Lesson 28
Kinematics of Elastic Collisions

Name:faculty Section:M2 Start Time:19:30:41 Instructor:pate Course:355

1) Let's take a step back and revisit this idea of the "Center of Mass" (CM) frame. The symbol V is used to represent the velocity of the CM frame as seen in the lab frame.

Consider a car and a truck facing one another as in the figure. Consider the car to be mass m1 and the truck to be mass m2, with m1 < m2. Please consider the following three scenarios and, for EACH one, try to describe (a) what V will be like (big, small, in which direction) and (b) what the motion of the car and the truck would look like from the CM frame:

  1. truck at rest in the LAB frame, car moving toward it
  2. truck and car traveling at the same speed toward each other in the LAB frame
  3. truck and car traveling toward each other, truck traveling faster than car in the LAB frame.




2) Suppose you come upon tracks that indicate the paths taken by two Acme billiard balls that collided elastically on a frictionless Acme pool table. The masses m1 and m2 are the same and you can tell from the tracks that mass m2 was initially at rest.

Given the information available in the tracks, estimate the ratio of m1's final kinetic energy to the initial kinetic energy of the system, and estimate the ratio of m2's final kinetic energy to the initial kinetic energy of the system (all as observed in the LAB frame).

Also, estimate at about what angle you think m2, whose path wasn't recorded clearly, must have come away from the collision.

Please, with each of these, explain what you are thinking, what equation you are using, etc. rather than just giving a numerical estimate. Thanks!



3) Which of the following statements is true?

  1. The initial kinetic energy in the CM frame is always the same as the initial kinetic energy in the LAB frame.
  2. The initial kinetic energy in the CM frame is always less than the initial kinetic energy in the LAB frame.
  3. The initial kinetic energy in the CM frame is always greater than the initial kinetic energy in the LAB frame.
  4. The relationship between the initial kinetic energy in the CM frame and the initial kinetic energy in the LAB frame depends on the masses involved in the collisions.



Below is a space for your thoughts, including general comments about today's assignment (what seemed impossible, what reading didn't make sense, what we should spend class time on, what was "cool", etc.):

You may change your mind as often as you wish. When you are satisfied with your responses click the SUBMIT button.

I received no help from anyone on this assignment.
I received help from someone on this assignment (document in comments section).