Momentum and Center of Mass

 

Run the PhET Flash simulation “Collision Lab”.  This program simulates collisions between objects in an idealized zero friction scenario.  Explore and discover some important properties of systems of interacting objects by taking the following steps.

 

1.     Note that there are three options.  Use the Explore 2D option.

2.     Play around with the program settings and controls.  Notice that you can also click and drag objects and vectors in the simulation in order to change the position and velocity of each object.  If you want to rerun the simulation that you have set up then click on the small blue “circle back” button.  Note that the larger orange reset button resets everything in the Explore 2D setup.

3.     After you have experimented with all of the program options click on the large orange reset button to restore the original settings of Explore 2D.

4.     Click on the Center of Mass option – this will show a gray "×" that marks the position of the Center of Mass of the system.  Run the simulation in 2 dimensions with the Reflecting Border and simply observe the Center of Mass as the objects collide with one another and also collide with the border “wall”.

5.     Try pausing the simulation at any point in time.  Where is the Center of Mass always located relative to the two objects?

6.     Adjust the simulation so that it shows two objects of equal mass.  Run and pause – where is the Center of Mass always located?  Experiment with different numbers of objects and different masses – what is a general conclusion about the location of the Center of Mass?

7.     Only certain types of collisions will have an effect on the motion of the Center of Mass.  Use the simulation to determine which type of collision makes the Center of Mass move at a different speed or in a different direction.  Note that you can slow the simulation down or even step forward or backward one frame at a time.

8.     Click on the Momenta Diagram option – this will generate a vector head-to-tail diagram that illustrates the total momentum of the system.  The blue arrows represent the momentum of each object and the black arrow is the total momentum of the system.

9.     Run the simulation and compare the total momentum black arrow to the motion of the Center of Mass.  What do you notice?

10. The total momentum of the system is only affected by which type of collision? Try viewing in slow motion and/or pause and step forward and backward to compare before and after a collision.

11. What is the effect of the Elasticity control?  Does adjusting the Elasticity change any of the above observations and conclusions?

12. Remove the Reflecting Border and try creating a collision in which the Center of Mass is at rest before impact.  What do you observe?  What has to be true of the masses, velocities, and/or momentums of the objects in order for the Center of Mass to be stationary?