# B Falling Feather and ball in vacuum

Tags:
1. Mar 26, 2017

### Ammar Ahmed

I saw some of these videos showing feather and some balls falling with the same speed in vacuum. But on normal atmospheric pressure they fall depending on their weight.

This is just blowing my mind, WHY is this ?

Does this prove Newtons Law of Gravity is wrong ? Then how is Einstein's Fabric of Time and Space correct ? Because that is the only other explanation i have heard of gravitational pull.

1.
Is it because of Archenemies Law (here air replaced by water), the reason for buoyancy for boats ? No air, no fluid to push upwards (drag). I am sorry i dont remember abbreviations but :

Weight of boat in water = Weight of boat on land - Weight of water replaced by submerged part of the boat

2.
Or simply because of air resistance ?

3.
Or something else i dont know ? But I really want to know.

Just another question, what happens if we compare feather with a helium balloon (if it can stay there, LOL) ?

Thanks in advance for the answer so that I can sleep tonight :) .

Ammar Ahmed

2. Mar 26, 2017

### Staff: Mentor

Yes.

3. Mar 26, 2017

### Orodruin

Staff Emeritus
Did you ever try driving a cab and raising your hands or to stick your hand out of a car window?

4. Mar 26, 2017

### Ammar Ahmed

Thanks Nugatory, but does it prove that Newton's Law of Gravity is wrong ?

@Orodruin, yes, so this is similar to drag, right ?

5. Mar 26, 2017

### phyzguy

No, it proves that Newton's law of gravity is right. Newton's laws say that F = ma = GmM/r^2, so the acceleration a = GM/r^2, which is the same for all objects, regardless of their mass m. This is why they fall at the same speed.

6. Mar 26, 2017

### Staff: Mentor

No. Newton's law of gravity says that the gravitational force between two objects is $Gm_1m_2/r^2$, and that's what the gravitational force is on all the objects in all the videos.

We add to that the force of air resistance (easy to calculate in a vacuum where the air resistance is zero, not so easy on earth) to get the total force on the object, and that's what you're seeing in the videos.

7. Mar 26, 2017

### Ammar Ahmed

Just to complete, do we have any equation to find gravitational force from Einsteins Fabric of a Time and Space ?

8. Mar 26, 2017

### ZapperZ

Staff Emeritus
It is still doubtful if you even have understood what you've been told here based on just simple Newtonian gravity. Have you?

https://www.physicsforums.com/insights/why-is-acceleration-due-to-gravity-a-constant/

I strongly suggest you figure this out first before jumping into "Einstein's Fabric of Time and Space", because Special Relativity and General Relativity are not exactly within your reach if you can't understand the simpler explanation yet.

Zz.

9. Mar 26, 2017

### Ammar Ahmed

I understood Newton's Law, but what confused me was the part that how feather had more speed in vacuum.

Look at me post, the first thing came in my mind was Newtons Law.

The 2nd thing came in my mind was Archemedes principle.

Einsteins explaination (as of i understood so far) is that fabric of time and space is the reason for gravitational force. Please correct me if i am wrong.

10. Mar 26, 2017

### phyzguy

Of course we do, but in the situation you are describing, the difference between Newton's laws and Einstein's GR is so small that it is unmeasurable. Newton's laws are all that is needed.

11. Mar 26, 2017

### Staff: Mentor

There is no "fabric of time and space" - that's just an analogy that authors use when they're writing for an audience that lacks the mathematical background to take on the real theory.

There is no gravitational force in general relativity, which is Einstein's theory of gravity - there is gravity, of course, but not gravitational force. However, if you solve the (much more complicated) equations of general relativity, you will find that they accurately predict the motion of objects in a gravitational field. However, there's no point in trying to take on general relativity until you have been through a serious college-level calculus-based physics program, and picked up a few years of math beyond that.

12. Mar 26, 2017

### Ammar Ahmed

Perfect. Now i feel quenched.

I am sorry for mixing up these things and that i havnt touched college maths for last 10 years.

I understand tha Einsteins fabric of ts is just an analogy and it is not what it will look like. But just for sack of discussion i used this term.

13. Mar 26, 2017

### ZapperZ

Staff Emeritus
You also need to understand that, under normal, ordinary situations, Einstein's General and Special relativity CONVERGES to the same description as Newtonian laws! In other words, if Einstein's theories predict something entirely different than Newton's under terrestrial condition, it would have been wrong!

So trying to use SR and GR in the situation you describe is like asking a structural engineer to use SR and GR to design a building. You'll be laughed at!

Zz.

14. Mar 26, 2017

### Ammar Ahmed

Lolz, yup. I am not dumb. In some causes I can expect a punch.