A spring connects two balls. You can vary the mass of each ball separately. As the simulation starts, a force is applied to the left-hand ball for a short time only. The magnitude of this force is under your control. After the force has been applied, there are no external forces acting on the system. Although neither ball moves with constant speed after the initial force, what does the center of mass velocity graph show about the motion of the center of mass? How does the velocity of the center of mass change when the force is changed?

Some things for you to try: View the simulation for an applied force of 2 Newtons. Note the velocity of each object, and of their center of mass.
 1 Predict what will happen to the center of mass velocity if the force is doubled in magnitude.
 2 Predict what how the center of mass velocity will change if the force is halved in magnitude.
 3 Stop the simulation at a time between one and two seconds. Record the velocities of the two objects. Calculate the total momentum of the two objects. Predict the momentum at a time exactly 1 second later. Run or step the simulation to this time, and determine the momentum at this time. Is the total momentum the same or different? After doing step 3, reset the simulation to the beginning. Determine the total momentum at this time. Is it the same as you determined earlier? Why or why not?
 4 Predict what would happen if the mass of the red ball is small compared to the mass of the blue ball. Use the simulation to check your prediction.