Moving in a lift and gravity. Yes the classic question but I can't understand

[scilearner]

Hello everyone,

 

Ok I thought I knew gravity and just realized I absolutely have a very poor understanding about it.

 

Ok let's say a person was moving in a lift

 

1. The blood would pool around the legs because of gravity, but my question is aren't we overcoming the gravity force to go up, why would it affect us? Ok the explanation is that upward force from the lift is not acting on the body. But if we are moving up how come it doesn't affect us.

2. Ok what about lift moving down, why don't we feel any weight? Ok the explanation is we are moving with gravity, but if the lift is not accelerating at 9.8 ms2 we should feel the gravity right.

3. About basic free fall without a lift?

Ok now we are accelerating because of gravity, but why don't we feel the force. Aren't we accelerated down because of that force, why don't we feel it. I basically don't understand the difference between force and acceleration and how one can exist without other

4. Why do we feel weight because of normal reaction force?

It is the force of body on the earth. So how come we need a normal reaction force to feel the gravity. I can understand how staying still would make you feel gravity but what does normal reaction force have to do with it.

5. What is gravity?

How is there an acceleration without a force. I know this is a big question but just give me a simple explanation, I have a long way to go before I want to really know the answer for this.

 

Thank you so much for anyone who is going to take their time and help me. It would certainly do my physics a world of good. Thanks

[swaha]

1. The blood would pool around the legs because of gravity, but my question is aren't we overcoming the gravity force to go up, why would it affect us? Ok the explanation is that upward force from the lift is not acting on the body. But if we are moving up how come it doesn't affect us.

 

which force doesnt affect us i didnt get.

 

2. Ok what about lift moving down, why don't we feel any weight? Ok the explanation is we are moving with gravity, but if the lift is not accelerating at 9.8 ms2 we should feel the gravity right.

 

who said we dont feel weight we feel lighter if the lift is accelerating down. that doesnt mean we dont feel weight.

 

3. About basic free fall without a lift?

Ok now we are accelerating because of gravity, but why don't we feel the force. Aren't we accelerated down because of that force, why don't we feel it. I basically don't understand the difference between force and acceleration and how one can exist without other

 

what do we feel is weight which is reaction due to the force of gravity. to give reaction we need something else than the force. force & acceleration dont exist one without the other. the difference is force = mass.acceleration

 

4. Why do we feel weight because of normal reaction force?

It is the force of body on the earth. So how come we need a normal reaction force to feel the gravity. I can understand how staying still would make you feel gravity but what does normal reaction force have to do with it.

 

we dont feel gravity we feel weight. gravity is only an acceleration produced. u see the light is required to see the object but it remains present there, y?

 

5. What is gravity?

How is there an acceleration without a force. I know this is a big question but just give me a simple explanation, I have a long way to go before I want to really know the answer for this.

 

there is a force which is equal to your weight, otherwise y would u fall?

[swansont]

It might help to draw a free-body diagram and identify all of the forces acting on you (or the object in question)

[Sisyphus]

You don't feel acceleration, you feel stresses within your body. You don't feel anything in freefall, despite your acceleration, because gravity is acting on every part of you at once in the same way. Standing on the ground, gravity is still acting on every part of you, but there is force being applied to the bottom of your feet, which leads to a compressive stress through your body. (Gravity pulls each atom downwards, but each atom is prevented from accelerating by the obstruction of the atom below it. Etc.)

[scilearner]

I have worked out the elevator problem but they say as you move up the elevator blood pooling increases in the legs.

 

For example when I stand for long time there is pooling of blood in the legs.

 

If gravity is balanced by the normal reaction force how can it have any effect in our body? Isn't gravity cancelled by NRF?

[Cap'n Refsmmat]

Yes. So when the elevator is going up at a constant speed, nothing changes in your body. But when it's accelerating, the reaction force is larger than your weight. (You can't accelerate unless there's a net force on you.)

[swansont]

I have worked out the elevator problem but they say as you move up the elevator blood pooling increases in the legs.

 

For example when I stand for long time there is pooling of blood in the legs.

 

Blood pooling has some added issues, even when standing still. You have to pump the blood back up to the heart, which represents an increase in potential energy. There is a pressure from the blood above it, i.e. you have to push not only the volume near your feet, but all the blood above it, too.

