1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

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

  1. Feb 18, 2010 #1
    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 :smile:
     
  2. jcsd
  3. Feb 18, 2010 #2

    Doc Al

    User Avatar

    Staff: Mentor

    If the elevator is accelerating upwards, then it's pushing on you with a force greater than your weight. You feel heavier. Sure it affects you.
    What you feel depends on the support force from the elevator, not gravity directly. You don't feel gravity directly; what you feel is the support force and the resulting tensions and stresses that it creates in your body. Gravity alone acts uniformly on every bit of you, so if gravity were the only force acting on you there would be no internal stress created and you'd feel 'weightless'.

    So if the elevator is accelerating downward with acceleration a, the force that the elevator must exert on you will be less than your normal weight, thus you'd feel a bit lighter:
    Upward Force on you = your weight - ma
    See above.
    What makes you think that one exists without the other? Note that it's the net force on an object that determines its acceleration.
    See above. The normal force creates stress in your body--that's what you feel. Remove that stress--as in freefall--and you feel 'weightless'. A strange feeling, since we are used to feeling those internal stresses.
    Please give an example where you think there's an acceleration without a force.
     
  4. Feb 18, 2010 #3
    Hey thanks Doc Al. In regard to first question, if the elevator is pushing you with a force greater than gravity why is there blood pooling in the legs?

    Also normal reaction force is body on the earth. How does it create internal stresses on you body?

    Wait does normal reaction force act on the body. How come it doesn't cancel the gravity force, does it only affect the area it is in touch with meaning legs, or can it give a force to move blood up?

    Also I'm certainly not the man to talk about relativity. But if you are moving at the same rate as acceleration of force is it like nothing is happening. In free fall blood is not pooling in the legs. That means gravity is not affecting the body? Why is that?

    Thanks a lot for the help so far.
     
    Last edited: Feb 18, 2010
  5. Feb 18, 2010 #4

    Doc Al

    User Avatar

    Staff: Mentor

    For the same reason that all the papers on the dashboard of your car fly backwards when you step on the gas, thus producing a forward force. (Or stuff in the back seat moves forward when you slam on the brakes.)
    The normal force on your body is a force exerted by the earth on you. Any force exerted on your surface will create internal stresses. Imagine someone shoving you in the face--your face scrunches up, doesn't it?

    All of this can be thought of as illustrations of the inertia of mass.
     
  6. Feb 18, 2010 #5
    Hey thanks again Doc Al. I think you posted a reply to my non edited post. That is ok. Still in regard to first question.

    Doesn't inertia stop after a while. But when you go up against gravity they say blood is pooling down in legs. Can't the force exerted by the elevator make the blood go up.

    Does normal reaction force affect only the area that is contact, is it a non uniform force.
     
  7. Feb 18, 2010 #6

    Doc Al

    User Avatar

    Staff: Mentor

    If you are not accelerating, then the normal force does equal the gravity force. Which means they add up to zero net force on you. But the normal force still exists and exerts a force on you.

    The forces that move blood up (within your body) are due to your muscles and blood vessels. They are used to supporting blood that has no net acceleration. But if you are in an elevator accelerating upwards, then more force is required to move the blood up--the vessels can't do that well and thus the blood pools lower in the body.

    When you say that gravity is not affecting your body, what do you mean? It is accelerating you, that's one thing it's doing. Internally, blood is like any other body part. When you are standing on the ground, your neck must exert a force to hold up your head. (Note that this is an example of an internal force--one of those stresses mentioned before.) When you are in free fall, no additional force is needed to support your head--so that stress is relieved.

    Make some sense?
     
  8. Feb 18, 2010 #7

    Doc Al

    User Avatar

    Staff: Mentor

    The elevator moves your whole body up, including your blood, of course. What you mean is move it up with respect to your body, which the blood vessels aren't able to do as well when you are being accelerated up.

    Imagine yourself pushing a box on a frictionless floor. You only exert a force on the surface, but that force creates internal forces in the box that ends up pushing the entire box. And those internal forces are not uniform and they end up squishing the box a bit.
     
  9. Feb 18, 2010 #8
    Thanks DocAl for your answers. But I still can't understand very well. When the net force is zero how do we feel any force acting on us. I'm rereading your answers constantly. Thanks a lot for taking your time to help me so far. I know I have some major misunderstanding.
     
  10. Feb 18, 2010 #9

    Doc Al

    User Avatar

    Staff: Mentor

    Imagine I place your head in a vise and tighten it. (Extreme example, I know!) You will agree that there's no net force on your head, right? (After all, you're not accelerating.) But you'll also agree that you'll feel forces acting on your head, right?

    Just because the net force is zero does not mean that the individual forces disappear. All it means is that they add to zero. They still act to produce pressure and stress.
     
  11. Feb 18, 2010 #10
    DocAl thanks again.

    Starting from normal reaction force? How is it created? If it is weight force on surface, why does weight force affect the body, I mean only force now affecting the body is normal reaction force? Does weight force affect you or the surface to create normal reaction force?

    If net force is zero why is blood accelarting downward when you are moving against gravity? Doesn't net force mean no acceleration
     
    Last edited: Feb 18, 2010
  12. Feb 18, 2010 #11

    Doc Al

    User Avatar

    Staff: Mentor

    Without gravity acting, there would be no normal reaction force of course. But gravity alone would not create any stress in your body.
     
  13. Feb 18, 2010 #12
    This is my misunderstanding.

    Ok a person is standing. There is weight force acting on the body and normal reaction force. Now netforce is zero but blood pools in the legs. I thought netforce means blood is not accelerating downwards?

    I hope it is not a big ask. But could you just name the force acting upwards and downwards in a lift when it accelerating up and accelerating down.
     
