According to Newton's first law, when standing.....

Action and Reaction are equal and opposite. (normally the Third Law)
Action...elevator...UP
Reaction....body parts...DOWN

Answer: Hands and Feet.

Fresco, he wants to know what part inertia plays in this. You didn't tell him that.

A body at rest has a tendency to stay at rest (the law of inertia). You could say that it "resists" acceleration.

F = MA, so M = F/A. So "mass" is just "resistance to acceleration." The more mass an object has, the more force it takes to move (accelerate) it.

Gravitational mass is, for unknown and seemingly strange reasons, equivalent to inertial mass. But that known fact led Einstein to conclude that acceleration and gravitation are basically the same thing. In most ways, they are indistinguishable.

When the elevator accelerates upwards, it has the same effect as would increasing the gravitational force that an object inside it is subjected to.

So now there is more "gravity" pulling the blood in your body down.

Put another way, when the motor which powers it starts accelerating the elevator, it causes the elevator to accelerate (but not your body--at least not directly). In effect, the floor of the elevator is moving toward the blood in your heart (or any other part of your body). Having inertial mass, your blood "resists" being accelerated.

It is always easier for your heart to pump blood to your feet than your head (if you are standing up). It has to "resist" gravity going to your head, while gravity "helps" the blood go to your feet.

The stronger the gravitational force you are subjected to, the more this is the case. So it will "feel" like blood is rushing to your feet if you accelerate upwards.

I read over all the responses here and everyone has it wrong. Because if you replace the person with a balloon filled with water the principle is the same. It has nothing to do with the heart or blood pressure. In fact it has nothing to do with gravity. Because you could put this same scenario in space on say a space station that is in orbit around the earth. This station has an elevator (for some odd reason) the situation would be identical to an elevator on Earth.

The reason is the object in the elevator wants to remain at rest. However; the elevator is moving (upwards) thus accelerating the object that is inside of it. But this object wants to remain at rest. So there is a force acting on the object in the opposite direction of the elevator's travel (downward). So the blood or water get's squished against the floor of the elevator due to this force.

This is why you would feel the sensation in your feet because all the blood is being squished towards the floor. If you had a balloon sitting there you would observe the balloon get shorter and fatter. Because the force is squeezing the balloon downward but it can only squeeze so far so the sides of the balloon get expanded outwards. If an object can not be squished any further it tends to expand along it's radial.

This is what happens, the blood in your body gets squished towards your feet which are all ready filled with blood so the blood in your feet start to expand along the radial as the blood above it pushes down.

The force I keep referring to is that of inertia. An object at rest wants to remain at rest. This is true even if you accelerate the object. The object takes on the momentum of what ever is accelerating it so if the momentum stops suddenly the object will want to continue moving along the tangent of that momentum. This is easy to experience if you have ever ridden on a bus while standing. When the driver suddenly pushes on the breaks the bus slows but your body wants to keep moving and it will if you don't apply a force in opposition to this momentum.

Krumple, you're saying its due to acceleration. If that is equivalent to gravity, as General Relativity presupposes, then it has to do with "gravity" (which is the same thing as acceleration).

Granted, you don't need GR to explain Newtonian mechanics. I was just pointing out the analogy (equivalence) of acceleration and gravity. The "fictitious force" caused by upward acceleration is the equivalent of a "downward" force of gravity.

In Newtonian mechanics, inertia is not considered to be a "force," as far as I know. You might call it a "counter-force," I suppose, but it is essentially mass.



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

This "apparent" force has been called an "inertial force" by some, but again, in Newtonian mechanics it is not a "true" force, but rather a "fictitious" one.

Krumple, in principle I think your explanation is correct, and it may well be the one most comprehensible to the OP. I do, however, think statements like these are somewhat inaccurate, technically speaking:



The reason it's called a "pseudo-force" is because there is no force "pushing down on" and "squishing" the balloon. The phenomenon is an artifact of motion (acceleration) rather than the application of a typical physical "force," such the head of a hammer hitting a nail.

Mash down on the gas pedal of your hot rod Lincoln and you will feel as though you are being pushed backwards (but there's nothing pushing you). Hit a tree in that hot rod while going 100 mph, and your head will go through the windshield. In neither case is there any tangible physical force which is pushing/pulling you.

But you're right that it is the existence of the inertial mass that creates this effect. It would not occur without inertia (which mass is said to measure). Even though we have a label for it, it seems no satisfactory answer to the question of the origin of inertia has been found.