szabo23: One thing you might like to think about: imagine an enclosed truck completely filled with water, and fish swimming around in the water, instead of a truck full of air and birds flying around. There's no fundamental difference - just a different fluid. Now, when a fish swims upwards, what happens to the weight? When it swims downwards, what happens? And should there be any difference for birds in the air?
I don't know if this adds any weight to the argument but I took this pic this evening and thought it was sort of appropriate for this thread. It's the flight tracks of silver gulls. Please Register or Log in to view the hidden image!
JamesR: "when a fish swims upwards, what happens to the weight? When it swims downwards, what happens? And should there be any difference for birds in the air?" Our scales would register the vertical accelerations more instantaneously, because of the far lower compressibility/elasticity of liquid water compared with air. What happens would be a lot like standing up straight on a scale and pumping your arms rigidly up and down: A good scale would register the vertical accelerations almost instantly. But this assumes heavier-than-water fish, which is not the norm. Fish in water or balloons floating on air would register displacements and wakes, but not accelerations in a fluid that they are nearly equivalent to in mass/volume. Back on your own scale, bend your knees and move your arms up and down in a rubbery way, and the readings are closer to the effect recorded in our hypothetical of a bird flying in a sealed truck sitting on a scale- Total (and constant) weight of the truck and its contents is displayed, + - the delayed/damped vertical accelerations of any birds flying within. Turn the walls of the truck into a cage allowing air to flow through the confined area, and most of the weight of each bird is removed with their takeoffs. Most (not quite all) because on the cage-truck, some bird wake and probably some droppings (stupid truck) may impinge aerodynamically on the despised prison. For me, opening it for their escape instantly takes a load off a real and every imagined mind.
Are you saying that "pumping your arms rigidly up and down" while standing on a scale changes your weight ? lol Please Register or Log in to view the hidden image!
Your mass remains the same, but your weight as registered on a scale would fluctuate with the vertical accelerations of your arms in this demonstration. When I fly aerobatics, my 170-lb personal weight in my seat varies between 680 lbs/309 kilos, through zero, to an upward pull on the seat straps of 340 lbs/155 kilos.
Isn't that impulse that registers, not weight ? Besides, it's moving up and down the whole system, not some tiny part of it.
We're touching on some particulars of terminology here: Weight is measured as a load, which is a summary of inconstant, inconsistent forces, especially in the case of parts in vertical motion being measured on a load sensor or scale. Strictly speaking, near the Earth's surface Weight and Mass are numerically (but not conceptually) equivalent. Mass is different, because it is a measure of a typically fixed amount of matter in a given object or volume of space, regardless of acceleration or gravitational forces acting on it.
But.. if you throw a rock on the ground it has more weight at the moment of impact than when it's just lying on the ground ? I disagree, it's impulse force.
Right you are- and the scale we were pondering in the Birds on a Truck thought experience would register it. I'm only pointing out that in our imagined sealed box with birds flying up and down within, the weight registered on a sensitive scale beneath would fluctuate.
Perhaps. But I would still say that the weight of the whole system didn't change, despite the changing readings on the scale.
That's right. The weight is technically constant, and the readings fluctuate with inertial changes of the package.
OK. Let's go back to the start (or almost the start): James, you forgot something here. The truck has air in it as well as birds (about 25 to 30 kilograms of air for a typical 16 foot truck, sorry about the mixed units). That air is quite important. More on this later. That's assuming a bit too much, IMO. The birds rise to fly. More on this later, too. This is key. A spring scale does not measure "weight". The weight of some object is tautologically defined as the object's mass times the acceleration due to gravity. This definition might be of interest to aircraft designers, but it is not very useful IMO because there is no way to measure an object's weight. It is also not very useful to this problem because the truck's actual weight does not change. Answering the question with a tautology is not a good answer, and it does not answer the intent of the question. A spring scale measures "apparent weight". The apparent weight of some object is the net sum of all real forces acting on the object. Gravity is not a real force; it doesn't count. (That gravity is a fictitious force rather than a real force is why there is no way to measure an object's actual weight.) There are a couple of implicit simplifying assumptions here. First and foremost is the assumption that the system center of mass does not change. This is not the case as the birds and air move around. Secondly, this argument assumes internal forces transmit instantaneously. This also is not true. The group velocity of the pressure waves induced by the flying birds is considerably less than the speed of sound. All bets are off if the truck is not enclosed. So assume the truck is enclosed, and airtight. This is a false analogy. Birds do not fly in a vacuum. The air is a very important part of the truck+birds+air system, and this analogy takes the air out of the system. That is an incomplete and inaccurate view of flight. Think about it from the perspective of Newton's third law. Some force has to act on a bird in level flight to counter gravity, and by Newton's third law, there exists some force that is equal but opposite to this force that buoyant force. Birds maintain level flight by directing air downward. The mechanism you are missing is the downward air flow. This flow creates a pressure imbalance inside the truck and a pressure wave that hits the floor of the truck. The pressure variations created by the birds' flapping will travel at slower than the speed of sound. You can see this in the Earth's atmosphere. Frontal boundaries travel very slow compared to the speed of sound. What one should see with a sensitive scale (and what the MythBusters did see; I watched that episode) is a slightly varying measurement (apparent truck+bird+air weight) as the birds flap around. The mean will of course be the apparent weight prior to scaring the birds into flight, and the variation will be a lot less than the total actual weight of the birds. The MythBusters saw these variations but hand-waved them away as "noise". One problem with this explanation: the scale measurement was a nearly a flatline prior to making the birds take off. The "noise" didn't appear until the birds took to flight.
Which, as I pointed out would not be a stable value. Only over a longer period of time would the average 'weight' be the same.
I think it is a good analogy, not at all false. The essence of the question / problem is to realize that a force must be applied to accelerate ANY mass (bird or cannon ball, etc.) inside the truck. By Newton's 3d law there is an "equal and opposite" reaction force on the floor (with the air being an intermediary) associated with the vertical acceleration. When this reaction force is transmitted to the floor of the truck by downward moving air intermediary, it is less obvious than if by the rails of the electromagnetic rail gun bolted to the floor. ALL analogies MUST change some aspect of the problem. As the air as the means of the 3d law reaction force pressing down on the floor is what is obscuring understanding, my analogy replaced that aspect of the original problem to make understanding of the fact there is a reaction force acting on the floor easy.
Understand too, that there is also aerodynamic interaction with the ceiling of the truck, and friction with the walls- all part of the closed system expressing Newtonian action/reaction and Bernoullian aeroelasticity. Enmos: "So the weight didn't change.. you agree ?" Yes. But a sensitive scale would register the averaged and damped vertical accelerations of the hermetically-captive birds. Or to simplify- a decent scale would also be able register an indication of the vertical motions (and emotions) of a single big fat unhappy bird. By the way, flying freight, I have on occasion transported carrier pigeons. I used to stack up their cages in the right seat. They always seemed very interested in the flight deck. I even caught some ogling the GPS when they thought I wasn't looking.
