# Birds in a truck

Discussion in 'Physics & Math' started by BenTheMan, May 20, 2007.

1. ### BenTheManDr. of Physics, Prof. of LoveValued Senior Member

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I heard this riddle somewhere and have been thinking about it. I wonder if yall will come to the same conclusion.

Suppose you have a cargo truck full of birds that are asleep. You weigh the truck. Now you wake the birds up and the all start flying about , so that no more of them are asleep on the ground. You weigh the truck again.

Does the truck weigh the same as it did before?

3. ### AbsaneRocket SurgeonValued Senior Member

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The truck weighs the same.

For every action, there is an equal and opposite reaction.

5. ### BenTheManDr. of Physics, Prof. of LoveValued Senior Member

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Yeah this is the conclusion I came to, too.

The lift caused by the bird's wings exerts a downward force on the truck.

7. ### temurman of no wordsRegistered Senior Member

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It depends on whether the truck is closed. If not, some of the force caused by the bird's wings can go off the truck. If it was not, imagine you are walking under a bunch of birds and you should feel they are pushing you down, even if they are far high.

8. ### 2inquisitiveThe Devil is in the detailsRegistered Senior Member

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The truck does not weigh the same. The birds are expending energy, by flapping their wings, to negate their weight.

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10. ### BenTheManDr. of Physics, Prof. of LoveValued Senior Member

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Well...to first order

The change in mass due to the birds burning calories is like 1/c^2. If you can find a scale to measure this (on top of the mass of the truck), then I'll concede the point

Yes, of course.

Good! I love MythBusters, but have never seen this episode.

11. ### 2inquisitiveThe Devil is in the detailsRegistered Senior Member

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BenTheMan,
But your OP did not say mass, it said weight. The weight of the truck.

12. ### temurman of no wordsRegistered Senior Member

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What Ben means is that the air becomes heavier when the birds are flying and if you carry a closed box with birds flying inside it the weight would be the same as the birds were sitting.

13. ### BenTheManDr. of Physics, Prof. of LoveValued Senior Member

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The two are proportional? Multiply by g, if you must...

14. ### BenTheManDr. of Physics, Prof. of LoveValued Senior Member

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You really are quite the nit-picker...

15. ### James RJust this guy, you know?Staff Member

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Complete explanation:

Take the (enclosed) truck plus the birds to be the system of interest.

The net external vertical force on the system as it sits on the scales is zero, since it is not accelerating. The external forces are the weight (downwards) and the reaction force from the scales (upwards). To get zero net force these two forces must be equal. The scales register the reaction force (actually the equal and opposite force of the system pushing down on the scales).

The same argument applies whether the birds are flying around or stationary in the truck. Any forces exerted by the birds inside the enclosed truck are forces internal to the system as defined, and so can have no effect on the registered weight.

If the truck is not enclosed, things may be different, since there can be unaccounted-for external forces in that case.

16. ### 2inquisitiveThe Devil is in the detailsRegistered Senior Member

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The birds displace the same amount of air whether they are flying or sitting. Weight is a measurement of force.
If I stand on a bathroom scale, then launch myself into the air, the scale will register the increase in force (weight) as I launch. It will then register zero force (weight) while I am in the air, even if I seal the bathroom doors (an enclosed system).
The initial launch of the birds from the floor of the truck will cause an increase in the weight of the truck, both from the force they exert as their feet push downwards, and the force of the air on the nearby surface. The birds could freefall back to the surface from a heigth. The truck would weigh less during their freefall.
Also, do you believe a scale covering the entire floor inside an enclosed room would not register a change as the bird left the floor and flew around the room?

17. ### James RJust this guy, you know?Staff Member

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Not if the truck is airtight.

18. ### iceauraValued Senior Member

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The birds are not freefalling, but maintaining their height by pushing down on the air, which in turn pushes down on the floor, which in turn pushes down on the scale.

Birds flying around a room with a scale on the floor are pushing down on air which is not all pushing down on the scale. Most of the weight of the birds is being supported by the floor of the room directly, not through the scale.

Consider: a truck with an airtight enclosed box holding a bag of balloons and a tank of helium. While the truck is sitting on the scale, the balloons are inflated and rise to the ceiling, which they push on. Does the truck weight change?

Can you tell just by weighing the truck whether the balloons are inflated or not?
This is so, but not the description of the problem.

Last edited: May 21, 2007
19. ### 2inquisitiveThe Devil is in the detailsRegistered Senior Member

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Consider: a truck with an airtight enclosed box holding a bag of balloons and a tank of helium. While the truck is sitting on the scale, the balloons are inflated and rise to the ceiling, which they push on. Does the truck weight change?
Consider: an airtight truck with a smaller balloon inside attached to a heating device, such as a bunsen burner. Ignite the bunsen burner to heat the air in the balloon, lifting the burner itself off the floor. Does the weight of the truck, weighed by scales placed underneath the tires, change?

20. ### 2inquisitiveThe Devil is in the detailsRegistered Senior Member

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Consider a fast moving car topping a hill. The tires can potentially lose contact with the surface of the road due to the acceleration force. No weight will be on the tires or the road surface at that point. The mass of the car will not change. Weight is frame dependent and can be offset, unlike rest mass.

21. ### James RJust this guy, you know?Staff Member

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In both cases, the answer is: the truck's weight doesn't change.

Agree?

22. ### PeteIt's not rocket surgeryModerator

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True, but not relevant to the OP, in which the truck is weighed while its center of mass is presumed to be not accelerating.

23. ### BenTheManDr. of Physics, Prof. of LoveValued Senior Member

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Sure, a scale INSIDE the truck. Not the scale UNDERNEATH the truck.

This is beside the point. The truck is at rest, there are no reference frames at issue here.

I will change my answer, too, about the change in mass (and thus weight) being proportional to 1/c^2.

If the birds burn energy to stay aloft, that energy must go somewhere---i.e. it heats up the truck. Energy lost by the birds = energy gained by the ambient air inside the truck. So there will be no change in mass of the truck (and therefore no change in the weight) if the birds are isolated.