Easy physics puzzles
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6 members have voted
July 6th, 2026 at 8:58:02 AM
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I saw this on my Facebook feed, which links to this page. Fair warning I don't know the answer, but have an opinion.
The question for the poll is who is your favorite member of the Manhattan Project?
"No great mind has ever existed without a touch of madness." -- Aristotle
July 6th, 2026 at 10:29:06 AM
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Quote: Wizard
I saw this on my Facebook feed, which links to this page. Fair warning I don't know the answer, but have an opinion.
The question for the poll is who is your favorite member of the Manhattan Project?
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Well, my first answer was No, the cart will not move. The action of the fan blades dragging on the air produces a net force backward which offsets the force of the air being driven into the "sail."
But I am a bit unsure. There must be an onboard power source - say, an electric battery that is powering the fan and putting energy into the system but that is not shown in illustration. So external energy is being applied to the cart and thus it is plausible that the cart is permitted to have net momentum.
Secondly, considering the air as a fluid, the action of the rotating fan is essentially causing a reduced air density at the point of the fan This creates a wake in the air to the left of the fan which will actually cause air movement - a slight breeze - going from left to right. Think of it this way: As the fan pushes air forward, a vacuum would occur around the fan unless air was moving into the fan. Thus air is sucked into the fan and the flowing air will tend to act on the structure of the fan and cart and tend to move the entire cart forward. This is consistent with the observation that the cart is not a closed system; the 'closed system' is the cart plus the upstream and downstream air plus the on-board battery.
However, I must point out that the screen/sail is so close to the rotating fan that there will be backflow from the screen/sail into the fan, inevitably causing some turbulence and possible disruption of the idealized aerodynamics of the fan. It might even affect the mechanical rotation of the fan blades if the distance between the fan and screen is small enough.
So, it is complicated.
So many better men, a few of them friends, are dead. And a thousand thousand slimy things live on, and so do I.
July 6th, 2026 at 10:35:16 AM
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No. The two cancel each other out.
July 6th, 2026 at 11:17:18 AM
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I think the cart will move to the left. Without the sail, there would be a recoil effect from the fan blowing right, as well as a vacuum, causing the cart to move left. This is partially mitigated by the sail. However, there will be some loss of air movement between the fan and the sail, for the same reason a hand held fan won't do much good if it is held six feet from your head.
"No great mind has ever existed without a touch of madness." -- Aristotle
July 6th, 2026 at 11:29:30 AM
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The screen/sail are bolted to the cart the fan is bolted to the cart any force put into the sail is canceled out by the thrust created in the opposite direction.
July 6th, 2026 at 1:42:20 PM
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Spoiler : Mythbusters tried this and you can see the result at https://www.youtube.com/watch?v=vVlsKdQ-o3I . It's so funny but I won't reveal their result!
July 6th, 2026 at 2:00:58 PM
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Quote: rainmanThe screen/sail are bolted to the cart the fan is bolted to the cart any force put into the sail is canceled out by the thrust created in the opposite direction.
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As I said in my spoiler box this would be true if the cart (or boat) were a closed system. But the cart/boat has a battery or combustion engine that is rotating the fan, so you are putting energy into the cart from the battery/engine and thus conservation of momentum need no longer apply.
The rotating fan sucks air into its rotating blades and actually creates a sustained wind or air flow that is not present when the fan is turned off. The entire 'closed system' includes the cart, the engine/battery and the air, not just the cart.
So many better men, a few of them friends, are dead. And a thousand thousand slimy things live on, and so do I.
July 6th, 2026 at 2:10:14 PM
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Quote: gordonm888Quote: rainmanThe screen/sail are bolted to the cart the fan is bolted to the cart any force put into the sail is canceled out by the thrust created in the opposite direction.
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As I said in my spoiler box this would be true if the cart (or boat) were a closed system. But the cart/boat has a battery or combustion engine that is rotating the fan, so you are putting energy into the cart from the battery/engine and thus conservation of momentum need no longer apply.
The rotating fan sucks air into its rotating blades and actually creates a sustained wind or air flow that is not present when the fan is turned off. The entire 'closed system' includes the cart, the engine/battery and the air, not just the cart.
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Took me awhile to get this got it now thanks.
July 6th, 2026 at 2:45:00 PM
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Quote: Wizard
I think the cart will move to the left. Without the sail, there would be a recoil effect from the fan blowing right, as well as a vacuum, causing the cart to move left. This is partially mitigated by the sail. However, there will be some loss of air movement between the fan and the sail, for the same reason a hand held fan won't do much good if it is held six feet from your head.
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Yes, I think you have it!
To create a clearer mental picture of it:
1. What if... the screen was just a pencil?
2. What if everything was the same size as pictured, except the cart was a mile long?
3. If the cart was a mile long, would it make any difference if the screen was a mile wide, was a pencil, or was as pictured?
