jaa101 t1_ja5plcr wrote
Reply to comment by mmmmmmBacon12345 in ELI5: Why does farming equipment require such low horsepower compared to your average car? by thetravelingsong
If pulling a plow requires 1 ton of force at 10 mph then the same plow in the same ground is going to need around 4 tons of force at 20 mph. This means that doubling the speed requires eight times the power.
purplepatch t1_ja73mnu wrote
Surely you mean doubling the speed requires four times the power, not eight.
jaa101 t1_ja7845f wrote
If the speed doubles and the force required quadruples, then the power goes up by a factor of eight. This is because power is proportional to force times speed.
The above is true for air resistance and water resistance, where drag is proportional to speed squared. I found a publication linked in this thread that says the same is true of plowing, but another commenter found a paper with experimental results showing the force required increasing much more slowly with speed. If so, plowing is not like fluid resistance and the power required increases even less than the square of the speed.
InsidiousTechnique t1_ja66r9u wrote
Can you source this? I doubt it's true
CollegeAnarchy t1_ja6a1ay wrote
Here is a link to an explanation:
https://www.quora.com/Why-is-air-resistance-roughly-proportional-to-the-cube-of-speed
I understand that link is for air, but the concept Is true for any “fluid”. For all purposes of farm equipment, the soil is a fluid because it flows around the implement.
Actually, a lot of solids can be modeled as fluids when in small pieces. Fluidizing flour, sugar, and sand is how it handled on an industrial scale.
InsidiousTechnique t1_ja6b1qj wrote
I understand the concept, I doubt it applies to dirt in the same way. There's probably some affect there, but surely not in the same cubic relation.
As an example, you can plow dirt and if you were to go over the same dirt right after and it would take much less force at a constant speed.
It's more about the mechanical bonding and friction than fluid losses in this instance. I'm calling in to question your assertion that dirt acts similarly as a fluid in this specific instance.
How much force does it take to pull a plow through dirt at zero speed? Meaning, if you put a plow in to the dirt, does it take greater than zero force to move it?
MortalTwit t1_ja6iwq0 wrote
Force = mass times acceleration squared. If you double your speed, you need x4 the force.
ViridisWolf t1_ja6nnza wrote
No, f=ma. No squared term.
jaa101 t1_ja6fja6 wrote
Note that air resistance is only proportional to the square of the speed, so the heading of the linked article is incorrect. Resistance is a force. It's power that goes with the cube, because it's proportional to force multiplied by speed.
Travianer t1_ja6pfq0 wrote
The whole truck isn't moving through the medium of dirt though so it's apples to oranges in this case.
kyrsjo t1_ja6vq2w wrote
That doesn't really matter tough. The force would be the sum of two terms that both goes like v^2, the plow drag and the body air resistance drag. So the total drag force still goes like v^2, and the power (force x velocity) like v^3.
jaa101 t1_ja6ej8o wrote
InsidiousTechnique t1_ja6gtqn wrote
So I read the paper, and saw it did assert that. But here's another paper (that looks more researched) that has draft force compared to speed, and there's definitely not a squared relation there although it does show an increase on draft force compared to speed it appears more linear.
jaa101 t1_ja6iqeq wrote
Looks like you're right, in fact it shows closer to a power of 0.33 than 1, and far from 2. Neither document goes into the physics involved.
twelveparsnips t1_ja6jgp2 wrote
Resistance increases exponentially in relation to speed. Kinetic energy is 1/2 M*V^2
InsidiousTechnique t1_ja7cwhb wrote
Check the paper I linked.
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