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| Thread: A hoax called gravity. |  This thread is pages long: 1 2 3 4 5 · «PREV / NEXT» |
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TheDeath
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posted July 01, 2008 05:41 PM |
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I understand that now but you don't explain it very well.
Basically, a heavier object has more force than a light one, but it also requires more force to be 'moved' -- therefore the 'extra' force goes into that extra energy needed (since it's heavier) and thus, the final acceleration or speed is the same.
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mvassilev
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posted July 01, 2008 05:45 PM |
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TheDeath
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posted July 01, 2008 05:50 PM |
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Well I know momentum (or inertia, how is it called?) had to do something with it but I had to ask anyway 
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Corribus
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posted July 01, 2008 06:02 PM |
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@theDeath
Gravity is a field, so the force due to gravity is inherently different than a force needed to move, for example, a car. A much better way to understand it is in terms of work and energy, rather than trying to compare very different types of forces.
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alcibiades
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posted July 01, 2008 10:37 PM |
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Death, what you do is one of the most classical mistakes of students of classical mechanics: You mess up the meaning of Force, Mass, Velocity and Acceleration. More important, you put an equality between the weight (force) of an item and it's fall speed.
Let me try to elaborate.
Let's look first on the weight issue
When you lift something, you feel it's weight because the gravitational acceleration makes it exhert a force on you. Let's consider two things: You hold a tennis ball above your head (mass = b ~ 0.1 kg) and you hold a cannonball above your head (mass = c ~ 100 kg). What does Newtonian mechanics say on this subject?
Well, one of Newton's laws state that:
F = m * a ; Force is equal to Mass x Acceleration.
In order to understand this, we need to address: What is acceleration? Acceleration means change in velocity per time, i.e. if an item has an acceleration of 10 m/s^2, that means the item gains velocity 10 m/s each second. If it starts out in rest, after 1 second it moves with 10 m/s, after two seconds it moves with 20 m/s, after 3 seconds it moves with 30 m/s, etc.
Now, I assume you're a strong man, you hold both items at rest, i.e. they don't move and don't gain velocity. Hence, a = 0, and thus F = 0. This means, that while you hold the items above your head, there is no (net) force affecting them! This might seem contrary to your experience: Clearly, the gravitational force is affecting them, that's why you'll probably feel pretty strained to hold up that canon ball for long!
But if we look at it, the NET force on the balls are zero, because the gravitational force pulls down, but you push up with the similar force to keep them at rest. That's why you get tired!
Furthermore, according to the theory of gravity, gravitational acceleration at Earth's surface is constant == g = 9.82 m/s^2 ~ 10 m/s^2. The gravitational force on the two balls are therefore, according to F = m * a:
F(tennisball) = b * g
F(cannonball) = c * g
To keep them at reast, you need to exhert the same force. Now, since the canonball mass (c = 100 kg) >> the tennisball mass (b = 0.1 kg), you need to exhert a much greater force on the cannonball to hold it up than you have to on the tennisball.
Now, let's evaluate the theory of gravity. Does it make sense that the gravitational acceleration is equal for all things at the surface? Well, let's concider what would happen if this was not the case: Imagine we take two 10 liter buckets and fill them with iron. Common sense says we would expect these two buckets to have the same weight, i.e. you need to exhert the same force to hold them up (F1 = F2). Also, since the buckets are similar in volume and content, we would expect them to have the same mass, i.e. m1 = m2. Hence:
F1 = m1 * g1 ^
F2 = m2 * g2 ^
F1 = F2 ^
m1 = m2 <=>
g1 = g2
Hence, common sense would leed us to conclude that gravitational acceleration should be the same for these two buckets. If this was not the case, one bucket would feel heavier than the other (F1 > F2)! The gravitational theory seems to make sense with what we know from everyday observation.
Now let's consider the fall speed issue
Imagine we go into the leaning tower of Pisa and drop said tennisball and canonball from the top. With DROP, I mean we release them with 0 initial velocity. Let's not consider resistance of air, we imagine this is negligable. This leaves us with the equation:
F = m * a, where the acceleration comes from the force of gravity alone, a = g. Hence, the force on the balls are different!
BUT!
What about the fall speed? Remember, acceleration is change in speed per time unit. And since they start out with no velocity, v = 0 (remember: We drop them), they experience the same growth in velocity, and hence, as they start out both with speed zero, at any time will have the same fall velocity at any given moment!
Since the ball falls with the same velocity, they should also be the same amount of time to reach the bottom! Notice that this argument holds because the fall velocity depends only on the acceleration and not on the force, and hence is independant of the mass! The latter - velocity depends on force - is a very common misconception, but it is wrong. Velocity depends on acceleration!
*ADVANCED PART*
Can we calculate the fall time? Yes, but this requires basical knowledge of differential algebra.
Acceleration is change in velocity per time, i.e. a = dv/dt (dv ~ delta v, i.e. change in v).
Now furthermore, velocity is defined as change in place (s) per time, i.e. v = ds/dt.
Hence, a = dv/dt = d/dt[ds/dt] = d2s/dt2
If we put a = g = constant, we can easily verify by differentiating twice that:
s = 1/2 * a * t^2
Notice, that from this equation we can calculate the fall time T for a given height s = h:
T = sqrt[2s/a]
Notice, that T is independant of the mass of the object, i.e. all objects will have equal fall time, given the condition that acceleration is the same (which means: No air resistance).
~ End of todays physics class ~
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Celfious
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posted July 03, 2008 12:52 AM |
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Quote:
If there wouldn't be gravity, we could lift objects without effort?
If so, why are heavy objects harder to lift? Air resistance?
maybe I'm missing something here
For example, with my 3 phased oxygen illiteration, Things sink in the middle phase. Some sink some float in the water phase.
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TheDeath
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posted July 03, 2008 03:51 PM |
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Quote:
The latter - velocity depends on force - is a very common misconception, but it is wrong. Velocity depends on acceleration!
But doesn't acceleration depend on force? 
I know that things are the way they are, but I did not understand how (until I figured out at the beginning of this page). I mean, we can say "mass" and "acceleration" are two different things, but why?
I know how acceleration works. I know how to do it with differential (calculus) math. But I didn't know why the heavy object don't fall faster, since they have greater force (cause of gravity) and thus greater acceleration...
however, as I have pointed out, they also require more 'energy' to be moved (I think it was called momentum) -- that is, it's a lot harder to make a train move than a ball -- and consequently much harder to stop a train than a ball (not talking about gravity).
Thus, the extra force that I believed should have gone to acceleration, only compensates for the extra energy required to move the heavier object. Thus, the speed/acceleration is the same, but the total energy is much higher!
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mvassilev
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posted July 03, 2008 06:06 PM |
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TheDeath
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posted July 03, 2008 06:49 PM |
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Yeah whatever, force is an abstract thing -- it's basically just energy applied on a direction
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Asheera
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posted July 04, 2008 02:15 PM |
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Question: Why heavy objects are pulled with a higher force by the gravity than the lighter ones?
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TheDeath
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posted July 04, 2008 02:24 PM |
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Quote:
Question: Why heavy objects are pulled with a higher force by the gravity than the lighter ones?
Cause God wanted it that way
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Minion
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posted July 04, 2008 03:14 PM |
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Hehe, please read Alchibiades post. It is almost right on top of yours. Can't miss it
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Asheera
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posted July 04, 2008 03:18 PM |
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Quote:
Hehe, please read Alchibiades post. It is almost right on top of yours. Can't miss it
I understand, but I want a "why" not just "because it's how we observed until now".
I mean, what's gravity and why it pulls objects with higher mass with a higher force?
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Minion
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posted July 04, 2008 03:40 PM |
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Edited by Minion at 16:14, 04 Jul 2008.
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Quote:
Quote:
Hehe, please read Alchibiades post. It is almost right on top of yours. Can't miss it
I understand, but I want a "why" not just "because it's how we observed until now".
I mean, what's gravity and why it pulls objects with higher mass with a higher force?
I don't think you have understood that it is mass that causes gravitation. The greater the mass, the greater the gravitational field it has.
But if we are talking about gravity on a planet like Earth, it has massive gravitational field. Unlike a pen. If you look from Earths perspective, it doesn't matter if an object weights 10kg or 1000kg. It's mass is huge enough to give any object the same acceleration. The same is not true with an object with the mass of a Moon, and distance matters too of course.
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TheDeath
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posted July 04, 2008 04:22 PM |
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It goes like this:
Object A attracts Object B, but also Object B attracts Object A, so they attract faster (if they have the same mass, that'll be 2x times (or squared times?) their attraction force compared to e.g: attracting light).
Or am I wrong?