    Last edited: Feb 18, 2010
  14. Feb 18, 2010 #13

    jtbell

    User Avatar

    Staff: Mentor

    While the blood is in the process of redistributing itself, the net force on it is not zero. As it pools downward, the upward force on it, exerted by the walls of the blood vessels, increases until it reaches a suitable value.

    If the person is simply standing (e.g. in a motionless elevator), that "suitable value" is equal in magnitude to the gravitational force on the blood.
     
  15. Feb 18, 2010 #14
    elevator.gif

    In this diagram I don't understand picture 2. Why is it 2 mg? Isn't gravity acting downwards?
     

    Attached Files:

  16. Feb 18, 2010 #15

    Doc Al

    User Avatar

    Staff: Mentor

    Your blood is not accelerating downwards, I hope! (If it was, it would be leaving your body somehow.)

    Since blood has weight, your vessels must exert a force to support the blood. Unless you are pumping your legs, the blood will have a tendency to pool towards the bottom. (For details of how your blood system works, we'll need to consult a biologist.)

    Only two external forces act on you: Gravity (down) and the normal force (up).
     
  17. Feb 18, 2010 #16

    Doc Al

    User Avatar

    Staff: Mentor

    What they are calling 'W' in those diagrams is the 'apparent' weight, which we have called the normal force. Yes, gravity acts downward. In picture 2 the elevator is accelerating upwards with an acceleration equal to 9.8 m/s^2 upwards. So the normal force is twice the usual value (when in a non-accelerating elevator.)
     
  18. Feb 18, 2010 #17
    If elevator is accelerating up why does normal reaction force increase? Doesn't these diagrams have 3 forces. This is my misunderstanding. In the 2nd diagram I'm thinking this.

    Upward force on the person= ma + Nr- W(from gravity)

    Tell me why this is wrong? Thanks!!

    Why does it show weight force acting down if it is normal reaction force? which should act up?
     
  19. Feb 18, 2010 #18

    Doc Al

    User Avatar

    Staff: Mentor

    Those diagrams are viewing things from the non-inertial, accelerating frame of the elevator. So they show inertial forces as well as 'real' forces. Unless your class is studying non-inertial frames of reference, I strongly suggest that you stick to an inertial frame of reference.

    From a non-inertial frame of reference (as shown in those diagrams) for diagram 2, the forces are:
    (1) gravity (mg) acting down
    (2) inertial force (equal to -ma = mg) acting down
    (3) normal force acting up

    These add up to zero, so: -mg -mg + N = 0, thus N = 2mg.

    Viewed from the usual inertial frame of reference, the forces are:
    (1) gravity (mg) acting down
    (2) normal force acting up

    These forces add up to ma = mg upward, so: -mg + N = mg, thus N = 2mg.
     
  20. Feb 18, 2010 #19
    Hey thanks a lot for coming back online and helping me DocAl. After rereading your posts a lot I think I understand the elevator question. Tell me if this is right.

    The reason why I get confused is I'm thinking where is gravity coming into all this. But gravity is not affecting the elevator right, it is electrical, then it makes sense, when you only consider the weight. Thanks for your equation they really helped.

    I just have one question now. Why is that when your weight is zero the blood doesn't pool in the legs, I mean if gravity is still acting why can't it pool blood in the legs. I'm not interested in the biology area of this I just like to think of blood like object with a fluid in this case. I know I'm asking the same question again but I understood the elevator question. Thanks!!
     
  21. Feb 18, 2010 #20

    Doc Al

    User Avatar

    Staff: Mentor

    I assume you mean when your apparent weight is zero--when you are in freefall. In that case, both your legs and your blood and every other part of you are falling together. They exert no forces on each other.

    Here's an example: Imagine a cup of water. When at rest, the cup must exert an upward force to hold up the water. But if you drop the cup of water so it is in free fall, the cup and water exert no forces on each other. The cup no longer has to hold up the weight of the water, since both cup and water are falling together.
     
  22. Feb 18, 2010 #21
    Ok thanks again DocAl, but when you are moving up in a lift, your normal reaction force is greater than gravity, that means there is a net force up acting on the body but blood doesn't go up, it still pools in your legs? Also if you overcome force of gravity and go up with some acceleration is that free fall in the reverse direction?
     
  23. Feb 18, 2010 #22

    russ_watters

    User Avatar

    Staff: Mentor

    But your blood DOES go up. It's not like you go up to the second floor but leave your blood on the first!

    The problem still is that you are mixing and matching reference frames. Your first "up" is your body wrt the groind and your second "up" is your blood wrt your body. Don't do that: Stay consistent!
     
  24. Feb 18, 2010 #23
    Oh I think I understand now. The normal reaction force is different to gravity right it doesn't accelerate everything at the same time.So there is inertia to overcome.

    If you do have normal reaction force like gravity if would be free fall in the other direction right?

    Did I get it right? Did I finally get out of my misery?
     
  25. Feb 18, 2010 #24

    russ_watters

    User Avatar

    Staff: Mentor

    Stay with the ground as the reference: The elevator accelerates up and applies a force to your legs, which accelerates your body up. Since your body is not rigid, it deforms when first beginning to accelerate, causing your blood to accelerate slower at first and pool up in your legs.
     
  26. Feb 18, 2010 #25

    russ_watters

    User Avatar

    Staff: Mentor

    The normal reaction force is the sum of the gravitational force and the force due to the acceleration (f=ma).
    I don't understand what you are saying there. Gravity makes you free-fall in the direction of the gravitational force.

    Consider another example: a glass of water sitting on a table. Push it. What happens to the water?
     
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook