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

Against the law

  1. Mar 3, 2007 #1
    I'm having trouble with a particular law, the one that says "heavier objects DO NOT fall faster" Now all experiments I've read were conducted or theorized in an atmoshere. This variable must be removed. To see the correct interaction of 2 different bodies of mass one must experiment in a vacuum, or in space. After all, the total mass of the earth is "all of it" from the core to the outer atmosphere. So to be accurate, you must be beyond the physical mass of the object. Wether it be solid, liquid or gas. Friction must not be an issue, and the gravity sphere of these objects is obviously greater than their physical dimensions.And another thing "To any object of mass, gravity and acceleration are exactly the same."

    If you understand me I can go on. Here is my experiment:
    We will fashion 2 objects, here on Earth. Both will be 1 foot in diameter. Object A weighs 100kg and object B weighs 1kg."here on Earth". Now let's take them both away from earth and close to something very massive. A neutron star perhaps. Now we will manuver both objects to a distance from the star that gravity begins to be felt. We go to a distance that each object feels the same attraction they felt back on earth. To keep these objects from "falling" we must use 100 times the force on object A as object B.

    Now this 100 to 1 ratio can be directly compared to acceleration rate or G forces. Now release them both. I must conclude that object A will be travelling faster than object B at various distances when compared to each other, or another way to look at it would be it takes less time for object A to travel 100 km than object B.

    Do you agree with this? If not, can you tell me logically why this is not the case?

    This next paragraph was taken from: Answered by: Dr. Michael Ewart, Researcher at the University of Southern California
    "If no air resistance is present, the rate of descent depends only on how far the object has fallen, no matter how heavy the object is. This means that two objects will reach the ground at the same time if they are dropped simultaneously from the same height. This statement follows from the law of conservation of energy and has been demonstrated experimentally by dropping a feather and a lead ball in an airless tube."

    What I say is: In this experiment there wasn't enough distance for the actual behavior to be observed.
  2. jcsd
  3. Mar 3, 2007 #2
    What exactly do you mean by this? Surely it isn't true since mass is mass, gravity is a force, and acceleration is the rate of change of velocity, so how are these exactly the same thing?

    Why would this be the case? Have you though about Newton's second law at all in considering this?
  4. Mar 3, 2007 #3
    "To any object of mass, gravity and acceleration are exactly the same." (To the mass) I'm trying to make it simple.
  5. Mar 3, 2007 #4
    That still isn't true. Force and acceleration are not the same thing.
  6. Mar 3, 2007 #5
    I'm saying that mass (a person can be mass) feels the same effect if we accelerate at 1g in an elevator in space(9.8 m/s2) or if we are standing on the surface of the earth, it feels the same. I understand newton.
    And try to be nice to the old bass player.
  7. Mar 3, 2007 #6
    Yes that's true, but it is not the same as what you were saying earlier. And you still haven't really justified you're conclusions you reach in your first post.
  8. Mar 3, 2007 #7
    Take it step by step so I can respond. I think I was logical and my conclusions are within established rules of physics. This is where I see a contradiction "heavier objects DO NOT fall faster." I say they do.
    Last edited: Mar 3, 2007
  9. Mar 3, 2007 #8
    I have no idea what you mean by that, I've read your post, but your logic is flawed if we have two masses say M and m where M>m, and we have some larger body of lets say mass B then it puts a force gravitational force of magnitude F_m on m and F_M on M with the following magnitudes, assuming that both M and m are the same distance r away from B.

    F_m = G*m*B/r^2
    F_M = G*M*B/r^2

    Which follows from Newton's law of gravitation, but we also have from Newton's second law that F= ma, but since all we really care about it a at the moment this is equivalent to a=F/m.

    let's call the acceleration of m a_m and likewise a_M for the acceleration of M.


    a_m = (F_m)/m = (G*m*B/r^2)/m = G*B/r^2
    a_M = (F_M)/M = (G*M*B/r^2)/M = G*B/r^2

    thus a_M = a_m and the two objects accelerate or "fall" at the same rate.
  10. Mar 3, 2007 #9


    User Avatar
    Gold Member

  11. Mar 3, 2007 #10


    User Avatar
    Gold Member

    It doesnt matter the distance they are allowed to fall. Both objects experience an acceleration, g, and both have a constant increase in velocity.
  12. Mar 4, 2007 #11
    Actually, you have it backwards. This experiment can/has been done inside an evacuated tube. http://web.mit.edu/fluids/www/Shapiro/ncfmf.html video had a demonstration, but its not currently working. You can see it for yourself (but ranger gave you a video that shows the equivalent anyways).

    What does the total mass of the earth matter in this calculation? We are finding the local acceleration due to gravity.

    Ok, I get what your saying, but kg is not a weight, its a mass. If it were weight, it'd be in Newtons, not kg.

    ok, fine.

    Gravity is felt by the objects no matter how far appart they are.

    But who cares if the attraction is the same or not, they will both experience the same acceleration (just different magnitude than on earth).

    Herein lies your problem. Yes, you have 100 times more force, but thats because you have more mass. The acceleration is the same, the force is what is different in order to provide the same acceleration to both bodies.

    It is the increase/decrease in force (related to mass) that compensates to have the same acceleration.




    Assume body 1 has force F and mass M. Body two has force 100F and mass 100m.

    (100F/100m)=(F/m)=g <- g never changed.
    Last edited: Mar 4, 2007
  13. Mar 4, 2007 #12


    User Avatar

    Staff: Mentor

    You already accept that larger masses have a larger weight, so you must also accept that all masses fall at the same rate, since that's how you calculate weight! F=ma: a= acceleration due to gravity!

