AP Physics 2 : Other Fluid Dynamics

Example Questions

Example Question #1 : Other Fluid Dynamics

Viscosity is defined loosely as internal friction in a fluid. Which of the following fluids would you expect to have the highest viscosity?

Molasses

Air

Water

Gasoline

Molasses

Explanation:

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.

Example Question #2 : Other Fluid Dynamics

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?

Explanation:

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:

Example Question #3 : Other Fluid Dynamics

A spherical ball of mass  and radius  is sinking in water. What is the terminal velocity of the ball?

Explanation:

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: