# Large Truck

sloppyb
Posers and Puzzles 30 Mar '10 00:03
1. 30 Mar '10 00:03
A large truck is crossing a bridge 1 mile long. The bridge can only hold 14000 lbs, which is the exact weight of the truck. The truck makes it half way across the bridge and stops. A bird lands on the truck. Does the bridge collapse? Give a reason.
2. 30 Mar '10 00:40
Yep, down it goes, the bridge is supporting truck+bird now.
3. AThousandYoung
30 Mar '10 02:51
No. Bridges are designed with some spare capacity, and 14000 lbs is only reliable to the closest 1000...the truck could really be 13997 lbs or 14006 lbs...the weight of the bird won't make a difference.

In an idealized reality, though, the bridge would collapse a la the straw that broke the camel's back.
4. 30 Mar '10 03:37
Originally posted by sloppyb
A large truck is crossing a bridge 1 mile long. The bridge can only hold 14000 lbs, which is the exact weight of the truck. The truck makes it half way across the bridge and stops. A bird lands on the truck. Does the bridge collapse? Give a reason.
Supposing that the bridge's capacity and the "exact weight" of the truck are equal at the point when the truck first enters the bridge; and then the truck makes it half-way across before the bird lands; then I guess the answer should depend on the comparative weight of the bird and the fuel expended while the truck is on the bridge. Hopefully, either the truck is a fuel guzzler; or it's not a big bird; or both.
5. wolfgang59
Mr. Wolf
30 Mar '10 10:07
Originally posted by LemonJello
Supposing that the bridge's capacity and the "exact weight" of the truck are equal at the point when the truck first enters the bridge; and then the truck makes it half-way across before the bird lands; then I guess the answer should depend on the comparative weight of the bird and the fuel expended while the truck is on the bridge. Hopefully, either the truck is a fuel guzzler; or it's not a big bird; or both.
Not enough information.

If the bridge can only take 14000 lbs then the we know that the [b]weakest[b] part of the bridge can take 14000 lbs. We know nothing about the rest of the bridge. The half-way point could be above a support.
6. 30 Mar '10 13:131 edit
lemon was on right track
7. 30 Mar '10 16:091 edit
Originally posted by wolfgang59
Not enough information.

If the bridge can only take 14000 lbs then the we know that the weakest part of the bridge can take 14000 lbs. We know nothing about the rest of the bridge. The half-way point could be above a support.
Word.
8. 30 Mar '10 19:57
Originally posted by LemonJello
Supposing that the bridge's capacity and the "exact weight" of the truck are equal at the point when the truck first enters the bridge; and then the truck makes it half-way across before the bird lands; then I guess the answer should depend on the comparative weight of the bird and the fuel expended while the truck is on the bridge. Hopefully, either the truck is a fuel guzzler; or it's not a big bird; or both.
Nice one lemon.
one US gallon of diesel weighs 7.15 pounds which is 3.3kg

A large truck does between 10 and 5mpg, so in 1/2 a mile of bridge it will burn between 1/20th and 1/10th of a gallon of diesel, i.e. between 165 and 330 grams

So, how much does the bird weigh?
A sparrow weighs between 24 and 32 grams
A pigeon weighs between 280 and 450 grams

So, the bridge will stay up if it is a little bird, but is likely to fall down if it is a biggish bird.

If he stopped because he got a flat though, he has a little more leeway, I can't find the dimensions a a typical large-truck tire (anybody know?) but I'm guessing that truck tires might hold 100g of air, meaning that the bridge is quite likely to survive if the truck is also of low efficiency.
9. forkedknight
Defend the Universe
30 Mar '10 21:26
Originally posted by iamatiger

If he stopped because he got a flat though, he has a little more leeway, I can't find the dimensions a a typical large-truck tire (anybody know?) but I'm guessing that truck tires might hold 100g of air, meaning that the bridge is quite likely to survive if the truck is also of low efficiency.
Yeah, but if the driver hops down from the truck to fix his flat, he's screwed.
10. 31 Mar '10 00:46
Originally posted by forkedknight
Yeah, but if the driver hops down from the truck to fix his flat, he's screwed.
The driver's weight is applied to the bridge whether he is in the truck, or outside.
11. AThousandYoung
31 Mar '10 01:59
Originally posted by geepamoogle
The driver's weight is applied to the bridge whether he is in the truck, or outside.
If he jumps down, though, more force than simply his weight is applied in reaction as the bridge's normal force decelerates him quickly. If it was only his weight applied, he would not decelerate when he hit the ground.
12. 31 Mar '10 21:211 edit
Originally posted by AThousandYoung
If he jumps down, though, more force than simply his weight is applied in reaction as the bridge's normal force decelerates him quickly. If it was only his weight applied, he would not decelerate when he hit the ground.
But the force might have to be applied for quite some time in order to break the bridge... BTW, Could someone measure the radius, internal radius and width of a large truck tire for me? (and look at what PSI it's rated for)
13. 06 Apr '10 22:02
Originally posted by iamatiger
Nice one lemon.
one US gallon of diesel weighs 7.15 pounds which is 3.3kg

A large truck does between 10 and 5mpg, so in 1/2 a mile of bridge it will burn between 1/20th and 1/10th of a gallon of diesel, i.e. between 165 and 330 grams

So, how much does the bird weigh?
A sp ...[text shortened]... ir, meaning that the bridge is quite likely to survive if the truck is also of low efficiency.
The exact weight of the truck is 14,000lbs .
It makes it half way across a bridge that can only take 14,000lbs .
Is there a driver ? ,because he would weigh enough to make the bridge collapse at the midway point .. ???
14. joe shmo
Strange Egg
06 Apr '10 23:30
Originally posted by iamatiger
But the force might have to be applied for quite some time in order to break the bridge... BTW, Could someone measure the radius, internal radius and width of a large truck tire for me? (and look at what PSI it's rated for)
I dont see how the load distribution will come into play here? (if thats what your getting at?)
15. 08 Apr '10 08:59
Originally posted by joe shmo
I dont see how the load distribution will come into play here? (if thats what your getting at?)
I was thinking it's presumably quite a big bridge, and I'm not sure how it would react to the oscillation set up when the guy jumps down, which will presumably travel along it as some kind of wave. I was probably wrong though and it will break whatever.