Momentum Unit

10 Comments

Bill Kearsley • 1 year, 9 months agologin to reply

Question 2 on the Momentum test 2 seems incorrect. Answer A seems like the only one that would be possible. The answer D that is said to be correct would have unequal y components. Am I missing something?

John Ennis • 1 year, 9 months agologin to reply

Bill, the problem with answer A, is that if you add those two vectors, they are much greater than the before collision momenta. D allows for the right x direction, and it was intended for the y components to be equal - they're close, but we need to redraw those vectors so it's more clear. We'll fix it and upload a new exam. Thanks for noticing this. John

Melissa Axelsson • 1 year, 8 months agologin to reply

Bill - new version is now posted. - Melissa

Liz Hensley • 1 year, 8 months agologin to reply

Question 31 on the Classwork-Homework Momentum MC may be incorrect. A head-on collision with two identical cars at same speed vs one car colliding with a wall at same speed. I think the damage on all three cars are the same.

Melissa Axelsson • 1 year, 8 months agologin to reply

Liz - thanks for the feedback, we agree. I just posted a corrected version. - Melissa

John Ennis • 1 year, 8 months agologin to reply

Liz, I agree - we will upload a corrected MC and answer key.

Daniel Rice • 9 months, 2 weeks agologin to reply

What is the problem solving technique for question 1E on the free response? First year teaching Mechanics and I am looking to get better.

John Ennis • 9 months, 2 weeks agologin to reply

Daniel, I'm providing the solution for 1C as that is the last part of question 1 - there is no 1E. Is that the one you wanted? Start with the 3rd Kinematics Equation, where v final = 0, delta x is 0.9 m and v zero is the velocity from part (a). Solve that for the acceleration which describes the block slowing from v zero to being at rest. Now use Newton's Second Law where the only force acting is friction which is mu times mass times g. That equals ma. Masses cancel out, and use the acceleration found in the previous sentence (a = -6.05 m/s^2). mu is then a/g = 0.62. If this wasn't the problem you wanted, please let me know. John

Daniel Rice • 9 months, 1 week agologin to reply

Thank you John for your response. It appears the edition of the FR set I am using is changed. There was a problem 1 that was removed that was about a bullet moving through a fixed block, and then finding the minimum thickness of the block needed to stop the block. The other questions are identical, but the question that I was referencing, which I posted part of below, has been removed from what I assume is an older edition. I finally arrived to the correct solutions. "The bullet emerges from the block with a velocity v0/2 when the block is held fixed. c. Find the change in kinetic energy of the bullet-block system. d. Assuming that the retarding force in both trials is the same, find the minimum thickness Lmin of the block in order to stop the bullet. e. Explain the difference in answers (b) and (c)." Thank you again, John!

John Ennis • 9 months, 1 week agologin to reply

Daniel, I'm glad you got the correct solution. We are always revising our work based on teacher/student input and our own checks. I keep a notebook of problems, exams, and other materials related to the units, and before I teach a new unit, I check my notebook against the website and if there are changes, I download the new materials. Thanks for your help and interest. John

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