While the balloons are accelerating at the start and end of their journey, the truck's weight will change by a tiny amount. The final weight is the same as the initial weight. The dynamics are more interesting here, but the final weight is still the same as the initial weight.
Pete, The 'weight' I am weighing is recorded by scales placed under the truck tires. I am only interested in that recorded weight while the balloon/bunsen burner combo is in the air, not the 'final' weight. I realize the mass of the system does not change, except by the consumption of fuel by the bunsen burner.
The reason the truck's weight is the same is simple: birds don't actually weigh anything. The reason that a duck is used in a standard witch-test is because it is essentially weightless, and so is she. If the subject were a normal woman, the scale would tip up and the woman in question would complain loudly about how she only had a salad for lunch. If the subject were, by contrast, a witch, then the scale would remain balanced since witches weigh nothing either, since they are composed of concentrated evil. So, it makes no difference if the birds are in the air or not, since they weigh nothing. I love science.
If the balloon and burner are hovering, then the weight recorded is the same as the initial weight. Actually, the waste gases produced will have the same mass as the fuel+oxygen consumed, so if the truck is airtight the mass won't even change with the fuel consumed.
Only on average. The exact "weight" at any given time depends on whether the birds are generally flying up or down at that time. Birds aren't uranium powered Please Register or Log in to view the hidden image!
It depends on whether the truck's center of mass is accelerating up or down. Once the birds are in flight, this difference would be much less than the mass of the birds, and could be ignored. Doesn't matter! Chemical energy equates to mass in the same way as nuclear energy.
I stand corrected. Are you sure? I read somewhere that total mass could change slightly in chemical reactions, but I'm pretty sure most of the energy released is just potential energy converted to other forms as chemical bonds are broken.
I wouldn't stake my reputation on it, but I think so. At the end of the day, it's all just heat anyway.
Pete, Are you sure, Pete? Please Register or Log in to view the hidden image! We shall see. What does that have to do with anything? We are talking weight, not mass, remember? All of you are getting confused by frames of reference. An airliner taking off from an airstrip has the same mass in the air as on the ground, but the weight recorded scales placed under its tires will change as the airliner gains lift. Weight is an acceleration force, not mass.
I had to make a trip to a store and it hit me while I was driving. I now agree with you. While the bunsen burner/balloon is in the air, it adds to the weight of the air of course. The greater weight of the air pushes down on the floor of the truck to a greater extent. The actual weight of the truck structure alone will be less, but the added weight of the air will equalize the total effect in a closed system. If the air inside the truck is allowed to escape due to the added pressure, then the weight of the truck would be less.
the truck is light but the difference is so small that it doesn't really matter. weight is a factor of the force of gravity acting on a mass the greater the distance between 2 objects the less the force of gravity is so the lighter it is but difference the few feet would make is like i said to small to truely make a diffrence
To someone's: "The initial launch of the birds from the floor of the truck will cause an increase in the weight of the truck," In post 14 James replied: I also think there is a transient increase in weight of the truck. Let me take a more extreme case to show this is true and you are wrong in your reply. (I think): The truck is carrying an almost exactly vertically pointed electromagnetic rail gun cannon, (No black powder or gasses from it burning.) with cannon ball attached to rails, ready to be fired. The box back part of truck is very tall, exactly 1.00 mile tall inside the box, but well sealed and even contains a perfect vacuum inside the box. The cannon is fired and cannon ball rapidly accelerated upwards by force much greater than its weight, but it only goes 0.99 miles high on this initial impulse, so truck remains sealed and evacuated. I think that while the cannon ball is being accelerated by this vertical electric rail gun cannon, the excess force accelerating the ball (above its weight) adds to the weight of the truck. (The weight of the ball does not as it is not being supported by the truck in any way, but ball was supported bytruck before firing. Hence, "the excess" is the increse in truck weight.) Once the rail gun current is again zero, just as the ball flies off then end of the rails, the weight of the truck drop below the original weight by exactly the weight of the ball and stays at this lower value until ball falls back into the sand pile, which is just beside the rail gun, inside the box. This de-acceleration of the ball by the sand causes the weight of the truck to again become briefly greater than the original weight, but the weight of truck drops back towards the original weight (as the sand slows the ball) and when the ball is at rest in the sand pile (and all the scattered sand is also at rest) the truck weighs what it did originally. If you agree, James, why did not the much smaller initial vertical ACCELERATION of the birds not produce an increase in the weight of the truck? Your argument seemed to focus on internal forces in a closed system and I agree that is a valid argument IF NO PART OF THE SYSTEM IS ACCERATING, but that condition is not satisfied by either the bird or the cannon ball. Comments? PS a significant fraction of my posts are efforts to correct someone. I am Sir "Knight,defender of the Truth." It is rare I get the opportunity to help you, but not the first time. Please Register or Log in to view the hidden image! Hope I have not stuck my lance in the ground of error as I sometimes do. In any case, "the truth will out" and that is what makes science fun.
You get no argument from me, Billy T. I held the same opinion, but strayed offcourse with the bunsen burner/balloon example, which has different dynamics.
My vote goes to Billy T. It seems to me that the mean weight of the truck is the same, but a bird accelerating up or down will temporarily change the weight.
I agree. If it's not obvious with birds, instead imagine an elephant jumping up and down inside the truck Please Register or Log in to view the hidden image!
Better (more simple at least) than my rail gun cannon. (I have been a physicist for too long, I guess.) BTW, the birds got out. They got organized, flew from side wall to side wall in resonance with the lateral sway frequency of the truck until it fell over and split open.
But what is the mechanism...??? Hi, I just heard about this question a few hours ago and there's one aspect of it that's driving me crazy, so I hope that by re-opening this thread, someone on this forum can set me straight. OK, so I'm basically satisfied that in a closed system (and for the sake of argument, let's call it airtight), it makes no difference whether the birds are flying or not -- the mass is constant and with no external forces acting, then the weight of the system should remain the same. But what I don't understand is how a bird beating its wings in the air generates a downward force on the floor of the truck that's equal to the weight of the bird. Given a sufficiently large truck (large enough for the birds to fly freely in horizontal circles) how is this reactive force propagated through the air? To elaborate somewhat: as far as I understand it, when a bird flies, air passes over its wings such that the air above the wing is moving faster than the air below the wing. This causes a localized pressure differential -- lower pressure above the wing, higher pressure below -- which results in a net lifting force on the bird's wings. As the bird flies on, it leaves eddies and turbulence in the air behind it. My question is: how does this wake of disturbed air then manage to produce a net downward force on the floor of the closed system? Or is there some other mechanism that I'm completely overlooking?
