Easy physics puzzles
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4 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?
"My life is spent in one long effort to escape from the commonplace of existence. These little problems help me to do so." -- Sherlock Holmes
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.
"My life is spent in one long effort to escape from the commonplace of existence. These little problems help me to do so." -- Sherlock Holmes
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.
"My life is spent in one long effort to escape from the commonplace of existence. These little problems help me to do so." -- Sherlock Holmes
