Workout: Momentum conservation


Read the web pages Restating Newton's 2nd law: momentum and Momentum conservation.


To give you some practice on working with momenta, go to the PhET simulation Collision Lab to perform a few tasks.

Set up

For this activity we will be studying the implication of momentum conservation for elastic collisions (where kinetic energy is also conserved). Momentum conservation plays a critical role in collisions in which energy is exchanged with internal degrees of freedom (chemical energy) as well.

Make sure that the tab that is showing is the Introduction tab. In the green options palette in the upper left, turn on Velocity Vectors and Show Values, leaving everything else turned off. This now shows the values of the velocity and momentum for each of the objects. For all our examples we'll keep the initial velocities equal to 1 m/s for mass 1 and 0 for mass 2.

Answer these questions

  1. Set the masses so that both masses are equal to 1 kg. Predict what you think will happen when mass 1 collides with mass 2. Did the result agree with your prediction? Was momentum conserved in this collision?
  2. Set mass 2 to 2 kg. What happens to the velocity and momentum in this case? Are either of them conserved? 
  3. Reverse the masses so that mass 1 is 2 kg and mass 1 is 1 kg. Is either velocity or momentum conserved?
  4.  Put mass 1 back to 1 kg and set mass 2 equal to 20 kg. Before you run the example, make a prediction. Did the result agree with your prediction? Is momentum still conserved? 
  5. What happens if mass 2 is very large – say 200 kg. Will momentum still be conserved? Describe what happens. (You may have to move mass 2 farther back. You can do this by pressing the "More Data" button and changing the positions.) 

Joe Redish 10/7/17

Article 537
Last Modified: February 7, 2019