# The Naked Scientists Forum

### Author Topic: Beginners question - The Equivalence Principle and Relativity?!?  (Read 1054 times)

#### Alinta

• Jr. Member
• Posts: 14
##### Beginners question - The Equivalence Principle and Relativity?!?
« on: 05/03/2013 05:56:22 »
The equivalence principle states that you cannot distinguish pure linear acceleration from gravity. An example of this in a video that I just watched is being inside a small elevator going downwards.

In theories of relativity (sorry, too much of a beginner to know if it is general or special) it is stated that you cannot tell you are moving if you are going at a constant velocity. But if you are travelling along the highway and apply more acceleration, you will be pushed back in your seat a little as you feel a force pushing you backwards (not quite sure what exact force that is... gravity?).

So: moving at a constant velocity = no feeling of movement.
Moving at a constant acceleration = feels like gravity.

But if gravity on earth makes all things fall at the same rate once they reach 9.8m/s/s, how is it possible for an acceleration to feel like gravity? Wouldn't they reach a constant velocity and thus cancel out a feeling of acceleration?

So when my teacher says it 'feels like gravity' I'm a bit unsure what this means. Right now, given the above, I am completely unsure as to what gravity could possibly 'feel' like at all...

(Sorry if I have written some of the details incorrectly)

Cheers
Alinta
« Last Edit: 05/03/2013 06:00:46 by Alinta »

#### JP

• Neilep Level Member
• Posts: 3366
• Thanked: 2 times
##### Re: Beginners question - The Equivalence Principle and Relativity?!?
« Reply #1 on: 05/03/2013 14:57:13 »
But if gravity on earth makes all things fall at the same rate once they reach 9.8m/s/s, how is it possible for an acceleration to feel like gravity? Wouldn't they reach a constant velocity and thus cancel out a feeling of acceleration?

I think you might be confusing acceleration and velocity.  9.8 m/s/s is a constant acceleration, not a velocity.  If you were sitting in a rocket accelerating at constant rate of 9.8 m/s/s (meaning your velocity is increasing by 9.8 m/s every second), you feel yourself pressed to the floor of the ship with the same force as if it were sitting on the earth's surface.

#### Alinta

• Jr. Member
• Posts: 14
##### Re: Beginners question - The Equivalence Principle and Relativity?!?
« Reply #2 on: 06/03/2013 01:58:30 »
Doh! Thanks JP! I was driving myself mad there thinking about it!

#### Pmb

• Neilep Level Member
• Posts: 1838
• Physicist
##### Re: Beginners question - The Equivalence Principle and Relativity?!?
« Reply #3 on: 06/03/2013 02:04:12 »
Doh! Thanks JP! I was driving myself mad there thinking about it!
To be precise, a uniform gravitational field is equivalent to a uniformly accelerating frame of reference. However if the field is not uniform, i.e. there are tidal gradients present in the gravitational fied, then this principle only holds locally, i.e. given the precision of your instruments and there is a small enough region and short enough time interval so that you can't tell tell the diffence between gravity and acceleration. A lot of people miss this part of the equivalence principle.

I recommend that you read my article on this subject. Show it to your teacher too.
http://xxx.lanl.gov/abs/physics/0204044

#### The Naked Scientists Forum

##### Re: Beginners question - The Equivalence Principle and Relativity?!?
« Reply #3 on: 06/03/2013 02:04:12 »