# I What is the condition of true weightlessness?

1. Nov 12, 2018

### shk

I think only happens when gravity is zero.
But as gravity of Earth won't be zero , we need to be somewhere between Earth and Moon so the g of moon and Earth cancel each other out . At this point we will have true weightlessness. Is this correct and enough?

2. Nov 12, 2018

### phinds

Well, do you think that the other planets and the sun have no effect in that position? Note I didn't ask if you think they have a SMALL effect, but if they have ANY effect.

On the other hand, think about astronauts in the ISS. Do you think they are not weightless?

What is the difference between "weightless" and "in free fall" ? Where does "in free fall" occur?

3. Nov 12, 2018

### BearY

If you have a distribution of mass, you may find a point where g = 0 through simple math. The question is do you have a distribution of mass? Probably not.

4. Nov 12, 2018

### shk

i understand but
Do you know what is the condition of weightlessness?

5. Nov 12, 2018

### phinds

6. Nov 12, 2018

### shk

I am a bit confused with this website. I have been trying to get the answer for my question but everyone is asking me a question instead of answering my question.
I would appreciate it if you can answer my question as it saves a lot of time for me. all i want to know is:
what is the condition of true weightlessness?

7. Nov 12, 2018

### Staff: Mentor

The condition of weightlessness is usually that a device which measures weight (a scale) read 0.

8. Nov 12, 2018

### shk

thanks but I think this the condition of apparent weightlessness!

9. Nov 12, 2018

### Staff: Mentor

Then please provide a scientific reference that explains this concept of “apparent weightlessness”. It is a term I have not seen.

10. Nov 12, 2018

### shk

1.True weightlessness

It occurs only when an object is not subjected to any gravitational force .For example , if the astronaut is very far from the Earth and other astronomical objects, then g = 0, and there is true weightlessness.

2.Apparent weightlessness

For the spacecraft, there are two special examples of apparent weightlessness.
The first is the obvious case of the spacecraft falling vertically downwards with acceleration g.

The second is the case of the spacecraft circling the Earth. Although the motion is along the circle , the acceleration is still downward and equal to g , provided that there are no other forces such as air resistance or engine thrust .

but I am still not sure about the true weightlessness as I am not sure if such a place exists

11. Nov 12, 2018

### Staff: Mentor

This appears to be your own personal definition, not one from an authoritative source.

12. Nov 12, 2018

### phinds

Yes, exactly. That's because our goal is to help people find answers for themselves, not just spoon feed answers. This is not a "Q&A" type forum where you just ask a question and get an answer.

13. Nov 12, 2018

### shk

I found it here:
http://www.skhlkmss.edu.hk/physics/Gravitation/weightlessness.htm

I have been googling this the whole day to see what true weightlessness really is. Someone asked me this yesterday and i am looking every where to find the answer. he said this is part of his A level homework.
It seem that apparent and true wheightlessness are 2 different things

14. Nov 12, 2018

### shk

probably I'm in a wrong place then . I just need the answer so I can help someone with physics. I am a Maths teacher but I occasionally help my students with physics

15. Nov 12, 2018

### Staff: Mentor

Ok, I have never heard of it before, but you have the definition and it seems pretty clear. According to this source true weightlessness is when g=0.

16. Nov 12, 2018

### shk

ok thanks
maybe I should accept this although I am not sure if in reality such a place exists.
I understand your definition of weightlessness but "True weightlessness" seems to be something different.
Thanks

17. Nov 12, 2018

### hmmm27

Weight has nothing to do with gravity.

Weight is a phenomenon of a contact force. If you're sitting on a hillside, the component of the normal force that is pushing your arse straight up is what causes "weight".

If you're in a rocket travelling through space, no matter how many planets and moons you swing by you will remain weightless until you actually contact a planet (or its atmosphere) or fire up the engine (or maneuvering thruster).

18. Nov 12, 2018

### Staff: Mentor

I also don’t think such a place exists, which is probably why most sources don’t bother with such a definition. I would prefer just to use the standard easily measurable definition.

19. Nov 12, 2018

### shk

I understand
Many thanks for your time. It helped a lot.,

20. Nov 12, 2018

### shk

so how would you differ true weightlessness and apparent weightlessness?