Is gravity just an illusion in the universe?

[Naveen3456]

Earth is revolving around the Sun at a great speed. Our Galaxy is also moving away from other galaxies at a great speed and so on and so forth there are many kinds of motions that we are subjected to ( Earth's rotation, galaxy's rotation etc.). Plz tell

1. Are all these motions uniform (steady) that we feel no force? A physics professor once told me (though unofficially) that there is no 'steady' motion in this universe.

2. Even If all this motion is 'uniform', what is the reason that we don't feel any kind of force and live as we are living on a stationary Earth ( i.e. we are not thrown off at tangents when the Earth encounters a curve in it's elliptical path etc.)?

Thanks.

 

[russ_watters]

Welcome to PF!

1. I wouldn't call the motions "steady". For example, the Earth speeds up and slows down in its orbit and its distance from the sun changes.

2. The reason we don't "feel" the force is that all of the motion that the Earth experiences, we experience it exactly the same way. The same force (the sun's gravity) that makes the Earth accelerate toward it makes us accelerate toward it at exactly the same rate, so the result is we and the Earth move together and we "feel" no changes.

 

[Naveen3456]

The same force (the sun's gravity) that makes the Earth accelerate toward it makes us accelerate toward it at exactly the same rate, so the result is we and the Earth move together and we "feel" no changes.

How come every point ( or person) on Earth is accelerated at the same rate? A person at the equator must be accelerated differently as compared to a person on the poles?

 

[Drakkith]

How come every point ( or person) on Earth is accelerated at the same rate? A person at the equator must be accelerated differently as compared to a person on the poles?

Yes and no. If you are on the equator and the Sun is overhead, you are nearer to the Sun than someone who's at the poles. In this case you would be attracted just a bit more towards the Sun than the person at the poles. The situation would be reversed when you are on the night side of the Earth, directly opposite of the Sun. And twice a day, right as the Sun is coming up or going down you are the same distance from it as the person is at the poles, and you would experience the same force.

 

[Naveen3456]

Welcome to PF!

1. I wouldn't call the motions "steady". For example, the Earth speeds up and slows down in its orbit and its distance from the sun changes.

If a space shuttle makes a sharp curve in space (considerably away from matter, gravitation), would the occupants feel force?

 

[Bandersnatch]

The funny thing about being in a state of free fall in a gravitational field(i.e.,every orbit), is that you don't feel any forces acting on you. It's like being in a falling elevator.
All you feel is when something tries to stop your free fall. Like the push of the ground beneath your feet.

If you're in a shuttle that's making a sharp turn(accelerates), then you feel the push of the shuttle's floor/seat on you due to the way the force supplied by the engine is transferred to the ships' occupants and components.
If the force were permeating the space and acting on everything equally(i.e., same acceleration), then you'd feel nothing.


As for the forces due to Earth's rotation, these are too small to detect by human senses. You can do some experiments to see their effect, like the Foucault's pendulum.

Forces due to tidal influence of the Moon and the Sun are likewise too small to notice - unless you're living by the sea and can experience tides.

 

[D H]

How come every point ( or person) on Earth is accelerated at the same rate?

They aren't. The difference between the acceleration of the Earth as a whole versus that of a point on the surface of the Earth is the driving force behind the tides.

 

[russ_watters]

If a space shuttle makes a sharp curve in space (considerably away from matter, gravitation), would the occupants feel force?

Yes.

 

[Naveen3456]

They aren't. The difference between the acceleration of the Earth as a whole versus that of a point on the surface of the Earth is the driving force behind the tides.

If water feels the force as in tides, why don,t we?

 

[Naveen3456]

Yes.

1. when Earth makes a curve along its elliptical path, why don't we feel force. For a small part of this curve we can say that the speed of the Earth increases ( i.e. it accelerates) along a straight line, so why no force is felt by us?

 

[Bandersnatch]

If water feels the force as in tides, why don,t we?

Because it's so minuscle. The acceleration difference is in the order of one part in ten million of a g(~10^-7g). The only reason it's noticeable during tides is because the oceans are huge bodies of (mostly)free-flowing particles, so the effect gets added up over the whole Earth's surface, not to mention that the presence of landmasses tend to exaggerate the effect in places.

1. when Earth makes a curve along its elliptical path, why don't we feel force. For a small part of this curve we can say that the speed of the Earth increases ( i.e. it accelerates) along a straight line, so why no force is felt by us?

Because the gravitational field permeates space almost uniformly* on scales much lower than the distance from the mass producing it, so every part of you gets accelerated simultainously. This makes experiencing acceleration impossible.
Look up the othilitic organ of the human inner ear to see how the perception of acceleration works, and ask yourself if it could register anything if all of its elements were acclerated at the same rate
Here's a link:
http://en.wikipedia.org/wiki/Vestibular_system#Otolithic_organ

*the divergence from uniformity is due to the fact that the farther you are from the central body the weaker the force(as in Newton's law of gravity). This is exactly what causes the tides, and as it's been mentioned before these are too small forces to notice by humans.

 

[Naveen3456]

Because the gravitational field permeates space almost uniformly* on scales much lower than the distance from the mass producing it, so every part of you gets accelerated simultainously. This makes experiencing acceleration impossible.

I don't get it, Earth's gravitational field also permeates space almost uniformly, yet when a plane negotiates a curve, we do feel acceleration.

 

[Lsos]

Earth's gravitational field pulls uniformly on each and every atom of your body causing them to accelerate equally. If they all accelerate equally, then there will be no forces exchanged between them and you won't feel anything. Think of being stuck in a traffic jam, and suddenly ALL the cars started accelerating equally at the same rate. No car would feel, or even touch any other car.

A plane is not like gravity. First the ailerons move which causes air to push on them, they in turn push on the wing, which in turn pushes on the plane, which in turn pushes on your seat, which pushes on the skin of your butt, which causes a cascade of pushes on each and every one of your molecules, from one to the next. You WILL feel that. Think of the same traffic jam as above, but instead of each car moving on its own, someone attempts to move them all by pushing the last car with a bulldozer. If the dozer is strong enough then the traffic jam will move, but the cars will definitely touch and feel each other.

 

[xAxis]

Naveen3456, I suggest you read "the happiest thought" of Albert Einsteins life.
In general relativity, gravity is not even considered to be force. Then the answer to your question is simple, we don't feel it because no forse actually acts on our bodies.