### All AP Physics 2 Resources

## Example Questions

### Example Question #1 : Fluid Dynamics

Turbulent flow is characterized by __________.

**Possible Answers:**

irregular and chaotic flow

regular streamlines

All of these

low flow rates

**Correct answer:**

irregular and chaotic flow

Turbulent flow is caused by sufficiently high flow rates, and is characterized by chaotic and irregular flow patterns. Streamlines represent the curves tangent to the point of the direction of flow; thus, if a flow is turbulent the streamlines will be chaotic as well.

### Example Question #1 : Fluid Dynamics

Which of the following is an example of turbulent flow?

**Possible Answers:**

The flow of water in the center of a calm stream

The flow of water through a hose

The wisps of smoke from a blown-out candle

Pouring olive oil into a pan

**Correct answer:**

The wisps of smoke from a blown-out candle

The wisps of smoke from a blown out candle start out a smooth, but eventually swirl and become chaotic and turbulent. The other examples can be intuitively thought of as smooth flow, and thus are laminar.

### Example Question #3 : Fluid Dynamics

Methanol is traveling through a cylindrical tube with a diameter of . What is the maximum velocity of methanol that maintains laminar flow?

**Possible Answers:**

**Correct answer:**

We will use the expression for Reynold's number for this problem:

Where is the hydraulic diameter, and for a cylindrical tube,

Rearranging for velocity:

For laminar flow:

Plugging in our values into the expression, we get:

Methanol has a relatively low viscosity, so it only has a very small range for laminar flow and quickly becomes turbulent

### Example Question #4 : Fluid Dynamics

What is the Reynold's number of water flowing through a circular tube of diameter at a rate of ?

Assume and

**Possible Answers:**

**Correct answer:**

We will use the expression for Reynold's number for this problem:

Where is the hydraulic diameter, and for a cylindrical tube,

Plugging in our values, we get:

### Example Question #5 : Fluid Dynamics

What is the Reynold's number of water flowing through a fully filled rectangle duct that is at a velocity ?

Assume and

**Possible Answers:**

None of the other answers

**Correct answer:**

We will use the expression for Reynold's number for this problem:

For a fully filled rectangular duct, the hydraulic diameter is:

Plugging in values:

Now we can plug our values into the original expression:

### Example Question #81 : Fluids

Honey is flowing through a rectangular duct with width at a velocity of . What is the depth of the honey if the Reynold's number is ?

**Possible Answers:**

**Correct answer:**

We will use the expression for Reynold's number for this problem:

Rearranging for hydraulic diameter, we get:

For a partially filled rectangular duct, the expression for hydraulic diameter is:

Rearranging for height:

### Example Question #12 : Fluid Dynamics

By what factor does the Reynolds's number change for water flowing through a circular tube as the cross-sectional area of the tube gradually triples?

**Possible Answers:**

**Correct answer:**

Let's begin with the expression for Reynold's number:

Dividing the second expression by the first, we get:

(1)

From the problem statement, we are told:

Also, for a circular tube:

Therefore:

(2)

Now we need to determine the change in velocity. From the law of continuity, we know that:

Rearranging:

Where:

Thus:

(3)

Plugging in expressions (2) and (3) into (1), we get:

### Example Question #13 : Fluid Dynamics

A baseball is thrown at a catcher with a high velocity and the ball passes right by the glove of the catcher. Which of the following scenarios will occur because of this?

**Possible Answers:**

The ball will be pulled toward the glove because of the increased air pressure.

The ball will be pushed away from glove because of the increased air pressure.

The ball will be pulled toward the glove because of the decreased air pressure.

The ball will be pushed away from glove because of the decreased air pressure.

None of these situations will occur.

**Correct answer:**

The ball will be pulled toward the glove because of the decreased air pressure.

As the baseball passes by the glove, the air surrounding the glove increases in velocity. The increase in air velocity will cause a decrease in pressure. The decrease in pressure causes the ball to be pulled towards the glove.

Therefore the correct answer is that the ball will be pulled toward the glove because of the decreased air pressure.

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