Student Exploration: Fan Cart Physics

Student Exploration: Fan Cart Physics

Vocabulary: acceleration, force, friction, mass, newton, Newton’s first law, Newton’s second law, Newton’s third law, velocity

Prior Knowledge Questions (Do these BEFORE using the Gizmo.)

1. Imagine a horse pulling a cart. What would happen to the speed of the cart if several bags of cement were added to the cart? The speed of the cart would decrease.

2. Suppose several more horses were hitched up to the same cart. How would this affect the speed of the cart? This would affect the speed of the cart because there is more force pulling the positive direction.
Although these questions may seem simple, they form the basis of Newton’s second law of motion. The Fan Cart Physics Gizmo™ can be used to illustrate all three of Newton’s laws.

Gizmo Warm-up
The Fan Cart Physics Gizmo™ shows a common teaching tool called a fan cart. Place fan A on the cart and turn it on by clicking the ON/OFF button below.

1. Look at the blue lines coming from the fan. In which direction is the air pushed? Negative

2. Press Play ([pic]) and observe the cart. In which direction does the cart move? positive direction By blowing to the left, the fans exert a force on the cart that pushes it to the right. This illustrates Newton’s third law: A force in one direction results in an equal force in the opposite direction.

3. The velocity (v) of the cart is its speed and direction. Click Reset ([pic]). Select the BAR CHART tab, and click Play. Does the velocity change or stay the same? Stays the same A change in velocity is called acceleration (a)
|Activity A: |Get the Gizmo ready: |[pic] |
| |Click Reset. | |
|Newton’s first law |Remove all fans from the cart. | |

Question: What happens to the cart when there is no force?

1. Form hypothesis: What will the motion of the cart be like when there is no force at all? (There is no friction in this model.) There will be no motion if there is no force.
2. Predict: Suppose a cart with no fans has a starting velocity of 2 m/s. What will be the velocity of the cart when it reaches the wall? 20 m/s.

3. Experiment: Check that there are no fans on the cart. On the DESCRIPTION tab, set the Initial velocity of cart to 2.0 m/s. Select the BAR CHART tab, and click Play.

What do you notice about the velocity of the cart? The velocity is constant.

Experiment: Click Reset. Place two fans on the cart, and point them in opposite directions. (Next to DIRECTION, click the [pic] button for one fan.) Turn both fans on, and click Play.

What do you notice about the velocity of the cart? The velocity is constant.

4. Analyze: Select the GRAPH tab.

A. What do you notice about the graph of position vs. time (x vs t)? It is a straight line, which indicates a constant relationship between position and time . B. What does the velocity vs. time (v vs t) graph show? The velocity vs time graph shows a constant velocity. C. What do you notice about the graph of acceleration vs. time (a vs t)? The acceleration vs time graph shows an acceleration of 0 which means the velocity is not changing.

5. Draw conclusions: Newton’s first law states that an object in motion will travel at a constant velocity unless acted upon by an unbalanced force. How do these experiments show this?

The experiments demonstrate Newton’s first law because the fans act as the unbalanced force. If the fan is blowing in one direction, it will make the cart accelerate, and if there are two fans blowing in opposite directions, the cart will remain at a constant velocity because it is a balanced force.

|Activity B: |Get the Gizmo ready: |[pic] |
| |Click Reset. | |
|Newton’s second law |Set the Initial velocity of cart to 0.0 m/s. | |
| |Place three fans on the cart, all blowing to the left. | |

Question: How do mass and force affect acceleration?

1. Experiment: Turn on the fans. Click Play and watch the cart, then select the TABLE tab.

A. Scroll to the bottom of the table. What is the final velocity of the cart? 4.8 m/s

B. How long did it take the cart to reach the end of the track? 4 s

2. Calculate: Acceleration is a measure of how much the velocity of the cart changes each second. To calculate acceleration, divide the final velocity by the amount of time it took to reach that velocity. The units of acceleration are meters per second per second, or m/s2.

A. What is the acceleration of the cart? (Include units.) 1.2 meters per second per second.

B. Check your answer on the TABLE tab. Were you correct? Yes.

3. Form hypothesis: a. How do you think changing the mass of the cart will affect its acceleration?

Changing the mass of the cart will affect its acceleration by causing it to accelerate slower than if there was no mass on the cart. a. How do you think the number of fans will affect the cart’s acceleration?

The number of fans will affect the cart’s acceleration by causing it to accelerate faster every time a fan is added

4. Experiment: Select the BAR CHART tab and turn on Show numerical values. For each of the situations below, record the acceleration of the cart.

|Load |Number of fans turned on |Acceleration |
|3 fans, 0 mass units |1 |.4 |
|3 fans, 0 mass units |2 |.8 |
|3 fans, 0 mass units |3 |1.2 |
|3 fans, 2 mass units |1 |.2 |
|3 fans, 2 mass units |2 |.4 |
|3 fans, 2 mass units |3 |.6 |

Activity B (continued from previous page)

5. Analyze: Look at the acceleration values.

A. How did doubling the force affect the acceleration of the cart? The acceleration to double as well. B. Compare the first and third lines of data. How did tripling the force affect the acceleration of the cart? It tripled the acceleration. C. A cart with two mass units and three fans has twice the mass as a cart with just three fans. How did doubling the mass affect the acceleration of the cart? It didn’t affet the acceleration of the cart the acceleration stayed the same.

6. Draw conclusions: Newton’s second law states that force is equal to mass times acceleration: F = ma. This law can be rearranged as a = F / m, or a = F ÷ m.

How does this experiment demonstrate Newton’s second law? This experiment demonstrates Newton’s second law of motion because you can use the data tables to work out the equations.

7. Challenge: The unit of force is the newton (N). One newton is the force required to accelerate a 1-kg object at a rate of 1 m/s2.

Suppose each fan supplies a force of 2 N. Use Newton’s second law and the Gizmo to find the following.

A. The mass of the cart: __________________________________________________

B. The mass of a fan: ____________________________________________________

C. The mass of one of the draggable mass units: ______________________________
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