[scilearner]

If we are moving up in a lift,

 

Normal reaction force is greater than gravity, meaning there is a net force up, but still blood doesn't go up, why is that? Also if you overcome force of gravity and go up with some acceleration is that free fall in the reverse direction?

Edited February 18, 2010 by scilearner

[swansont]

If we are moving up in a lift,

 

Normal reaction force is greater than gravity, meaning there is a net force up, but still blood doesn't go up, why is that?

 

Actually the blood does go up, just not relative to the veins.

[Mr Skeptic]

I think that it will benefit you to forget gravity for a while. Suppose you are in an elevator in outer space, far from any gravity. The elevator is accelerating in a direction we will call up. You are accelerating up as well, since you are in the elevator. But, you are not accelerating evenly: the elevator floor pushes your feet which pushes the rest of you. This will feel like gravity, but in this case rather than you standing still and gravity pulling your whole body down, your feet are accelerating and your body's inertia is what produces the effect. Gravity has nothing to do with any of this, but if you also have gravity you just add up the effects since they are indistinguishable to you.

[Sisyphus]

The blood is going up. If it wasn't, it would be pooling at the bottom of the elevator shaft while your exsanguinated corpse continued inside the rising elevator.

 

You can't look at your whole body as one entity. Look at one blood cell. It is rising. Why? Because the blood cell below it is pushing on it? Why? Because the one below that is pushing on that? Etc., down to the floor of the elevator. Yet each one also has inertia to be overcome, so there is going to be drag as each part pushes the next. This is true in any object, but it's going to happen more in your fluid blood than in your relatively rigid bones, muscles, etc.

[scilearner]

The blood is going up. If it wasn't, it would be pooling at the bottom of the elevator shaft while your exsanguinated corpse continued inside the rising elevator.

 

You can't look at your whole body as one entity. Look at one blood cell. It is rising. Why? Because the blood cell below it is pushing on it? Why? Because the one below that is pushing on that? Etc., down to the floor of the elevator. Yet each one also has inertia to be overcome, so there is going to be drag as each part pushes the next. This is true in any object, but it's going to happen more in your fluid blood than in your relatively rigid bones, muscles, etc.

 

lol Ok thanks both of you for your replies. Now what I'm thinking is that gravity is what makes your blood pool. I don't understand the inertia you are referring to. Is inertia caused by gravity pulling down blood, and you suddenly accelerate up, but blood still wants to go down. Wouldn't this last for only few minutes though!!

[Sisyphus]

How about this. Imagine two asteroids freely floating in deep space, connected by a spring. Push on one of them. It doesn't matter with what. Call it a rocket named the U.S.S. Elevator Floor. The rocket pushes on one asteroid, which then pushes on the other. As long as the rocket is accelerating the one, the spring is compressed. The forward asteroid has intertia that is resisting the accelerating from the rear asteroid pushing it.

 

Now imagine there is a rocket attached to each asteroid, such that they can be accelerated equally. As this happens, there is no stress on the spring, right? This is the same as how gravity works.

[scilearner]

Oh I see. What I was thinking is that elevator provided a force like gravity uniform in all parts to the person in it. I understand now. Thank you guys!! Thanks for everyone who helped.

 

"The blood is going up. If it wasn't, it would be pooling at the bottom of the elevator shaft while your exsanguinated corpse continued inside the rising elevator."

 

Wouldn't the create a great avater? lol

[swansont]

Oh I see. What I was thinking is that elevator provided a force like gravity uniform in all parts to the person in it.

 

That physics likes to treat extended objects as points or monoliths sometimes backfires.

[Mr Skeptic]

That physics likes to treat extended objects as points or monoliths sometimes backfires.

 

http://everything2.com/title/spherical+horse

A gangster assembled an engineer, a chemist, and a physicist, and ordered them to find a way to guarantee that his horse would win in the next week's races. The day before the race, they reconvened.

 

Gangster: Engineer, what have you got?

Engineer: I've invented a way to weave metallic threads into the saddle blanket so that they will act as the plates of a battery and provide electrical shock to the horse.

Gangster: That's very good! But let's hear from the chemist.

Chemist: I've synthesized a powerful stimulant that disolves into simple blood sugars after ten minutes and therefore cannot be detected in post-race tests.

Gangster: Excellent, excellent! Physicist?

Physicist: Well, let us consider a spherical horse in simple harmonic motion...

[swansont]

You can use the point-cow approximation, too.

http://blogs.scienceforums.net/swansont/archives/5