ELIF: How does drafting work? Is it possible for humans to run behind a large vehicle that’s traveling at speeds faster than they are capable of reaching?
In: Physics
drafting creates a vacuum effect behind whatever it is that is moving, the wider the back side the stronger the effect, same for speed of the moving thing. you would have to be able to catch up to the speeding thing before getting caught in that vacuum, or be able to maintain speed with the thing until it goes fast enough to make a good enough vacuum. a human cant run fast enough to take advantage of drafting. if you drop a human behind a thing going fast enough to make a usable vacuum for drafting, they wouldnt be able to move their legs fast enough to keep up and the draft would probably effect them just enough to make them tumble for longer after they fall. [a person on a bike can do it](https://www.reddit.com/r/WTF/comments/41mh2p/cyclist_drafting_behind_a_truck_at_50_mph/)
Drafting is a way of taking advantage of another object’s work to make your work take less energy. For example, if person ‘A’ is running, the force they have to exert to move forward is the amount of force to move their body plus the amount of force necessary to push the air ahead of them out of the way. Now the air that gets pushed out of the way swirls around person ‘A’ and fills in a semi-vacuum behind them. If person ‘B’ can run close enough behind person ‘A’, they won’t have to exert as much force to move the air in front of them, and can possibly even get a little push from behind from the air filling in behind person ‘A’ Now ‘A’ and ‘B’ are running at the same speed, but ‘B’ is using a lot less energy than ‘A’ to maintain that speed. When ‘A’ eventually tires and slows down, ‘B’ can now kick in that energy that was conserved by drafting ‘A’ and continue running for a longer period of time.
Team races, like bicycle racing, use this as a strategy. A team of racers will draft in a single line, rotating the lead position among the lower ranked members of the team to preserve the energy/endurance of the star member, who will be unleashed toward the end of the course to sprint into the lead and win that leg of the race.
It’s also why birds fly in a ‘V’ formation. The trailing birds are drafting in the wake of the lead bird and they’ll rotate that role among themselves allowing the group to fly further in a day than any individual bird could alone.
Drafting reduces the air drag forces by moving within a pocket of low pressure air left behind a vehicle or object moving forward in front of you. As a big truck (for example) drives along a road it has to push the air out of its way and it takes some time for the air to collapse back in behind it as it moves. So there’s a slipstream in its wake (kind of like the wake left in water as a boat moves through it) that has less air in it. Since drag opposes forward motion, then it allows for a car behind the truck to move forward faster with the same engine RPM and fuel consumption (or the same speed with less engine RPM and fuel consumption).
In theory the idea of drafting would work with runners as well, but the effects are likely negligible (Not only do runners not run fast enough for drag to be a big factor is slowing them down, but also the object in front needs to either be moving very fast or be very large to create a big enough area behind it that you could fit inside and get the benefits from the slipstream). Race Cars take advantage of it from like-sized other cars because they’re moving so quickly. Highway driving takes advantage of it because of not only speeds, but the large size of the big trucks behind which you can draft. Two runners on a track probably don’t get MUCH benefit (but technically should get SOME).
There was a recent record attempt sponsored by a running shoe company (the one hour marathon?) using runners instead of a vehicle. As it wasn’t allowed for official sporting records they made up their own rules and swapped runners in and out like the geese example mentioned in another post. They had real trouble coordinating the runners, so I reckon you should recommend your idea to the company as an improved method!
When predicting drag, you look at the cross sectional area of the object, the drag coefficient, and the speed^2 of the air flowing at it. You can’t change area or drag coefficient externally, but the speed of the air can be changed. If the air behind a bus is half the speed of the bus, then that’s 1/4 the drag. That’s why running downwind is easier also.
The limit to human running speed is how fast we can move our legs forward and back. If you hung from a rope and moved your legs as fast as you could forward and back then the speed your feet reached would be about your max running speed.
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