Question

If air is a fluid, why do we not include the buoyant force caused by the displacement of air by the objects in this experiment? Explain your answer carefully. (Hint: Pwater = 1000 kg/m^3)

Answer 1

Archimedes' principle allows us to find that the reasons why the thrust is not written when a body is in the air is:

• The thrust of air is about 800 times less than the thrust of a fluid

• In general the other forces (weight, tension) are much greater than thrust

Archimedes' principle establishes that the thrust is equal to the weight of the dislodged liquid (fluid)

        B = ρ g V

Where B is the thrust, ρ and V the density and volume of the fluid, respectively, g the acceleration due to gravity.

In the attachment you have a diagram of a system in equilibrium in air and water, we can see that in the two cases for a system in equilibrium

                         B -W = 0

                         B = W

Let's find the value of the thrust in each case and compare

               $B_{air} = \rho_{air} g V_{air}$

               $B_{water} = \rho_{water} \ g V_{water}$

Used the density

               $\rho_{air} = \ 1.26 \ kg/m^3 \\\rho_{water} = 997 kg/m^3$

               $B_{air} = 1.26 \ g \ V_{air}\\B_{water} = 997 \ g \ V_{water}$

Suppose that the volume of the two bodies is the same

               $B_{water} = 792 B_{air}$r

           

We can see that the thrust in air or other gas is about 800 times less than the thrust in liquids. This is the reason that in many problems the thrust is not written when the body is in the air.

In conclusion, using Archimedes' principle, we find that the reason why the healed thrust is not written for a body is in the air is:

• The thrust of air is about 800 times less than the thrust of a fluid

• In general the other forces (weight, tension) are much greater than thrust

Learn more about Archimedes' principle here:

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