Other Fluid Dynamics - AP Physics 2
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Viscosity is defined loosely as internal friction in a fluid. Which of the following fluids would you expect to have the highest viscosity?
Viscosity is defined loosely as internal friction in a fluid. Which of the following fluids would you expect to have the highest viscosity?
The correct answer is molasses. Since this liquid is notably thicker and more difficult to move, it is intuitively sensed that is has the highest viscosity of this group of fluids.
The correct answer is molasses. Since this liquid is notably thicker and more difficult to move, it is intuitively sensed that is has the highest viscosity of this group of fluids.
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A spherical ball of mass 
and radius 
is sinking in water. What is the terminal velocity of the ball?
A spherical ball of mass
We will start with Newton's second law for this problem:
When the ball reaches terminal velocity, we know that the net force on the ball is equal to 0:
There are three forces acting on the ball: gravitational, buoyant, and drag. If we designate any downward force as positive, we get:
(1)
Where:
(2)
Where:
(3)
(4)
Substituting expressions (2), (3), and (4) into (1), we get:
Now we just need to rearranging for velocity.
We have all of these values, so time to plug and chug:
We will start with Newton's second law for this problem:
When the ball reaches terminal velocity, we know that the net force on the ball is equal to 0:
There are three forces acting on the ball: gravitational, buoyant, and drag. If we designate any downward force as positive, we get:
Where:
Where:
Substituting expressions (2), (3), and (4) into (1), we get:
Now we just need to rearranging for velocity.
We have all of these values, so time to plug and chug:
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An object with mass 
and volume 
and reference area of 
is sinking in water with a constant velocity of 
. What is the drag coefficient of the object?
An object with mass
We will start with Newton's second law:
Since the block is sinking at a constant velocity, we know that the net force is 0:
There are three forces acting on the ball: gravitational, buoyant, and drag. If we designate any downward force as positive, we get:
(1)
Where:
(2)
Where:
(3)
(4)
Plugging expressions 2, 3, and 4 into 1, we get:
Now we need to rearranging for the drag coefficient:
Plugging in our values, we get:
We will start with Newton's second law:
Since the block is sinking at a constant velocity, we know that the net force is 0:
There are three forces acting on the ball: gravitational, buoyant, and drag. If we designate any downward force as positive, we get:
Where:
Where:
Plugging expressions 2, 3, and 4 into 1, we get:
Now we need to rearranging for the drag coefficient:
Plugging in our values, we get:
Compare your answer with the correct one above