Lecture 16: Collisions - Elastic and Inelastic - Center of Mass Frame of Reference

author: Walter H. G. Lewin, Center for Future Civic Media, Massachusetts Institute of Technology, MIT
recorded by: Massachusetts Institute of Technology, MIT
published: Oct. 10, 2008,   recorded: October 1999,   views: 6182
released under terms of: Creative Commons Attribution Non-Commercial Share Alike (CC-BY-NC-SA)

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1. 1D Elastic Collisions:

A mass with given speed collides with a second mass (initially at rest) in a one dimensional collision. Momentum is conserved. If kinetic energy is also conserved, the velocities of both objects after the collision can be calculated. Three limiting cases are explored analytically, and then demonstrated. The equations are used to predict the outcome of some air track experiments.

2. Brain Teaser - Elastic Collision with a Wall:

A tennis ball bounces off a wall elastically. The momentum of the wall changes, but the kinetic energy of the wall remains zero. How is that possible? Something to think about!

3. Center of Mass (CM) Frame of Reference:

A 1D elastic collision is considered as seen from the CM frame of reference (where the total momentum is zero). Using the velocity of the CM in the Lab frame, you can transfer between the two frames.

4. 1D Inelastic Collision and Internal Energy:

A 1D inelastic collision is considered from the laboratory and the CM frame. The kinetic energy is calculated in both frames and it is shown that the initial KE in the CM frame is the maximum KE that can be converted to heat (this is called the internal energy of a system). The equations are used to predict the results of an air track experiment.

5. Newton's Cradle Demonstration:

Professor Lewin solicits an analytical proof of his demo showing a lineup of colliding balls.

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Reviews and comments:

Comment1 Asaad Shaikh, February 4, 2009 at 7:54 p.m.:

Good for Knowlegde..Thankxxxxxx


Comment2 dev, September 3, 2010 at 7:31 p.m.:

sir,i wanted to know..what r centre of mass frame of refrence & laboratory frame of refrence? and the difference between the 2?


Comment3 anand, October 12, 2011 at 12:01 p.m.:

its wonderful to know something related to the subject in depth...


Comment4 jonathan, March 6, 2012 at 1:02 a.m.:

Whats the answer to this?

A tennis ball bounces off a wall elastically. The momentum of the wall changes, but the kinetic energy of the wall remains zero. How is that possible?

is it just that the wall with infinite mass has KE that approaches zero?


Comment5 Davor form VideoLectures, December 18, 2017 at 10:29 a.m.:

Hi all!

We have translated this entire course for you from English into 11 languages.

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