Better than Archimedes

Archimedes is said to have declared: "Give me a place to stand and I will move the earth". The figure explains the way he meant to do it.

Does it really? Let's invite the Mathematician and the Physicist to discuss the question in more detail. Here's how the discussion could have looked like.

Mathematician: Although ingeniously simple, Archimedes' idea has some serious drawbacks. Here's one of them. Let A be the point, at which the lever supports the earth as shown in the figure, and let B be the fulcrum, i.e. the point at which the lever rests upon a stable support. We may agree for that for practical reasons the segment AB should have some reasonable length, say, not less than 1 mm. But then the length of the entire lever, if it were to accomplish its task, would have to be monstruous. If we use F to denote the force to be applied at A to move the earth, and G to denote the force applied by Archimedes at the point C at the other end of the lever, then by the well known laws of physics the balance of these two forces implies the following relation:

      F\times AB = G\times BC

Let's assume the following values for the magnitudes involved: AB = 1mm, F= 6 . 1024 kG, G=10 kG, quite reasonable, I believe, for the case considered. Now you can compute the distance BC and the length of the lever (which the interested reader will certainly do).

Physicist:Why do you think the value of 6 . 1024 kG is reasonable for F? This is exactly the value of the entire earth mass and we have no reason whatsoever to claim that the force would be numerically equal to the mass. If the earth were replaced in the figure by an object with mass equal to 6 kG, then - knowing that we are on earth - it would justified to say that F is equal to 6 kG, i.e. 58.8 newtons (it seems that the use of newtons is safer, since it prevents confusion between the lower case "g" and the upper case "G" in kg and kG; 1 kG = 9.80665 newtons). The problem would be simple. But in our case neither Archimedes nor you have said anything about the position of the fulcrum.

M: Come on, don't hang on details. What interests me is the problem of using a lever to lift a very huge mass. After all, I can assume that I am on Jupiter and I want to lever up a ball of the size of the earth.

P: But Jupiter...

M: I know, I know. Jupiter consists of helium and nitrogen and I cannot "lay" the earth on it. But it's the problem that attracts mu attention and not the relation between earth and Jupiter.

P: OK, though I would prefer to bring the problem somewhat closer to reality. The earth moves along a free orbit and has no weight, just like cosmonauts flying on an orbit around the earth. But, anyway, I can accept Jupiter. Do you know what the weight of the earth would be on Jupiter?

M: I don't, but you should.

P: I can compute it. The general gravitation law says that the force of attraction between two masses is proportional to the product of the two masses and inversely proportional to the square of the distance between them. If the earth and Jupiter are tangent, this force is approximately

          1.29 . 1025 kG.

M: So, we have finally reached an agreement. Now I can compute that to lever up the earth on Jupiter using a pression corresponding to the force of 10 kG, I must use a lever of the length of 1.29 . 1018 km.

P: I don't agree that we agree. I may accept, however, that the length of 1.29 . 1018 km is sufficient to counterbalance the force of attraction on Jupiter's surface with a force of 10 kG.

M: Aren't you being fussy? If you can counterbalance the force of attraction, then with a slightly greater force you can also lift the earth.

P: In what time and by how much would you want to lift the earth to consider the job done?

M: The time is of no significance to me and I would be satisfied with moving the earth just 1 mm off the ground.

P: Well then, I consent to 1 mm, but the time is very important for me.

M: Why would you care about time?

P: Because it may happen that an entire lifetime would be too short to carry the task out, as I will prove to you now. Let's assume that we can afford to dedicate 100 years to lift the earth 1 mm off the surface. By applying a constant force we shall make the earth move with uniform acceleration of 2 . 10-22 m/s^2. To attain this effect we need a rather small resulting force of 123 kG. Unfortunately, we must first counterbalance Jupiter's force of attraction, which is equal to 1.29 . 1025 kG. Therefore the lever must provide a force of 1.29 . 1025 + 123 kG, which is approximately equal again to 1.29 . 1025 kG. In the conditions you proposed the length of the lever must have one arm of length 1.29 . 1018 km. Let's forget about the fact that there is no technical possibility of constructing such a lever. Anyway, if the point A is to cover the distance of 1 mm in 100 years, the point C must traverse the distance of 1.29 . 1015 km in the same time, which yields a mean velocity of 400 000 km/s -- much greater than that of light.

M: Well, but the idea was correct, wasn't it?.

P: I am still in doubt. Can an idea correct for the mass of 1 kg be considered correct for arbitrarily large masses?

M: But then, can the earth be moved at all?

P: Why, yes, but no lever nor place to stand is needed for that. It would be sufficient to launch rockets into space while the earth is at some specific positions, so as to act on it with a mean force of 123 kG in a fixed direction for the time of 100 years. This would move it by 1 mm and endow it with an additional speed of 6.3 . 10-13 m/s.

M: I must conclude that I like Archimedes' idea better. At least it's unfeasible! The earth shouldn't be played with...

Tomasz Hofmokl (P)