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The Collisions1D app is a basic physics lab, in which two objects (a red ball and a blue ball) collide with one another in one dimension. You can set the mass of each of the objects, the initial velocity of each object, as well as the elasticity of the collisions between the objects. In a perfectly elastic collision (elasticity = 1), the total kinetic energy is conserved. In a completely inelastic collision (elasticity = 0), the two objects stick together after the collision. The settings screen is accessible by pressing the info button at the bottom right of the main screen.

Using this app, you can explore the conditions in which momentum (linear momentum) is conserved in a collision like this, as well as the conditions under which kinetic energy is conserved in a collision.

In addition to colliding with one another, the balls collide with the walls. These collisions are always perfectly elastic. Accounting for these collisions with the walls, as well as the collisions of the balls with each other, it is interesting to think about when momentum is conserved in this app, and when it is not. The law of conservation of momentum states that the total momentum of a system is conserved as long as no net external force acts on that system.

You also have the option, in the settings, of showing the center of mass of the two-ball system. It is very interesting to observe the motion of the center of mass. In particular, pay careful attention to what happens to the motion of the center of mass when the balls collide with one another. Is this the same, or different, from what happens to the motion of the center of mass when a ball collides with a wall? Bonus points if you can understand why.

On the web site that accompanies this simulation, there is a worksheet you can use as you work with the app to understand the physics of one-dimensional collisions. The worksheet has a collection of equations you can apply to predict the outcome of a collision between the two balls, and suggests a set of basic experiments to try out with the app. The worksheet tells you what settings to use, and then asks you to predict what the balls will be doing after they collide. Predict what you think will happen, and then see if you're correct by running the app.

Note that there are three buttons to work with on the main screen. The “restart” button starts the app from the beginning (keeping all the settings). The “speed up” button is used to run the simulation faster, and the “slow down” button runs it slower. The “slow down” button is especially useful if you're trying to read the momentum and kinetic energy values after a collision.

If you've ever done collisions in one dimension on an air track, this app is a basic simulation of that process, just with no friction at all. Have fun with it.