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Lecture 10 

 

Newton’s laws of motion (cont’d)

  • Pushing two blocks
  • Friction
  • Two exercises with friction
  • Pulley: An exercise
  • Free body diagrams for selected problems in HW4. 

 

Reminders:

  • Bring your questions on HW4 to today’s Instructor’s Informal Session, RLM4.102, 4-5pm.

 

  • Re-work midterm problems:

o        To encourage you to take another hard look at problems in the midterm01, after the exam  please download corresponding “reexam01” set, which is your midterm, except for a different version number. 

o        Treat reexam01 as a homework set. Work on the set and submit your answers to the TT-system.

o        To avoid the time-conflict with HW4, the due-time for reexam01 will be at 1am on Monday, 9/24 (or Sunday evening).

o        Your Midterm01-score and reexam01-score will not be posted until after the due time of the reexam01.

o        For those who have made 85% or more in the reexam01, extra credit will be earned in the TA session. (See your TA for details)

 

  • There will be no attendance-test for the week immediately after the midterm.

 

Notes from Professor Markert: Re: The dash symbol in table 6.1

 

Because wax and snow are somewhat soft and can deform, the behavior for static friction depends a lot on conditions.

  • When there is not too much weight, and thus little deformation, the static coefficient is not too much different from the kinetic one, that is ms (at -10oC) ~ 0.2- 0.3   and    ms (at 0o C) ~ 0.1.
  • But if there is appreciable weight, or if one waits long enough for the crystals of snow to "sink into" or "dig into" the wax, the effective coefficient can become two or more times larger.  Thus the coefficient is not really well defined; it depends on the history of the surfaces.

 

If one wants values of ms, one can use the values above, but it depends on the circumstances.