eli5 Why are tractors able to pull hevier stuff than cars/trucks with 3x more horsepower?
In: 7
It’s all about mechanical advantage. Horsepower is a measure of power, 1 hp = 746 W, and power is a measure of energy (or work) over time, and work is force applied over a distance W=F•d
Mechanical advantage is how much you sacrifice the distance moved for more force. Since work is a measurement of energy, it must be conserved, so if we want to double our output force, we would need to use our normal input force over twice the distance the output would move.
You can apply it to any of the simple machines (pulleys, levers, ramps, etc) but here we are using gears. Calculating the mechanical advantage is as easy as calculating the ratio of teeth on two connected gears. A 20 tooth gear rotating a 40 tooth hear would spin the gear half as many rpm, but with twice the force, MA = 2.
If we do the reverse, 40 spinning 20, the gear would spins twice as fast, but only with half the force, and our MA = 1/2.
If you have lego technic stuff, this is super easy to play around with.
A tractor has so much more pulling force, because it uses a lower gear to get more mechanical advantage, but as a result, it can’t go as fast. Trucks can go 65 down the highway, but a tractor can’t for the same reason. It’s a tradeoff between speed and power. I answered a [recent post](https://www.reddit.com/r/explainlikeimfive/comments/vvbnr7/eli5_how_do_gears_in_a_bike_work/ifitybs?utm_medium=android_app&utm_source=share&context=3) about bike gears, and it’s the exact same thing, and can also apply to car gears. If you’ve ever driven a car with a manual transmission, you’ll know it’s very hard to go fast in a low gear, again, it’s mechanical advantage.
“With a big enough lever, I could move the Earth.” Archimedes
Fun fact: on roads you may see “steep grade, trucks use lower gear. This is so the truck, while coasting down the hill, doesn’t have to use the breaks as much. When the wheels turn, the internal resistance of the engine, multiplied by that mechanical advantage will help slow the truck down. This is because the pneumatic brakes on the truck don’t like being used rapidly over and over again in fast succession at high speeds, and this reduces the chance of failure.
Fun fact: if you shift your car down to a lower gear while moving very fast, the engine makes a very loud noise because it is now spinning much faster than it was a few seconds ago and you slow down a lot without your brakes. (This is very bad for your engine and transmission, but people do it to be “cool”)
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