# Buoyancy and Positive Lift

#### 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?

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#### jcsd

Science Advisor
Gold Member
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

Staff Emeritus
Science Advisor
Gold Member
Yup. Pound-force.

- Warren

#### krab

Science Advisor
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.

#### Syed F. Karim

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

Mentor
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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