Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Brian Greene, spinning bucket and acceleration

  1. Jan 19, 2012 #1
    Hello Forum,

    I have been reading a book, the Fabric of the Cosmos, by Brian Greene. It talks about Newton investigating the behavior of water in spinning bucket...

    Zero motion (at rest) and uniform motion (constant velocity) are two types of motion that cannot be distinguished: we cannot tell if we are at rest or moving....

    But when we are accelerating we surely know we are, we feel that we are. Why? What makes acceleration so special that we are able to realize we are accelerating, while we are unable to tell if we are at rest or moving at constant velocity?

    Greens talks about accelerated motion as a curve trajectory n spacetime while uniform velocity as a straight line in spacetime....that seems a very mathematical answer... Absolute space does not exist (As Newton believed)...

    Is there a more intuitive explanation of why we feel accelerated motion?

  2. jcsd
  3. Jan 19, 2012 #2
    Well, F = ma can also be stated as F = dp/dt, which is that force is equal to the change in momentum over time. Momentum is thus another way to think about how you "feel" acceleration or force. Maybe momentum is a better way to think about it for you?

    it works out because a = dv/dt, and so mdv/dt = dp/dt. Since the mass is a constant, or at least is in this case, it can be stuck inside or taken out of derivatives to change the derivatives like I did just then. Since p = mv, dp/dt = mdv/dt, and there we're back where we started :D
  4. Jan 19, 2012 #3


    User Avatar
    Gold Member

    No. If you try to explain it you just get a tautology.

    In curved spacetime objects in free-fall ( no external forces) move on geodesics which are the 'straightest' of the possible paths. Any external force takes the object off the geodesic.
  5. Jan 19, 2012 #4
    Free fall (which is seen in Newtonian physics as motion under the force of gravity) represents instead the motion under no forces at all, no acceleration. We only feel gravity when we resist to it. In free fall we are sort of neutralizing gravity: a scale under our feet would measure zero....
    But we can deny we are feeling that free fall, even if it is not an accelerated type of motion according to space time, correct?

    From the book: "...You feel gravity's influence only when you resist it. By contrast, when you fully give into gravity, you don't feel it...."

    While something is in free fall, it is all the other things that are accelerating upward....

    So why do we feel it, while we don't feel constant velocity motion or zero velocity motion?

  6. Jan 19, 2012 #5


    User Avatar
    Gold Member

    You might as well ask why Newton's laws of motion are correct. You feel forces because of inertia. In free fall the inertia is cancelled because the gravitational 'charge' is exactly equal to the inertia. That is one of the equivalence principles, thought to be valid nearly always.
  7. Jan 20, 2012 #6
    It seems like a mystery, but if you look closely it makes sense.

    When you are free falling in a gravitational field, you are increasing your speed of fall... you are accelerating toward the ground for example. If you step off a tall building you will certainly accelerate from 0 to 32ft/sec in 1 second.
    What is important to see is that each individual part and particle of your mass is being accelerated individually and identically (if you don't take micro-tidal effects from the gradient of the field strength into account). Since each and every part of you is being accelerated INDEPENDENTLY, these parts don't "feel" any differential force, pressure, or pull among themselves... the effect is balanced and the same throughout, so there is nothing to "feel".

    When you stand in a gravitational field on the surface of the Earth, there is a chain of influence or effect. There is a pressure between the ground and your feet, between your feet and your ankles, your knees, thighs, on up...in effect, your free fall is being stopped - first by the ground under your feet, which stop your legs, which stop the rest of you... from feet to head. All your parts feel this stopping force or pressure between themselves...
  8. Jan 23, 2012 #7
    Thanks everyone.

    A few more doubts....Consider two observers, in two different inertial frames of reference, traveling at a constant relative speed with respect to each other....

    In special theory of relativity, observer A in its own frame of reference A sees everything as normal: length, time interval are all proper.

    what looks abnormal is what happens in the other frame of reference. But because of symmetry, observer B in frame of ref. B see everything normal in its own world....

    In summary, things look strange when we peep into someone else frame of reference....
    Why do that? Because at the end we are all living in the same spacetime and the actions of someone is a different frame of reference must sometimes be measure or have an impact on things that are in a different frame of reference....
    Even if an observer in its own frame sees everything as normal, what happens there can impact things in another frame of reference in unexpected ways....
    Still weird: it is hard to detach from certain absolutes.....

    As far as free falling, I am very Newtonian: if we fall we feel the acceleration, we speed up if we are measuring our position from a lab frame of reference. I still can't grasp the idea that an object in free fall is not experiencing the force of gravity in a sense, simply because it is not resisting it....Surely our breath is taken away when we free fall.....we do feel something....
  9. Jan 23, 2012 #8


    User Avatar

    Staff: Mentor

    Do we really?
  10. Jan 23, 2012 #9
    on one hand, there is no 'intuitive' explanation...but if you have studied forces and acceleration, then you might perhaps find it 'intuitive'.