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Asheera
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posted July 04, 2008 04:48 PM |
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Yeah but why two objects attract each other? Does it have anything to do with the fact that protons attract electrons or not?
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TheDeath
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posted July 04, 2008 05:01 PM |
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Quote:
Yeah but why two objects attract each other? Does it have anything to do with the fact that protons attract electrons or not?
I don't think so (but may be wrong) because mass has nothing to do (necessarily) with 'charge'.
If you want to test how much force does an object use to attract something, you'll need to test it against light and see how it bends it (because light has no mass)
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alcibiades
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posted July 04, 2008 05:51 PM |
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Intervention, intervention!
Ok, let me try to see if I can make things a bit clearer by addressing a few statements since my last post.
Quote:
Quote:
The latter - velocity depends on force - is a very common misconception, but it is wrong. Velocity depends on acceleration!
But doesn't acceleration depend on force?
I know that things are the way they are, but I did not understand how (until I figured out at the beginning of this page). I mean, we can say "mass" and "acceleration" are two different things, but why?
I know how acceleration works. I know how to do it with differential (calculus) math. But I didn't know why the heavy object don't fall faster, since they have greater force (cause of gravity) and thus greater acceleration...
however, as I have pointed out, they also require more 'energy' to be moved (I think it was called momentum) -- that is, it's a lot harder to make a train move than a ball -- and consequently much harder to stop a train than a ball (not talking about gravity).
Thus, the extra force that I believed should have gone to acceleration, only compensates for the extra energy required to move the heavier object. Thus, the speed/acceleration is the same, but the total energy is much higher!
I think you explain things pretty well yourself in the last two paragraphs: There are three parameters in the equation, and maybe the best way to look at Newton's law is like this:
a = F / m
rather than the habitual way: F = m * a.
What you need to understand is, that the fact that the acceleration of the two items is the same has nothing to do with Newton's law. The fact that they accelerate the same (and hence fall equally fast) is a consequence of the theory of gravity, which states that all objects feel a constant gravitational acceleration from Earth, or more generalized.
Like I said above, the best way to understand why this must be so is to consider it logically. Let's consider we have three balls, one weighing 2 kg and two weighing 1 kg.
First, we need to accept that nature acts equally on the two 1-kg balls, i.e. we discard chaos. That's reasonable, one 1-kg ball should feel as heavy as the other. Thus, for each of the three balls we write:
Ball 1a (1 kg) = ball 1b (1 kg): F1 = m1 * a1
Ball 2 (2 kg): F2 = m2 * a2
Now furthermore, from everyday experience, we will want the collected force of the two 1-kg balls to be equal to the force of the 2-kg ball:
F1 + F1 = F2 => m1*a1 + m1*a1 = m2*a2 => (2*m1)a1 = m2*a2
Now, since 2*m1 = m2, it follows that: a1 = a2.
What does this mean? This means that, if we want "2 times 1 kg" to be equal to "2 kg" in terms of weight, i.e. the force the balls exhert on your arms if you hold them, then we need to have the gravitational acceleration on the two be the same. Otherwise, we accept chaos: Two objects of similar mass have feel different, i.e. have different weight. The latter is against logic and everyday experience.
One more note: Notice that whereas mass is an INTERNAL property of the object (i.e. the object has a mass which depends on the object alone), acceleration is an EXTERNAL property: It depends on the surroundings. the balls don't have an acceleration in themselves, the acceleration comes from outside: Gravitational acceleration from the field of gravity but possibly also acceleration from other things (we can give it acceleration by throwing it, for instance). Unfortunately, I have to go, but may elaborate on this later.
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Minion
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posted July 04, 2008 06:34 PM |
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Quote:
Yeah but why two objects attract each other? Does it have anything to do with the fact that protons attract electrons or not?
That is Electromagnetic force, and it is very different from gravitation. It works between electrically charged particles only but describes almost all phenomena of our everyday experience
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alcibiades
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posted July 04, 2008 08:46 PM |
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To elaborate on Minion's post, the gravitational force is one of the four fundamental forces in physics: The electromagnetic force, the gravitational force, the strong nucleonic force (is that the proper english term? It deals with the particles inside atomic nuclei), and the week nucleonic force. It is not completely understood how gravitation works, for instance, there has been theories about so-called "gravitons" which would work as thansmittors of this force (much like photons transmit the electromacnetic force), it's not something I know a lot about, but as far as I know, there's no indication such particles actually exist.
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