lewin has the same equations only they are not between vectors, but between magnitudes. Hence \(\vec F_S= -m\vec g\) becomes \(F_S= m g\)

Answer 4

Yes, I can understand that but he's defining that as weight, doesn't he ?

Answer 5

Yes, this is the correct definition of weight, and all the point in the video is to highlight the fact that it is NOT equal to gravitation.

Answer 6

Gravitation ?

Answer 7

What do you mean by gravitation, isn't \(W=mg\) ?

Answer 8

Gravitation is \(m\vec g\), whereas weight is \(-\vec F_{scale}\).

They coincide in an inertial reference frame, but watch this video as a lecture and it will tell you how they are different in a non inertial frame.

Answer 9

Weight vector is toward earth.

Answer 10

I've confused actually.

Answer 11

Q: "Weight vector is toward earth."
A: No, it's not.

I cannot explain here in a few lines, what takes 50 minutes of lecture and demonstration by Walter Lewin.

Please, watch the video and follow his steps. He says at the beginning it is non intuitive. I would add, it is also confusing because it goes against the definition of weight that you were given in your high school years.
What he explains is a whole new approach to what weight actually is.

Answer 12

When you watch the video, I would advise you to ignore part from 6:44 to 20:26.
You would be even more confused and this part is not really necessary.

Answer 13

Yea..
in the syllabus i teach

i usually define weight = mg
and apparent weight = the weight that YOU FEEL.. = the normal reaction :D

Answer 14

Yea, in this case weight and force of gravitation is the same.
I prefer to use "apparent weight" for situations where buoyancy counteracts weight.

Anyway, non inertial frames are tricky and counter intuitive!