How do I draw all forces on a stone sinking in water?

[Karagoz]

Homework Statement

How do I draw all forces on a stone sinking in water? And what are the names of the forces? And will the stone reach terminal velocity and be sinking in constant velocity?

Homework Equations

In some websites they just draw buoyant force and gravity force. But in other websites they say that both buoyant force and fluid drag force that push the object upwards. Is it only the buoyant force that push it upwards, or both buoyant force and fluid drag force that push it upwards?

The Attempt at a Solution

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[Dr.D]

If it is continuing to fall, there is fluid drag as well as the buoyant force acting upwards while the weight acts down.

 

[Karagoz]

If it is continuing to fall, there is fluid drag as well as the buoyant force acting upwards while the weight acts down.

Then is this the correct way to draw the force vectors?

 

[Abhishek kumar]

Then is this the correct way to draw the force vectors?
View attachment 215313

If body moves in downward viscous force(fluid drag force) acts vertically upward

 

[CWatters]

You would normally just draw one upward arrow for the buoyancy force.

If it is falling at a constant (terminal) velocity then the net vertical force is zero so

Fgravity + Fboyancy + Fdrag = 0

If it's still accelerating then the sum won't be zero.

 

[Dr.D]

What you have labeled "buoyant forces" are actually the pressure loads. The true buoyant force is the vector sum of the pressure loads.

 

[Nik_2213]

How much difference will vortex shedding affect drag ?

 

[Dr.D]

How much difference will vortex shedding affect drag ?

This is a part of the overall drag, so the answer to your question is simply, "some."

 

[Karagoz]

What you have labeled "buoyant forces" are actually the pressure loads. The true buoyant force is the vector sum of the pressure loads.

I have used pressure loads instead of buoyant force. Then this is the correct?

Is the fluid drag force bigger than the buoyant force when the object is in terminal speed?

 

[jbriggs444]

I have used pressure loads instead of buoyant force. Then this is the correct?
View attachment 215327

Is the fluid drag force bigger than the buoyant force when the object is in terminal speed?

Consider two cases.
1. There is a bowling ball with a density of 1.01 times that of water settling slowly to the bottom of a swimming pool. Is the buoyant force greater than or less than the drag force?

2. There is a very small small lead pellet (density approximately 11 times that of water) settling slowly to the bottom of a swimming pool. Is the buoyant force greater than or less than the drag force?

 

[CWatters]

There is more than one way to draw the forces depending on what you are interested in. If you were interested in the forces that effect the velocity of the stone then I would draw a Free Body Diagram something like this... The buoyancy force is equal to the weight of the fluid (eg water) displaced by the stone.

If the stone sinks at a constant velocity the three forces (vector) sum to zero..

Fg + Fb + Fd = 0

 

[CWatters]

Minor correction: By convention Free Body Diagrams only show ONE object and the forces acting on that object. So as we are only interested in the forces on the stone it's not quite correct/bad practice to show the water as well.