    If that isn't enough, the other thing you are missing is that mass is what resists acceleration. When you push a car, you are not pushing against its weight, you are pushing against its mass. So while you know your heavier object feels more force, it also has more mass, so it is harder to accelerate. And the relation for that phenomena is still f=ma.

    That Apollo expirment clip is pretty neat. When I was in junior high, my science teacher had an evacuated cylinder with a rock and a feather in it to demonstrat the same thing.

    It almost sounds like you think your experiment is necesary to prove this. It shouldn't be that hard to see that there are an enormous number of phenomena we deal with on earth that depend on this physical principle being true. If heavier objects fell faster than lighter ones, objects of different mass would require different orbital speeds, for example. And when an astronaut left the space station to go on a spacewalk, he'd be flung out into space.
    Last edited: Mar 4, 2007
  14. Mar 4, 2007 #13
    You guys maka me crazy, but it's fun

    I just disagree with fundamentals. For instance: When we stand on the Earth's surface we feel 1g, as if we are accelerating up. That leads me to conclude that if I'm not physically accelerating away from earth then the fabric of space is moving towards the center of G of earth "locally". How else could my mass feel if it were accelerating. We know it's not magnetic. Back to my original point.

    I know that there is an average attraction between two objects. Take object A and the star. When we release A it falls towards the star, and the star falls towards the object A. The same thing happens to object B and the star. The point I'm trying to make is that in the "A drop" the total mass of the star and object A is > Star mass + object B mass.

    I'm thinking that in" A drop", A and the star are a closed system. The same for the "B drop" scenerio. Total mass of the two masses in each scenerio is what determins velocity ( with respect to the star in each scenerio).

    Star mass+ A mass > star mass + B mass so, scenerio A has greater newtons.

    What if A was Jupiter and B was our moon? Can you see it then?

    cyrusabdollahi wrote:
    "Gravity is felt by the objects no matter how far appart they are.

    But who cares if the attraction is the same or not, they will both experience the same acceleration (just different magnitude than on earth)."

    I think "The effect of gravity between 2 objects of mass is effected by distance.
    Last edited: Mar 4, 2007
  15. Mar 4, 2007 #14
    The ratio of mass between the earth anything we put up is too great to notice this, but if you look at planetary orbits their mass increases with distance
    Last edited: Mar 4, 2007
  16. Mar 4, 2007 #15


    User Avatar

    Staff: Mentor

    Yes, but now you are asking a completely different question. If that is actually, what you meant from the start, then be specific: tell us exactly how much faster a heavier object will fall. Show us your math.

    Your orignal post implied that you thought an object's acceleration rate is exactly proportional to its mass. And using as an example a massive neutron star just makes the mass of your two test objects all the more irrelevant (since they are a smaller fraction of the mass of the neutron star than of the Earth).
    Huh? Are you saying the larger a planet is, the further it has to orbit. Clearly, not true...

    I feel compelled to ask: are you screwing with us here? Is this a serious question?
    Last edited: Mar 4, 2007
  17. Mar 4, 2007 #16
    You yourself said you need 100 times more force to hold up the larger mass.

    Similarly you need 100 times more force to accelerate the object.

    Using simple units.

    Large Mass: 100= 100*a therefore a = 100/100 = 1

    Smaller mass: 1 = 1 * a therefore a = 1/1 = 1

    a = 1 for both, therefore they are both equal, do you understand now? I've tried to explain it simpler then the others.
  18. Mar 5, 2007 #17
    richbass, you need to read carefully what d_leet told you in his post. It already answered your question.

    D_leet just showed you that the mass of the object falling does not effect the acceleration. What matters is the separation distance and the mass of the PLANET.

    Apart from that, there is nothing else any of us can tell you...-because that is the right answer.
    Last edited: Mar 5, 2007
  19. Mar 5, 2007 #18
    I think I get it now,

    You guys are saying that the ball and the feather fall at the same time because we take the sum of the mass of the ball, the feather and the earth. All three make up the total attraction. This is a closed system of three objects. f=ma is small mass observed from a larger stable mass.

    What I'm saying is this: I want to drop each separately while the other is beyond the influence of the earth system. Of mass the sum of the Earth + ball is greater than the sum of the Earth + feather. The ball would fall faster. Or I'm just out of my mind.

    Just tell me if I'm nuts.This is the last comment. I don't want to waste your time on it any further. But I'm cool if you guys think I'm crazy. Thanks.............. I have another one about the moon
  20. Mar 5, 2007 #19


    User Avatar
    Staff Emeritus
    Science Advisor

    No, nobody said that (neither that you are nuts nor that "all three make up the total attraction"- gravitational force happens "two things at a time"). The gravitational force on a bowling ball or a feather is proportional to the mass of the bowling ball or the feather. If the bowling ball has 100 times the mass of the feather, then it feels 100 times the gravitational force, just as you said. However, everyone has been saying that an objects response to force, its acceleration, is inversly proportional to its mass. The two effects cancel out- the bowling ball and feather fall at the same rate.

    It is also not true that "all experiments were conducted or theorized in an atmoshere." (I can't speak for what you've read.) Certainly, as has also been previously mentioned in this thread, one such experiment was conducted on the surface of the moon in a pretty hard vaccuum. It may have been (I don't remember now) by Neil Armstrong on the landing on the moon- he dropped a golf ball and a tennis ball and both fell at the same rate.

    Sorry, I've already used my quota of invective!
  21. Mar 5, 2007 #20


    User Avatar

    Staff: Mentor

    Also, if you're trying to work in a new idea about the force of the bowling ball pulling on the earth, it is true that a bowling ball pulls earth toward it, but since the earth is many, many orders of magnitude larger, that is typically ignored because the net effect is insignificant.
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?

Similar Discussions: Against the law