When I'm in a car and it moves I can't feel that I'm moving; I'm moving at the same speed as the car. I understand that this is true for all things, like being in an airplane or spaceship. However, when the car speeds up I am pushed backwards. Why does this happen if I'm going at the same speed as the car? Why do I feel this acceleration?

This is good old Newton's famous F = ma; Force = mass x acceleration. When you are accelerated or decelerated by something, that something has to exert a force on you to do so, which you feel as a push or a pull. Conversely, when a body has NO net force acting on it, it continues in a straight line at constant speed. This is why you feel nothing in such circumstances.

The car goes faster before you do. A force must be applied to you to keep you moving with the accelerating car. The force is what you feel.

When the car accelerates, you are not moving at the speed of the car. The seat exerts a force on you that accelerates you until your speed matches that of the car. That's why racing cars have deep bucket seats and harnesses to hold you in.

Just as a matter of interest we are now traveling at over 2 million miles an hour on Earth through the universe.

Fortunately we're not accelerating, so we don't feel it. Please Register or Log in to view the hidden image! We're also traveling at about 1,000 miles an hour (depending on latitude), along with the earth's surface as it rotates around its axis. Since this is a circular motion rather than straight-line, it does exert an extremely small acceleration force on us. But it's a centrifugal force that counteracts the effect of gravity, so all we feel is the net of the two. This centrifugal force is so tiny, in relation to gravity, that it has no practical effect anyway. Take a scale with you to the North Pole--even a very precise scale--and you won't see any difference between your weight there and your weight at the Equator. I'm not positive, but I think a difference in your distance from the earth's center might have more effect on your weight than a difference in latitude. On top of Mt. Everest versus Death Valley--can anybody do the math? I'd really love to, but I have to go to a meeting right now. Please Register or Log in to view the hidden image!

as along as you ignore the oblateness of the Earth, and assume a constant radius of 6378 km, then the centrifugal acceleration at the equator is ~ 33.9e-3 m/sec^2 Using the same base radius for sea level, the difference in acceleration due to gravity between Death Valley and Mt. Everest is ~27.5e-3 m/sec^2. So centrifugal acceleration wins here. If you however, work out gravity due to the difference between equatorial and polar radii, it works out to be a difference of ~65e-03 m/sec. S ohat difference due to the oblateness of the Earth work out ot be more that due to the centrifugal acceleration. Bt then again, that oblateness is due to the centrifugal acceleration.

hi. more fundamental question is regarding inertia and acceleration. why is one frame of reference accelerating when the other is not. there is not currently a satisfactory fundamental explanation. it must simply be measured. the answer to "Why do I feel this acceleration?" given by Feynman's father is "nobody knows" ( can't post link. Google "INERTIA: DOES EMPTY SPACE PUT UP THE RESISTANCE?" )

There is no resistance. The underlying reason why you "feel" acceleration is because it is locally equivalent to the presence of a gravitational field, which is in turn due to the geometry of space-time in your frame. This is aptly called the "equivalence principle".

Marcus, GR and space-time geometry do not explain the underlying mechanism of inertia. They don't even explain the underlying mechanism of gravitation. What they do is describe the geometry of the gravitational interaction of objects. Above you just state that acceleration is equivalent to gravity. You give no real mechanism for why or how, the two are equivalent. Again, GR and space-time geometry are descriptions of what is happening, not explainations of how or why it happens. MincedLunch, The question of inertia's underlying mechanism is still pretty much up for grabs. Though it has never been fully incorporated into GR, GR assumes that inertia is Machian in its origin. That is that inertia is the result of the gravitational influence of all of the objects in the universe, on individual components/objects. Everything in the universe pulls gravitationally on an object in a manner that is locally uniform.., or all local objects are pulled in every direction, by the rest of the objects in the universe... That balanced pull, only becomes noticeable when an object is under acceleration, at which time it's acceleration introduces an imbalance, which can then be felt and/or measured. Thought of in this way, inertia is not really a resistance to a change in motion as much as, when an object's motion changes, it feels the pull of gravity, from that portion of the universe, it is moving away from. But this remains a theoretical explanation. There is no proof that this is the mechanism. There has been some work in QM that suggests that inertia and perhaps even mass are emergent phenomena. Both essentially emerging from the interaction of an object's acceleration through the ZPF of empty space. In this approach the inertia of an object is the result of resistance, or an exchange of momentum between the object and the virtual particles of the ZPF. Here there is a resistance to the change in motion of the object. Instead of being pulled back by gravitation, its change in motion is being resisted. This assumes that virtual particles are real and that they have some intrinsic momentum of their own, which when they interact with an object results in their resisting the object's acceleration relative to the ZPF... This also remains a theoretical explanation, and though the description of inertia, within this context, is pretty straight forward, no transition from inertia to gravitation has yet been achieved, that includes a description of the geometry equivalent to GR. (The success of GR, as a geometrical description of gravity, sets the bar for all attempts to explain the underlying mechanism, of gravitation.) The problem is that there are differences in the way things like "empty space" are defined in GR and QM... And some of the assumptions that form the foundation of both are likely, less than wholly accurate descriptions of reality. Here is a link to the top hit from your reference above, INERTIA: DOES EMPTY SPACE PUT UP THE RESISTANCE?. The conclusion that no body knows is accurate, but I do believe we are getting closer.

This thread is not about the "mechanism of inertia", it is simply about why we can detect (proper) acceleration.

Huh? I'm curious about the derivation of this number. Is that a summation of earth rotation, travel, and galaxy motion through the universe? Or an objective factor of universal expansion on our position?

So I am no longer on your ignore list. Perhaps you could benefit from a little research into work on the subject, not found in a text book on general relativity... Like the dozens of papers by Puthoff, Hairsh, Rueda and others, which form the basis of much of the article published in Science in the link above. GR is not all there is in the world of science. Since this is a discussion forum, why not discuss as in, if you do not agree with something I post, post your specific objection, say it as you see it, instead of dropping the kind of unsupported personal opinion/comment above. Talk about the science if you know it.

because you have same speed as the car not same acceleration as the car.there fore when it accelerates you are pushed back.. i think it will explain thoroughly any questions?