Putting those 3 things together, you'll see why the assembly moving left, albeit not very efficiently, is likely.
A word missing from all this is "thermalized." The thermal energy in a gas is (1/2)mv2, where m is the mass of the gas and v is the RMS velocity of the gas molecules, and the higher the temperature the faster that RMS value is. The direction of motion of each molecule is random, and that motion is called Brownian motion. As the molecules coming from the fan collide with the molecules in the ambient, momentum must be conserved, so the rearward-moving molecules end up being batted around by the ambient, changing direction, changing velocity, sharing momentum, until they are barely distinguishable from the ambient and at that point we say the air from the fan has been thermalized.
July 6th, 2026 at 9:13:50 PM
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Quote: charliepatrickSpoiler : Mythbusters tried this and you can see the result at https://www.youtube.com/watch?v=vVlsKdQ-o3I . It's so funny but I won't reveal their result!
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This argues that the cart would go to the right. So, it seems I was wrong. Perhaps after wind hit the sail, the air went back and filled the void behind the fan, leaving less of the vacuum for the cart.
"No great mind has ever existed without a touch of madness." -- Aristotle
July 7th, 2026 at 3:39:05 AM
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I’d say that the above is not a physics problem, if the system is in an air environment. It’s a fluid dynamics problem. However, it’s a good physics problem, if it is in a vacuum environment. How will the system move in vacuum?
July 7th, 2026 at 5:21:03 AM
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Quote: acesideI’d say that the above is not a physics problem, if the system is in an air environment. It’s a fluid dynamics problem. However, it’s a good physics problem, if it is in a vacuum environment. How will the system move in vacuum?
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aceside,
In a vacuum the fan is useless, so the cart remains stationary.
Dog Hand
July 7th, 2026 at 8:04:53 AM
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Favorite: Emilio Segrè, because I once picked him up at the airport to bring him to a colloquium. Segrè's doctoral thesis advisor was Enrico Fermi, so I was two degrees of separation from one of the great physicists.
Least Favorite: Donald William Kerst, because he gave me an F in a physics course even though I aced all of the exams for that semester because my lab writeups were quite superficial.
An interesting related topic: https://en.wikipedia.org/wiki/Blackbird_(wind-powered_vehicle)
Least Favorite: Donald William Kerst, because he gave me an F in a physics course even though I aced all of the exams for that semester because my lab writeups were quite superficial.
An interesting related topic: https://en.wikipedia.org/wiki/Blackbird_(wind-powered_vehicle)
Gambling is a math contest where the score is tracked in dollars. Try not to get a negative score.
July 7th, 2026 at 8:07:28 AM
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Here's an interesting fact about windmills. It is impossible to take all the kinetic energy out of the wind and convert it into rotational energy (and eventually into electricity or useful work) because if you took all the kinetic energy out of the wind then the air would stop moving and you would be unable to have any more wind pass through the blades of the windmill. For this reason it has been determined that any given windmill has a theoretical limit: at least 41% of wind energy must remain after passing through a windmill.
So many better men, a few of them friends, are dead. And a thousand thousand slimy things live on, and so do I.
July 7th, 2026 at 10:07:11 AM
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Quote: DogHandQuote: acesideI’d say that the above is not a physics problem, if the system is in an air environment. It’s a fluid dynamics problem. However, it’s a good physics problem, if it is in a vacuum environment. How will the system move in vacuum?
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aceside,
In a vacuum the fan is useless, so the cart remains stationary.
Dog Hand
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No, probably not right. The fan has a mass and a spin and thus an angular momentum. The base has a mass and thus applies a torque on the spin; therefore, there is a movement for this system in vacuum.
A MIT professor demonstrated this using a bicycle wheel that was attached to a vertical rope at one wheel axis. You can just search the video to find the motion.
Last edited by: aceside on Jul 7, 2026
July 7th, 2026 at 10:23:25 AM
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Quote: acesideQuote: DogHandQuote: acesideI’d say that the above is not a physics problem, if the system is in an air environment. It’s a fluid dynamics problem. However, it’s a good physics problem, if it is in a vacuum environment. How will the system move in vacuum?
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aceside,
In a vacuum the fan is useless, so the cart remains stationary.
Dog Hand
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No, probably not right. The fan has a mass and a spin and thus an angular momentum. The base has a mass and thus applies a torque on the spin; therefore, there is a movement for this system in vacuum.
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Perhaps a movement, but not a linear translation. What would cause a linear translation in a vacuum? The lunar rovers worked just fine, but we used the motor to drive wheels in that case. The wheels themselves were not in vacuum; they were pressed against matter, the surface of the moon. If the rover was floating in space the wheels wouldn't do anything there.
