A cargo of birds weighs nothing when they take flight, and then double

[lincourtl]

If you're driving a truck full of birds, can you bang the truck and lighten your load as the birds launch into the air? Mythbusters concluded "no" when they tackled this question several years agoâ€â€but that's because they didn't have equipment as high-tech as these Stanford engineers'. The real answer, it appears, is a bit more complicated.

The answer is "yes," but only briefly and then the birds end up weighing twice as much for a brief moment, too, as the downstroke of their wings generate vertical force.

Via Factually.

If you have a rocket which accelerates at 1G (so our hypothetical astronauts don't get crushed), can you marginally lower your delta-v cost to orbit by having the astronauts do jumping jacks? They're exerting downward force on both the jump and "landing" after all. ;-)

[Albert VDS]

The average weight still is the same as when the bird is sitting still.

About your jumping jack suggestion. If they are jumping up and down then they are doing the "same thing" as the bird.

[Mr Shifty]

Hmmm. Not quite what the experiment shows. It doesn't actually measure the force due to the bird's weight at all, just due to variations in air pressure created by its wings. In other words, note that the force at both the beginning and end of the experiment (when the bird is resting on the bars) is zero.

[merendel]

If you have a rocket which accelerates at 1G (so our hypothetical astronauts don't get crushed), can you marginally lower your delta-v cost to orbit by having the astronauts do jumping jacks? They're exerting downward force on both the jump and "landing" after all. ;-)

It would be at best neutral on the fuel mass to orbit and probably detrimental for a variety of reasons. Not the least of which is the human body is not the most efficient converter of mass to energy. The physical exertion of doing the jumping would require bringing along more food so you'd need even more fuel to push the extra food mass into orbit. the jumping itself would produce no measurable gains.

[Agumander]

Doing jumping jacks as the rocket accelerates is a bad idea, for the same reason you don't want to be standing on a skateboard on a subway car as it changes speed. If your weight isn't part of the load, then you aren't being accelerated. You'll find yourself quickly hitting the back wall as the vehicle accelerates around you.

[Mr Shifty]

Doing jumping jacks as the rocket accelerates is a bad idea, for the same reason you don't want to be standing on a skateboard on a subway car as it changes speed. If your weight isn't part of the load, then you aren't being accelerated. You'll find yourself quickly hitting the back wall as the vehicle accelerates around you.

In precisely the same way that you hit the ground after jumping into the air on Earth's surface.

[Silver_Swift]

In precisely the same way that you hit the ground after jumping into the air on Earth's surface.

Well, twice as hard. If you are accelerating at 1G near earth's surface you are experiencing 2G (1G from the acceleration and 1G from the earth's gravity). Should still be pretty doable I guess.

[lincourtl]

About your jumping jack suggestion. If they are jumping up and down then they are doing the "same thing" as the bird.

Note the ;-) at the end of my question.