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Let's model the collision of two balls that are in the air near the earth. The top ball has a mass of 3.0 kg and an initial velocity of 2.0 m/s upwards, and the bottom ball has a mass of 5.0 kg and an initial velocity of 6.0 m/s upwards. They make a one-dimensional collision.
Copy the Interactive Physics program P405CollisionsAndGravity1.ip onto your computer and open it. It shows two balls flying through the air and colliding. Run the simulation and answer the following questions.
(a) Consider the system of the two balls. Is the net external force zero? If not, name the force and give its direction?
(b) Record the time and momenta for the following times. Be sure that you clearly indicate what the signs of the numbers mean.
| Time |
Momentum of Top Ball |
Momentum of Bottom Ball |
System Momentum |
| 0 s |
|||
| 1 s |
|||
| Just before |
|||
| Just after |
(c) The strict version of the Law of Conservation of Momentum requires the net external force to be zero. How can you rewrite the law so that it is approximately correct in this example?
(d) Under the World/Gravity, turn gravity off. Is momentum conserved for all times now?
(e) Now copy P405CollisionsAndGravity2.ip to MyDocuments and open it. This is a two dimensional collision of balls in the air over your head. Run the simulation.
(f) For the system of the two balls, is there a net external force, and if so in what direction?
(g) Use your Modified Law from part (c) for this collision. Does your modified Law work? Give specific evidence.
(h) OPTIONAL Viewing collisions from the Center of mass.
Turn tracking in the menu World/Tracking/Every 32 Frames. Show the location of the center of mass in the menu View/System Center of Mass. Run the simulation. Describe the motion of the center of mass.
If you rode along with the center of mass, you would see the collision differently. Use CTRL-1 to switch to the Center of Mass viewpoint and run the simulation again. How does this run compare with the previous run?