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Please take a look at the picture below before read my question:

enter image description here

As you see in the picture the object A is floating on the water because the $F_{drag}\gt F_{grav}$ obverse the object B is sinking in the water because the $F_{drag}\lt F_{grav}$.

But the problem is: How can the drag force $F_{drag}$ (Water resistance force) for the object A be greater than gravity force $F_{grav}$ while the speed of it equals to $v=0\,m/s$ (rest object) and thus the drag force $F_{drag}$ (Water resistance force) should be less than gravity force $F_{grav}$ and the object should sink ?

Example: If the density of the fluid (water) $\rho=998\,kg/m^3$, speed of the object A $v=0\,m/s$ (rest object), the cross-sectional area $\mathrm A=3\,m^2$, the drag coefficient $C_d=0.47$ and the mass of the object A $m=0.005\,kg$.

$F_{drag}=\frac{998\times0^2\times0.47\times3}{2}=0\,\mathrm N$

$F_{grav}=m\,a=0.005\times 9.8=0.049\,\mathrm N$

$F_{drag}\lt F_{grav}$ (object A should sink)

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You need to consider the buoyancy force also. What you really want to do is consider the force $|F_{grav}|-|F_{buoyancy}|$. If this is larger than zero, the object will begin to sink. Then you can look at applying the drag. – Will Jul 16 '13 at 16:14
Objects float in water when their weight is balanced by the upthrust from the water. The object will sink until the weight of the water it pushes out of the way is the same as the weight of the object. if it sinks its density is much more higher than the fluid – Hash Jul 16 '13 at 16:27
In other words, the diagram should have $F_{grav}<F_{submerged~buoyancy}$. – Will Jul 16 '13 at 16:28
To take this even further and make it really obvious that there are two forces at play, if you submerged a buoyant object and it began to rise (due to the buoyant force), then the drag would act in the other direction, slowing its rise. – Kyle Oman Jul 16 '13 at 16:44
@Will Thank you – Mohammad Fakhrey Jul 16 '13 at 17:00

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