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There are three coins lying flat on a table, arranged in a straight line with their edges touching. Call the coins A, B, and C (B is in the middle). Here are the rules:

You can touch and move only Coin A.

You can touch Coin B, but it cannot move.

You cannot touch Coin C.

Coin A cannot touch Coin C.

The table is immovable (no tilting or shaking the table).

How can you move Coin C without touching it, using only the provided materials and your own two hands?

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  • $\begingroup$ You have tagged this Puzzle with "Physics" & the Answer by user ibanezplyr seems to use "Physics" : I think that Answer is what you want , though it is not "Physically" valid & it is using a trick ! $\endgroup$
    – Prem
    Nov 3, 2023 at 6:36
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    $\begingroup$ @Prem I suppose we can clarify that "not moving" does not preclude negligible microscopic movement between atoms, or else we need to assume the entire apparatus is at the unachievable temperature of absolute zero. Macroscopically, Coin B may not move. $\endgroup$ Nov 3, 2023 at 12:44
  • $\begingroup$ "or else we need to assume the entire apparatus is at the unachievable temperature of absolute zero" - in which case touching coin B is a rather bad idea. $\endgroup$ Nov 3, 2023 at 15:24

5 Answers 5

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Place and hold one finger on Coin B to keep it from moving. With the other hand, slide Coin A into the side of Coin B forcing Coin C to move in the same direction. Like a Newton's Cradle.

enter image description here

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    $\begingroup$ Placing the finger is unnecessary as Coin B cannot move. $\endgroup$
    – mathlander
    Nov 3, 2023 at 2:19
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    $\begingroup$ When Coin B can not move , I do not think it can transfer the force from Coin A to Coin C. Imagine a hard wall with a ball on one side. I do not think we can make that ball roll away by throwing a ball on the other side. This answer is "Physically" not going to work , though I think this is what OP wants. In Coin Case , our hand is not immobile & when we hit with Coin A , then Coin B will move to transfer the force to Coin C & then our hand will make Coin B move back to Original Position. It will be so quick that we do not see it , though it will move. $\endgroup$
    – Prem
    Nov 3, 2023 at 6:27
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    $\begingroup$ @Prem Get 3 coins and try this out for yourself - it absolutely works! This is a trick my mum taught me when I was little. It's best if all 3 coins are the same denomination (we used to use UK 2p coins). Any movement of B is negligible. $\endgroup$
    – Stiv
    Nov 3, 2023 at 6:51
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    $\begingroup$ That will not work "Physically" & theoretically , @Stiv , when you say "movement of B is negligible" , it means "B will move" , contradicting the Criteria. It apparently works , because "movement of B is negligible" & it will not work when B is immobile ! $\endgroup$
    – Prem
    Nov 3, 2023 at 7:06
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    $\begingroup$ @Prem Force is transmitted as a pressure wave through Coin B. You can argue that the individual atoms of the coin are moving, but they do that all the time anyway. Macroscopically, the coin does not "move and then move back" - the only force your finger applies to the coin is directly down. I agree there must be some elasticity in the coin, but I think most people would very comfortably say the coin is "not moving". If you look close enough, there really is no such thing as something that's "not moving". $\endgroup$ Nov 3, 2023 at 12:37
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Touch Coin A directly at its center from the top so hard that the coin penetrates through the table because physics. Pick up the coin and place it at the area near the hole that you just caused. Do the coin penetration around both Coin B and Coin C until the area inside falls. It can be concluded that Coin C is moved. Here's an example of the A holes with Coin B and Coin C.

AAAAAA
A BC A
AAAAAA
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    $\begingroup$ That seems like a brute force attack. $\endgroup$ Nov 3, 2023 at 13:39
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    $\begingroup$ Creative, although I think it runs afoul of the "immovable table" rule, as it requires that the table surface can, in fact, move. $\endgroup$ Nov 3, 2023 at 13:47
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    $\begingroup$ "Coin B cannot move" - I think it would require significant amount of doublethink to say coin C has moved and coin B has not. $\endgroup$ Nov 3, 2023 at 15:22
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Put your finger on coin B and wait.

If your hand is warm enough, the coin will be heated (a bit...), so it will expand.

Due to the increase in size it will push the coin C

...which will cause the coin C to move

(a bit).

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Let's assume an entirely frictionless, flat table.

Move coin A so it is close to coin C, and next to coin B. The three coins should make a nearly equilateral triangle. Hold coin B in place, being careful not to move it.

Now, we just have to wait.

As in the Cavendish Experiment, the gravitational force of coin A will slowly drag coin C towards it.

It's as simple as that!

Depending on the size of the coins, I'd estimate this would take hours to days. The original experiment took about 7 minutes for full movement, and it used a 300kg mass to apply the gravitational force. A US quarter is about 50,000 less massive, so we'd need to wait about 200 days to see the same magnitude of motion. However, this is sped up by the small size of the coins. Moreover, we can declare success after just a bit of movement is detected, perhaps via a microscope.

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  • $\begingroup$ Except that coin C will be more attracted by you than by coin A. $\endgroup$
    – Florian F
    Nov 13, 2023 at 12:04
  • $\begingroup$ @FlorianF No rules against using our own gravity to move coin C $\endgroup$
    – isaacg
    Nov 13, 2023 at 22:39
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Yeah, honestly I'd just do a fanning pattern.

Even just a small swipe is enough to move the coin some distance.

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  • $\begingroup$ Was my idea too. Not sure if I’m being too creative, or not enough given what the "real answer" is. $\endgroup$
    – adentinger
    Nov 3, 2023 at 3:51
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    $\begingroup$ Please specify what you mean by "fanning maneuver". Do you mean fanning the air above C? Or something similar to the previous answer? $\endgroup$
    – bobble
    Nov 3, 2023 at 5:13
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    $\begingroup$ The table is not frictionless, you cannot move the coin by applying arbitrarily little force. Have you tried putting a coin on a table and fanning it off with your hand? I find it unlikely this would actually work. $\endgroup$ Nov 3, 2023 at 12:35

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