According to Relativity, gravity and acceleration are the same thing.
When I stomp on the gas in my car, I feel the G forces pin me against the seat…I see myself moving forward.
When I lie on my back on the ground, I feel one G on my back…but where am I moving forward to?
Furthermore, doesn’t acceleration imply a constant increase of speed? Since I cannot attain the speed of light, wouldn’t gravity accelerate me forward into “Whatever” until just below the speed of light, then just stop?
Gravity is a force, acceleration is the result of a net force. The sum of all the forces on a body gives you the net force acting on that body. If all the forces acting on a body cancel each other out, that body still feels these forces, but the net force on the body is cero thus there is no acceleration.
This is what happens when you lie down on the ground, gravity is cancelled out by the force that the ground applies on you, thus you don´t accelerate but you still feel the G force.
In reference to your other question, acceleration is not a CONSTANT increase of speed, far away fron the earth gravity is very weak and it also decreases as you near the center of the planet becoming cero right in the center.
You can never reach the speed of light but neither will you just stop. The faster you go the greater mass you will possess so the harder you will be to accelerate. If it is an object somewhere out in front of you accelerating you via gravity you will eventually out mass the object and attract it to you more than you to it. In the end you might get arbitrarily close to the speed of light but you will never reach it.
Your body applies a force downward to the ground. The ground applies an equal force upwards to your body. The two forces cancel out, which is why you don’t move up or down.
Gravity will accelerate you until you reach terminal velocity. At this point your acceleration would become zero.
I’m no expert, but that doesn’t sound right to me, Nanu. Gravity becomes weaker, the further an object is from the centre of the Earth. Doesn’t it follow that the near to the centre of the Earth, the stronger the gravitational pull?
By the way, here’s an interesting link explaning more about gravity and acceleration:
Let’s say that you’re in free fall wrt Earth. Even though you’re accelerating wrt to Earth you’re actually following a geodesic and you’re in an unaccelerated frame of reference wrt to space-time.
When you’re lying on the ground you’re in an accelerated frame because you’re no longer in free fall, and you’re no longer following a space-time geodesic. In other words you’re not going anywhere with respect to earth but you are accelerating wrt to space-time.
Sorry for all the wrts but I got get to bed.
Gravity gets weaker the further you are from the Earth, not the center of the Earth. At the center of the Earth, you feel no gravity from the Earth itself. This is because the stuff that makes up the Earth is all around you, and the gravity of that stuff cancels out, save for non-uniformity in the Earth’s shape and density.
You can make the approximation that all gravitational forces are generated by a point mass at the center of a spherical body only if you are outside that body. It makes the math alot easier.
I have a question thats maybe abit of a hi-jack but its still along the lines of the original question…so…
I’ve read that gravity on earth attracts any mass towards it at more or less 10m/s2 if ignoring frictional forces…if thats right, what would happen if i put “two planet earths” close together…what would be their acceleration towards each other?
10 m/s2 is, IIRC, a rule of thumb based on the universal gravitation law applied to objects near the surface of the earth.
The universal gravitation law says:
F = (G * m1 * m2) / (r * r)
where G is a universal gravitation constant; m1 and m2 are the masses of the two objects in question, and r is the distance between the centers of the two objects.
In the case of objects falling near the surface of the Earth,
F = G * m(e) * m(o) / (r(e) * r(e))
where m(e) is the mass of the Earth, m(o) is the mass of the object, and r(e) is the radius of the Earth.
Thus, acceleration = F / m(o) = G * m(e) / r(e) * r(e) = approximately 10 m / s2
If you have two earths near eachother,
F = G * m(e) * m(e) / (2r(e) * 2r(e))
so acceleration = F / m(e) = G * m(e) / 4(r(e) * r(e))
= approximately 2 1/2 meters per second squared, the rate at which each earth is accelerating towards their mutual center of gravity.
It seems to me that the “mutual acceleration” - essentially the change in the rate of the closing of the gap between the two earths - would be double that number - approximately 5 meters per second squared.
(Physicists, feel free to correct me!!)
I am assuming you’re talking about GR, since there’s no gravity in SR.
First thing you have to realise that relativity doesn’t work with accelerating objects and frames. That’s how you break the Twin Paradox.
Second thing you have to realise is that there is no gravity in GR. Or rather, gravity is not a force but a curving of space-time around masses.
Thirdly, when you stomp on the gas in your car, you don’t move forward, you move backwards. This is due to inertia, not acceleration.
Forthly, acceleration does not denote a constant increase in speed. In fact, acceleration is the derivative of velocity (a vector measurement) against time.
Very Good Answers above, except that all things described above are for conservative force fields i.e. the initial and final points of a particle decide the energy difference and is path independent.
Now reality is different from this.
There was a famous physicist who in Newtonian times played a dirty trick. I don’t remember the name of the physicist - some fellow doper will enlighten us for sure :). But he refused to pay the porter who carried his baggage for him at the train station. When the porter asked for the money, he sued him with a lawsuit where he said physics says that since there was no change in the potential energy of the luggage the porter has done no work :d and hence should’nt be paid 
Well, i think the physicist lost the case (nothing new there).
So altough while u’r body is not doing any work when it is acting against gravity, the muscles are stretched and since they are not totally elastic, you lose energy and also muscles generate somekinda acid (formic or oxalic, donno which) which gives the feeling of tiredness.
Urban Ranger,
Ok…you’ve lost me.
Need help understanding :
2)Second thing you have to realise is that there is no gravity in GR.
3)Thirdly, when you stomp on the gas in your car, you don’t move forward, you move backwards. This is due to inertia, not acceleration.
thx for any help
Relativity? Did you mean to say, special relativity? OTOH, special relativity works fine with acceleration. GR just makes it equivalent to gravity.
Nothing to do with football:
http://mentock.home.mindspring.com/twins.htm
http://mentock.home.mindspring.com/twin2.htm
http://mentock.home.mindspring.com/twinrdux.htm