It is my understanding that upthrust from a liquid on a body is due to pressure difference on the top of the body and the bottom of the body. How, then, is this fact used in order to derive/work out that the upthrust on a body is equal to the weight of fluid displaced? (For example, if a ball bearing is dropped through water then the upthrust is equal to $\frac 43\pi r^3\rho_{water}.g$, where $r$ is the radius of the ball bearing and $\rho_{water}$ is the density of water.)

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    $\begingroup$ Please search the site before asking questions. This has been asked before many times. $\endgroup$
    – DanielSank
    Apr 5, 2015 at 2:53

1 Answer 1


Imagine your object to be made up of a lot of infinitesimally small "straws" - little cylinders.

Each cylinder has an area $dA$ and a length $\ell$. You know that the volume of such a cylinder is $\ell dA$.

Now look at the pressure difference between the top and bottom of that cylinder: at the bottom, the pressure will be greater by $\rho \ell g$ (where $\rho$ is the density of the liquid, and $g$ is the gravitational acceleration) - that's just the way water pressure works, the pressure exactly supports the weight of the column of liquid above it. The difference in pressure is proportional to the difference in depth, which is $\ell$. So with an area at top and bottom of $dA$, the force on the cylinder is $\rho \ell g dA$ - difference in pressure, times area. But if the volume is $\ell dA$, then the force can be written as

$$F = \rho g V$$

Now if an object is made up of many such cylinders, each experiencing a force equal to the weight of the liquid it displaced, then the entire object will also experience a force equal to the weight of the displaced liquid.

  • $\begingroup$ Thanks, thats just what I was looking for. Is it reasonable to say that water pressure is proportional to depth of liquid because as you go deeper the amount of matter/number of molecules above that depth increases proportionally (more or less)? $\endgroup$ Apr 4, 2015 at 17:10
  • $\begingroup$ Yes that is right - the pressure of a column of water has to be such that it supports the weight of the water above it. And the weight per unit area is $\rho g \ell$ $\endgroup$
    – Floris
    Apr 4, 2015 at 17:22
  • $\begingroup$ @santimirandarp - no because their weight exactly balances the upward force. $\endgroup$
    – Floris
    May 3, 2018 at 0:53

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