View Poll Results: If there are birds in a truck, will the truck weigh more when the birds are at rest than when they a

Voters
10. You may not vote on this poll
• Yes

3 30.00%
• No

7 70.00%

1. I was watching the mythbusters 8) and this was a myth they were testing.

no. They weight the same

Why is this? I thought that the birds weight would stop affecting the weight when they flew but it doesn't.

2.

3.

4. Originally Posted by shawngoldw
I was watching the mythbusters 8) and this was a myth they were testing.

no. They weight the same

Why is this? I thought that the birds weight would stop affecting the weight when they flew but it doesn't.
Aha !!! I see, so I am not 104 kilogrammes then, I am whatever my weight REALLY is plus however much air is pushing down on me. Great news !!! ill tell my wife !

5. she may hurt you...

6. Wow, from a pedagogical standpoint I find it very interesting how difficult it is to explain problems like this, where the physical effect is very fundamental, but the magnitude of the effect is very small due to the nature of the problem. The effect is so small (although measurable) that people just don't have a feel for it (what't the weight of a pigeon compared to that of the truck?). You'd have to know the basic principles of physics to understand what's going on, because intuition will just as likely confuse you as help you, depending on your background. Ideally, you'd just have to say "actio = reactio", and everyone would be happy.

By the way, if anyone doesn't feel like scanning through the large and wildly distracted thread that william linked to: The bottom line is that birds do not cease to affect the truck when they fly. In wing-powered flight they are supported by the reaction of the momentum their wings impose on the air. The vertical thrust (aka lift) is equal to their weight and imposes the same effect on the truck as the sitting bird.

(More precisely: The effect on the truck will on *time-average* be the same as by the sitting bird, i.e. wing-up/wing-down, and bird-up/bird-down motions will generate a time-varying vertical momentum, but since the birds do not leave the truck, the time-average force is equal to their weight.)

7. Well, I did scan through the thread and it looks to me like you have to make the assumption that the truck is closed in so that there is no net force acting from the outside. When they tested it on Mythbusters, what kind of bed did the truck have?

8. the truck was a cargo truck with a full bed and enclosed sides and top. They added an agitator to the floor to keep the birds from landing but it was a completely enclosed environment. They latter flew a remote control helicopter in the same (or similar) truck with the same result.

9. looks to me like you have to make the assumption that the truck is closed in so that there is no net force acting from the outside
No net force from the outside? Whatever do you mean by that? The truck is subject to atmospheric pressure (among other things), regardless if open or closed. Regarding the effect of birds on the truck, it doesn't matter (much) if the top is made of chicken wire or solid. Sides and bottom need to be solid, though, unless the truck is standing and there is no wind. In fact, the truck will also be affected by any bird flying *over* it, not just the birds inside.

However, there is a reason why you would want it to be closed: It keeps the birds from flying away. :wink:

10. No net force from the outside? Whatever do you mean by that? The truck is subject to atmospheric pressure (among other things), regardless if open or closed.
The atmospheric pressure acts on the top and bottom so there is no net force.

To affect the weight of the truck the birds have to be able to exert a force on something outside the truck. Flying around inside the truck they can exert forces on the truck but no net forces.

11. The atmospheric pressure acts on the top and bottom so there is no net force.
Not really. The pressure underneath the truck is higher than the pressure on top, which results in hydrostatic lift. Just like in water, an object immersed in the atmosphere experiences lift according to Archimedes' principle (keyword: buoyancy). The variation of pressure with altitude is causing this effect in air as in water or any other fluid. I admit it's a very small force acting on the truck (certainly not enough to lift it), but it's not zero. It's equal to the weight of the air the truck displaces.

According to Newton, any object moving at constant velocity has zero net force, i.e. the sum of all forces is zero. For the truck this is true at least in the direction normal to the road surface (on time-average). Maybe this is what you mean to say.

The forces the birds exert on the truck are only components of the net force, and they are not zero by themselves. For example, a bird sitting in the truck exerts a force on the truck equal to its weight, which is compensated by (a part of) the normal reaction force the road exerts on the truck in the opposite direction. Likewise, a bird flying closely over the truck exerts a force on the truck equal to its aerodynamic lift. Again this is counter-acted by the normal reaction force exerted by the road.

12. According to Newton, any object moving at constant velocity has zero net force, i.e. the sum of all forces is zero. For the truck this is true at least in the direction normal to the road surface (on time-average). Maybe this is what you mean to say.
Nope.

I know from your responses you understand the physics of the situation very well, so we are just getting hung up on semantics.

When I said net force, I meant the sum of the upward and downward forces exerted by the bird on the truck, such as to make the axle weight greater or less depending on the birds' activity. After all, that was what the original question was about.

13. When I said net force, I meant the sum of the upward and downward forces exerted by the bird on the truck, such as to make the axle weight greater or less depending on the birds' activity. After all, that was what the original question was about.
Ok, but that's not zero. Removing the birds from the truck entirely (fly out a hole in the wall) will result in a different reading on a scale the truck stands on. The net effect of the birds on the truck is not zero, but equal to the bird's weight. What you can say about the effect: It is the same whether the birds are sitting on the truck platform or flying around inside the truck (and that was the answer to the original question, and the result of the myth bust). Do you agree? If you do, I am still misunderstanding your "semantics".

14. dose this mean that if an areoplane is flying above me, my weight is equal to the weight of plane plus my body weight?

15. Originally Posted by anandsatya
dose this mean that if an areoplane is flying above me, my weight is equal to the weight of plane plus my body weight?
LoL, only if all the weight focuses straight down, which it doesn't. You're forgetting about all that air between you and the plane.

16. LoL, only if all the weight focuses straight down, which it doesn't. You're forgetting about all that air between you and the plane.
Correct. The effect of the airplane high above you is diffused over a large mass of air (mostly behind and below the airplane). However, if you are taking a nap at the end off a runway while an A380 takes off just skimming over you, you will definitely feel a little heavier than usual, at least for the moment.

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