    Physics often requires some new perspectives, new ways of thinking about things, new 'intuitions'. That's what made Einstein so great.

    You 'feel' forces, and forces result from acceleration.... changes in inertia. Such changes can be 'felt" when you change speed or change direction....both qualify as 'acceleration' because forces act as vectors, havig a magntitude and a direction.

    It does not have to be that way, but it happens to be the way things work in this universe. Such forces hold things together, like the atoms of which we are composed. No forces, then no universes like this one.
  11. Jan 23, 2012 #10
    It may your use of the term "feeling" is literal here.

    Don't at all consider "what would I feel in this situation?".

    It is a matter of measurement / detection ecetera.

    I think this is the point where gravity is called a "fictitious force".

    Standing on Earth, you are not "experiencing" the force of gravity as much as the force of the ground under your feet "stopping" you from "accelerating". From a strickly physics perspective, if the path gravity would take me is geodesic, I would guess when the ground stops me from traversing that path it is then I am accelerating. So I would say, when we stand we feel acceleration, when we free fall, we don't feel acceleration. Maybe this is counter intuitive because of relative motion.

    I went on a tandem skydive once. 0-100mph or so in a few seconds.

    Did not even in the slightest feel like accelerating (just a falling feeling during the initial acceleration). In hindsight, I really only remmeber the speed getting faster & faster (the sense of falling was there for the first 5 or so seconds)

    In free fall (in a vacuum) what is it that the free fall object is inertial with? Asked differently (and idealizing), when we left the plane, we were accelerating away from the plane, but to the geodesic path we were decelerating weren't we, becoming inertial with what? And asked again, -> free fall is inertial with what>? the local spacetime?

    Is it more appropriate from a physics perspective to say; when we fall we are decellerating?
    Last edited: Jan 23, 2012
  12. Jan 23, 2012 #11
    There is much confusion here...

    If you free fall with an accelerometer (a spring scale with a mass attached will do), you will neither feel nor will you measure any magnitude of acceleration because all the parts of the accelerometer and all the parts of you are moving perfectly together... no relative movement, pressure, push, or pull among the parts.

    If you stand on the ground with the accelerometer, your feet are being pressed upwards, and the spring under the mass is being pressed upwards... in both cases there is compression... which you feel, and the accelerometer measures.
  13. Jan 23, 2012 #12
    So, when we are in free fall, from a Newtonian standpoint, we still have our weight W=mg.

    We sense, perceive that our apparent weight has disappeared, which is the measurement given by a scale attached under our feet....

    Anyway, according to relativity, are we accelerating or not? I am still confused clearly...

  14. Jan 23, 2012 #13


    User Avatar

  15. Jan 23, 2012 #14


    User Avatar
    Staff Emeritus
    Science Advisor

    According to relativity, a free falling object has no proper acceleration.

    Proper acceleration is a property of an object, independent of any coordinates that you might use to describe the motion. People have talked about how you can conceptually measure proper acceleration with a scale or accelerometer you carry along. Ballistic missiles use (or used to use) such systems with accelerometers, these systems were known as "inertial guidance systems".

    Your "coordinate acceleration depends on your choice of coordinates, and is not a property of the object itself, but a property of the object and the choice of coordinates. Formally, coordinate acceleration is the second derivative of the position coordinate with respect to the time coordinate.

    I suspect you may be conflating these two concepts. It may vary by context, but most of the time most physicists will be talking about "proper acceleration" when they are talking about acceleration. If you are mentally thinking "coordinate acceleration" when the physicist is talking about proper acceleration you'll be getting confused, which is what I suspect is happening here.
  16. Jan 23, 2012 #15

    D H

    User Avatar
    Staff Emeritus
    Science Advisor

    There is no way to sense that weight in the form of a local experiment. You don't feel gravity. What you feel when standing on the floor isn't gravity pulling you downward. What you feel is the floor pushing you upward. You feel this upward force on your feet. That upward force propagates somewhat unevenly throughout your body. You feel this uneven propagation throughout your body.

    Imagine taking a ride on a modern roller coaster, one that is specifically designed to mess with your mind. When the car goes over a rise and then drops parabolically that upward force disappears. There is almost no change in gravitation, but that is not what your stomach tells you. Your innards are used to that uneven propagation. The lack of that internal tension confuses your senses. Modern roller coasters magnify this sensorial confusion with zero g rolls. Your inner ear says up is one way while your legs and guts say it is exactly the opposite direction.

    You feel all this while gravity isn't changing.
  17. Jan 23, 2012 #16
    so a car's acceleration is coordinate and "free fall" is proper?

    Opps, i just re read your post, specifically the first line. It's the other way around
    Last edited: Jan 23, 2012
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook