Question about gravity's existence

I've just read in Marcus Chowns book that gravity does not exist per se. It's actually inertia. And also, gravity is acceleration.
I don't quite understand that.

He gave the analogy of someone blasting off in a space rocket, accelerating up at 9.8 m/s/s and to them, if they dropped something, it would fall, exactly as it would with gravity.
Now, I can't see how this can work- I mean, the earth's not accelerating in all directions at once is it? Could someone explain this to me please?
My valuable rep is up for grabs :biggrin:

Reply 1

olu

Wow!

Just stick to Isaac Newton's writings on gravity. Don't bust your brain

Reply 2

IWantSomeMushu

17

http://en.wikipedia.org/wiki/Gravity

Reply 3

Elementric

10

FatboyGinger

I've just read in Marcus Chowns book that gravity does not exist per se. It's actually inertia. And also, gravity is acceleration.
I don't quite understand that.

He gave the analogy of someone blasting off in a space rocket, accelerating up at 9.8 m/s/s and to them, if they dropped something, it would fall, exactly as it would with gravity.
Now, I can't see how this can work- I mean, the earth's not accelerating in all directions at once is it? Could someone explain this to me please?
My valuable rep is up for grabs :biggrin:

What? If you were accelerating upwards at 9.8m/s^2, and you dropped something, it would just experience a downward acceleration of 9.8m/s^2 and would be exactly the same as if you just threw it up in the air, I don't see your point. The object doesn't stay still...

Reply 4

Ben77mc

14

FatboyGinger

I've just read in Marcus Chowns book that gravity does not exist per se. It's actually inertia. And also, gravity is acceleration.
I don't quite understand that.

He gave the analogy of someone blasting off in a space rocket, accelerating up at 9.8 m/s/s and to them, if they dropped something, it would fall, exactly as it would with gravity.
Now, I can't see how this can work- I mean, the earth's not accelerating in all directions at once is it? Could someone explain this to me please?
My valuable rep is up for grabs :biggrin:

In space where there is effectively nothing around, there is negligible gravity. This means that if you accelerate at 9.8ms^-1 then it is like the earth's gravitational field.

So, if you were to let go of something, the floor of the elevator would accelerate up to it at 9.8ms^-1, meaning that to them it would seem like it had fallen as it does on earth.

Srry if this isn't what you wanted, but i dunno how to explain it any clearer.

Also, don't listen to the guy who said stick to Newtonian gravity.

EDIT: This might help http://www.astronomy.net/forums/blackholes/messages/3470.shtml

Reply 5

Harmful

When you drop the thing out of the rocket, it is no longer experiencing the upwards force provided by the rocket.
So, to a person insde the rocket, the ball would appear to fall away, since the rocket continues to move upwards.

Reply 6

clad in armour

16

OP just wants kudos for questioning gravity

Reply 7

Ben77mc

14

clad in armour

OP just wants kudos for questioning gravity

But it's a stupid question, so he won't get any kudos from me.

Reply 8

username279036

OP

Ben77mc

But it's a stupid question, so he won't get any kudos from me.

Thanks. It's not my problem if you don't understand it either.

Chown actually says that Einstein's theory of special gravity must mean that Newton is incorrect. The thing about large masses attracting other large masses = false.

To quote Chown
'We concoct the force of gravity to explain things such as Apples falling from trees... because we ignore the truth- that our surroundings are accelerating relative to us. Things move merely as a result of their inertia. The force of gravity does not exist!'

Can someone explain the above passage. Thanks for the help so far, but it still seems a bit... counter intuitive.

Reply 9

Ben77mc

14

FatboyGinger

Thanks. It's not my problem if you don't understand it either.

Chown actually says that Einstein's theory of special gravity must mean that Newton is incorrect. The thing about large masses attracting other large masses = false.

To quote Chown
'We concoct the force of gravity to explain things such as Apples falling from trees... because we ignore the truth- that our surroundings are accelerating relative to us. Things move merely as a result of their inertia. The force of gravity does not exist!'

Can someone explain the above passage. Thanks for the help so far, but it still seems a bit... counter intuitive.

Read something on GR and you will understand what he means.

Reply 10

clad in armour

16

Ben77mc

But it's a stupid question, so he won't get any kudos from me.

general relativity ftw

Reply 11

Ben77mc

14

clad in armour

general relativity ftw

very true

Reply 12

KSP

9

FatboyGinger

Thanks. It's not my problem if you don't understand it either.

Chown actually says that Einstein's theory of special gravity must mean that Newton is incorrect. The thing about large masses attracting other large masses = false.

To quote Chown
'We concoct the force of gravity to explain things such as Apples falling from trees... because we ignore the truth- that our surroundings are accelerating relative to us. Things move merely as a result of their inertia. The force of gravity does not exist!'

Can someone explain the above passage. Thanks for the help so far, but it still seems a bit... counter intuitive.

I have no idea what he means either, but i'm guessing it has to do with einstein's theory of general relativity since i've never heard of special gravity. But erm...you don't learn about general relativity until towards the end of a physics or maths degree so i guess we'll just have to accept it.

Reply 13

Emc2

FatboyGinger

I've just read in Marcus Chowns book that gravity does not exist per se. It's actually inertia. And also, gravity is acceleration.
I don't quite understand that.

He gave the analogy of someone blasting off in a space rocket, accelerating up at 9.8 m/s/s and to them, if they dropped something, it would fall, exactly as it would with gravity.
Now, I can't see how this can work- I mean, the earth's not accelerating in all directions at once is it? Could someone explain this to me please?
My valuable rep is up for grabs :biggrin:

Hmm, let's see if I can make this simple (it's not). In Physics there's this thing called a fictitious force (or pseudo force). Basically, it's a force that we use to explain a particular phenomenon, but that doesn't actually exist. Indeed, if we were to look at the same phenomenon from a different frame of reference, we would see that what causes it is something else entirely.

The classic example of a fictitious force is the centrifugal force. Imagine a car making a right turn: every object inside the car (including the driver) will be pushed towards the left hand side of the car. If we look at this from the point of view of the driver, we can explain it by way of the centrifugal force that results from the change in direction along a curved line. However, if we look at it from an external point of view, we see that the reason why this happens is that the objects inside the car naturally tend to follow the direction of the velocity (the red line in my pathetic attempt at drawing).

What this means is that the centrifugal force doesn't actually exist, and we can explain its effects by simply looking at the scene from a different frame of reference. One particular property of fictitious forces is that all objects are accelerated equally, regardless of their weight: whether you have a bottle of water or a large heavy box in your passenger seat, they will both experience exactly the same acceleration when you make a turn.

Now, what do we know about gravity? Well, Newton and Galilei (among others) found out that two objects in a vacuum fall to the ground in exactly the same time regardless of their weight, when released from the same height. Einstein realised that this meant that gravity, like centrifugal force, was also a fictitious force, because it had the same effect regardless of the mass of the object.

Einstein therefore concluded that gravity was a "wrong" way to explain certain phenomena, and that we should put ourselves in a different frame of reference if we were to explain them properly. He then postulated that gravitational attraction was in fact due to concavities in the space-time created by an object's mass (the greater the mass, the deeper the "hole" ), which like a bowling ball on a mattress, tends to attract everything that is close enough to it.

Hopefully this will make it a bit clearer for you. I should probably mention that my knowledge of general relativity is extremely limited, so I'm sure there are plenty of flaws that could be found in everything that I just wrote. I hope that any Physics student reading this will be kind with me :smile:

Reply 14

ashy

17

The centrifugal force does exist. It appears quite plainly in equations if you change your frame of reference. The idea that it doesn't exist is a myth propagated by physics teachers.

Also, I wouldn't argue that Newton was wrong, more that his approach is an approximation, one which works very well under sensible conditions. General Relativity makes corrections for where the Newtonian approximation breaks down.

Reply 15

Emc2

ashy

The centrifugal force does exist. It appears quite plainly in equations if you change your frame of reference. The idea that it doesn't exist is a myth propagated by physics teachers.

Also, I wouldn't argue that Newton was wrong, more that his approach is an approximation, one which works very well under sensible conditions. General Relativity makes corrections for where the Newtonian approximation breaks down.

I must confess I was tempted to include that very xkcd strip in my explanation above :biggrin:

And yes I know the centrifugal force does exist, but I was trying to make it as simple as I possibly could (the idea of a fictitious force isn't exactly trivial). As I said, I know my explanation was probably rubbish but I gave it my best :smile:

Reply 16

ashy

17

emc2

I must confess I was tempted to include that very xkcd strip in my explanation above And yes I know the centrifugal force does exist, but I was trying to make it as simple as I possibly could (the idea of a fictitious force isn't exactly trivial). As I said, I know my explanation was probably rubbish but I gave it my best