In this lesson, you will use a scientific model similar to the one you developed in the previous lesson. Using this model, you will observe the effects of pumping air into a bike tire.

The simple bike tire model will help us understand:

• Learning about the Kinetic Molecular Theory and its main assumptions
• Defining pressure (P) in a gas container
• Thinking about the role of gas pressure in real-life objects and events
• Considering the trade-offs of making simplifications when constructing computer-based models

In Lesson 1, we tried to hypothesize about how gas particles behave in a container. We did so by writing our hypotheses verbally, sketching, developing static computational representations, and finally defining the behavior of the gas particles within a block-based programming environment.

The Kinetic Molecular Theory (KMT) is the scientific theory that is used to explain and predict the behavior of different objects that use air pressure to function such as these:

The main assumptions of Kinetic Molecular Theory (KMT) are the following:

1. A gas is composed of a large number of identical molecules moving in random directions, separated by distances that are large compared with their size.
2. Collision between gas particles occur like collisions between billiard balls (i.e. the total energy is conserved; see elastic collision 🔗). Otherwise, they do not interact. There are no attractive or repulsive forces between the particles.
3. Any energy the particles have is because of their motion only (i.e. kinetic energy).
4. These assumptions are simplifications that describe a theoretical "ideal gas". Most real gases behave qualitatively like an ideal gas.

Before we move on to the implications of KMT, let's first reflect on how these assumptions compare to our gas particle models from the Lesson 1.

Study a physical bike tire and a pump (or equivalent objects chosen by your teacher) in the classroom. Try to observe what happens when you pump air into the tire. As you are doing this, keep the assumptions of KMT in your mind.

Then, familiarize yourselves with the simple computational bike-tire model below. To run it, first click the  button, and then click the  button. If you would like to pause the model, you can click the "➤➤ go" button again.

For now, just experiment with the model freely as you experimented with the physical bike tire and pump. Observe what happens when you run the model with different settings. Please do not spend more than 4 minutes on this task.

Pressure 🔗 is defined as force per unit area. It is represented by the ratio of force to area:

(P)ressure = (F)orce / Surface (A)rea

This means that the pressure on a surface depends both on the force that is applied to the surface and the size of the surface. A larger force on a surface leads to larger pressure. A larger surface with the same force against it leads to a smaller pressure.

What does this definition of pressure imply for our Bike Tire model? What constitutes the "surface area"? Where does the force come from? How do we calculate the pressure in a fixed volume gas container like the one in our model?

We will use our simple Bike Tire model to find answers to these questions. Run the model once or twice without changing any parameters. Just observe the "pressure over time" plot for a while (≈30 ticks). Then answer the questions below. 
Please do not spend more than 4 minutes with this task.

Please set your computer aside briefly (do not close this page) and join the classroom discussion that your teacher is going to moderate.

Note: If your teacher did not initiate the discussion yet, please start answering the first question below as you are waiting.

In the case of gas particles in a container (e.g., a real bike tire, the air duster can, and the Bike Tire model below) the pressure of the system is created by the particles hitting the walls of the container.

The pressure at a given instant is calculated as the total force applied by the particles hitting the walls at a given time and the surface area is the area of the walls.

That is also why we observe the fluctuations in the "pressure over time" plot: at a given tick, slightly different number of particles may hit the walls, and the total force applied by them might be slightly different. 

Now let's explore what is the difference between the fluctuations in the plot, changes in pressure, and the stability of the system: 

1. Run the model with 50 particles by clicking the  button, and then clicking the  button.
2. Wait until the system stabilizes (≈20 ticks). 
3. Add 150 more particles using the   button three times.
4. Observe the changes in the system until the system stabilizes again.

In the beginning of this lesson, you experimented with a real (physical) bike tire and then you also experimented with a computational bike-tire model. A distinct feature of our model was that it simplified some aspects of the real world object. For example, the walls in our computational model did not expand or shrink. 

Before ending this lesson, let's reflect on the differences between the two and why might we (and scientists) want to use such computational models.

Please set your computer aside briefly (do not close this page) and join the classroom discussion and the physical experiment that your teacher is going to moderate.