Hacker News Reader

Comment by whatever1 | Hacker News Reader

According to Wikipedia a godwit weighs ~0.5kg and it flew 7000 miles. If my calculations are right we are looking at 1.5kwh of energy and 0.2 wh per mile?

Even if we assume that the bird is almost all fat and fat can store ~3500kcal per pound, the maximum stored energy is 4kwh. That gives us 0.8wh per mile. So we can safely say that the bird spent [1.5-4kwh] to do this trip.

Teslas that are very efficient are in the order of 200 wh per mile.

What the heck we have a lot of work to do.

View on HN

I don't think comparing a car to a bird is a good comparison. Compare the bird to a glider. The world record appears to be 3000km without a power source (except for the launch, obviously). Surely as a bird or a plane you can take advantage of lifts, i.e. warmer air rising up, then just glide.

Wow that sounds insane. How much luck do you need to achieve that though ? Can I consistently fly from point a to point b using air streams?

It needs a lot of planning. You can’t consistently do these trips.

Some birds also eat flying insects as a source (if not primary source sometimes) of food. I don't know if this is true of the godwit or not. For example the swift eats airborne insects and has been recorded to stay up in the air for 10 months [0]

[0] https://www.npr.org/sections/thetwo-way/2016/10/27/499635084...

Even more incredible since the article points out that "Unlike albatross or other long-flying seabirds, godwits are active flyers, not gliders—their wings are moving the whole time." I wonder if they can use the active-flying equivalent of regenerative braking in favorable wind conditions to actually generate energy while flapping their wings. Not sure if any animals can generate energy while walking downhill.

The body's only energy store is ATP, which you can't generate from nothing

This isn't quite true. When muscles stretch, they store energy like a spring does. But for very long, of course, but if the cycle is repeated many times that's still many times more efficient than not storing it. Don't know if that applies here though.

Same for humans and running, storing energy in the achilles tendon if running barefoot or in shoes without damping.

The Law of Conservation of Energy heartily disagrees

I suppose they could swoop down for a quick bite or two, right? Some of these birds have a really good vision and can spot fish easily from quite a distance and if they are flying over the ocean, there isn’t any shortage of fish there.

The record for really small "trains" is 0,52 Wh/km (0,8 Wh/mile) https://www.delsboelectric.se/resultat

Godwits aren't carrying a bunch of fat mammals around.

Try normalizing by mass.

A kilo of tesla cannnot travel a single mile. Half kilo of this bird can travel 7000 miles non stop.

It makes no sense to normalize by weight because it does not scale by weight.

Mass isn't great either, due to scaling laws. Physics simply behaves differently at different scales.

An ant can lift a hundred times its body weight, but if you scale it up to human size it collapses under its own weight and immediately dies.

You can drop an injection moulded plastic toy car a hundred times its height into the ground and it won't even dent. If you make a car frame out of the same materials and with the same techniques it will likely fall apart before it's off the conveyor belt.

If you make a scale model of a planetary, you'll struggle getting it to start spinning around its center of mass through gravitational forces.

a good explainer video for scaling of mass: https://www.youtube.com/watch?v=f7KSfjv4Oq0

This is true, but irrelevant here. The bird is moving much less mass, so it takes less energy per mile traveled. I was trying to point out that if you normalize the energy consumption of a Tesla by the mass it is moving, you get a much more favorable (and sensible) comparison.

Reply on news.ycombinator.com