I am working one-on-one with a student in a special classroom. his name is Ryan. He is not an academically inclined student, but his is a willing participant in our sessions in which I am to deliver physics instruction to him. I have been weaving Snap! into my instruction and his assignments. He knows about basic motion and Newton's laws. We reviewed the physics he knew before starting what I will now describe.
This was my preamble for today's lesson that started with us looking at an empty Snap! browser with the exception of a gold circle (sprite) on a black stage in the display....
So, let's play God. I have created an object in the void of space. Nothing else exists. As God, we want to create a universe in which we can apply a force to this object and have it respond according to the physics you know. To do this we need to identify the necessary aspects of reality that we can represent with a number. This means to make variables that we will use in the universe. For example, time is such a variable we have used in motion simulations, so let's make a variable called "time". And, we are accustomed to making our time increase inside loops by an amount we call "delta-t". So, we will make that that variable, too.
"What other variables should we make to define our universe?", I asked.
"Mass?", he offered.
I did not expect this to be his first thought. I thought mass would be the one I would have to suggest. I was immediately encouraged.
"Yes! Excellent," I said.
"What else?", I asked.
"Acceleration", he said. I could tell he had Newton's 2nd Law on his mind. Perfect.
We continued until we had this set of variables: time, delta-t, mass, acceleration, force, and velocity.
I framed the next step. "Okay, now we have defined the important variables of our universe, now we have to make something happen. We have used repeat blocks as the core of previous simulations. Let's do that here, too."
I put a change-y-by block inside a forever block. I reminded Ryan that we wanted to put an equation in for the argument, rather than a simple hard number, as we had done when making sprites move at constant velocity and with uniform acceleration. With some prompting, Ryan suggested velocity*delta-t as an expression to represent displacement, which is correct for constant-velocity motion. I then reminded Ryan of the graphical area of a velocity vs. time graph being displacement. I pointed out that velocity*delta-t does not account for the displacement caused by the increasing velocity during the time interval. I showed him the several algebra steps that leads to:
We realized that nothing would happen, at first, because the force was zero. We entered a force and watched the sprite accelerate upwards on the stage.
This was good, but we did not quite feel like God, yet. I showed Ryan how to make a key control the force. We made an up arrow increase the force and a down arrow decrease the force. We discussed the sensitivity of the sprite to these controls. When asked how we could make the response more sluggish, Ryan replied, "Increase the mass." Perfect.
We ended up setting the force to zero whenever a key was not being pressed.
This enhanced the control of the sprite. And, God was pleased.