Buoyancy and Positive Lift

Let's say that I rig up a truck to a giant helium balloon, but I load up the back of the truck with enough stones so that the truck has reached a state of neutral buoyancy--its just floating in mid-air, not rising not falling, just there. Now let's say I take away a one-pound rock from the truck's bed. Does the truck now have a positive lift of one pound? And will it now rise just like a small balloon with one pound of positive lift?
 

jcsd

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Originally posted by Syed F. Karim
Let's say that I rig up a truck to a giant helium balloon, but I load up the back of the truck with enough stones so that the truck has reached a state of neutral buoyancy--its just floating in mid-air, not rising not falling, just there. Now let's say I take away a one-pound rock from the truck's bed. Does the truck now have a positive lift of one pound? And will it now rise just like a small balloon with one pound of positive lift?
Yes, when the truck has neutral buoyency the forces acting on it perpendicalr to the ground are in equilibrium, by removing 1 pound you are removing 1 pound* from the downward force which means that there will be a net lift of 1 pound*.

* Isn't it pound-force? I I've never used anything other than the SI system in mechanics.
 

chroot

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Yup. Pound-force.

- Warren
 

krab

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Originally posted by Syed F. Karim
Let's say that I rig up a truck to a giant helium balloon, but I load up the back of the truck with enough stones so that the truck has reached a state of neutral buoyancy--its just floating in mid-air, not rising not falling, just there. Now let's say I take away a one-pound rock from the truck's bed. Does the truck now have a positive lift of one pound? And will it now rise just like a small balloon with one pound of positive lift?
Yes it has positive lift of one pound, but No it does not rise just like a small balloon. It still has very large mass, so that one pound lift results in very small acceleration.
 
Yes it has positive lift of one pound, but No it does not rise just like a small balloon. It still has very large mass, so that one pound lift results in very small acceleration.

If a small balloon with a positive lift of one pound has an ascent rate of 1000ft/min, would a 1000lb-truck with the same one-pound positive lift have an ascent rate of just 1ft/min? Is it an inverse relationship? How do you calculate the rate of ascent of massive bodies? (This is not a homework problem, I am an inventor.)
 

russ_watters

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Originally posted by Syed F. Karim
If a small balloon with a positive lift of one pound has an ascent rate of 1000ft/min, would a 1000lb-truck with the same one-pound positive lift have an ascent rate of just 1ft/min? Is it an inverse relationship? How do you calculate the rate of ascent of massive bodies? (This is not a homework problem, I am an inventor.)
a=f/m
 

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