E None of the above How can the block get hotter if no work was done on it

# E none of the above how can the block get hotter if

• 21

This preview shows page 16 - 21 out of 21 pages.

E. None of the above. How can the block get hotter if no work was done on it? Reading Question - Sec. 8.6

Subscribe to view the full document.

Point Particle System: All external forces are applied to the center of mass; correctly describes the motion of the center of mass. ! K translational = ( F " f ) d Since v=constant, no change in K, so: Can this tell us anything about internal motions of the system? r F = r f Analysis of Point Particle System
! K trans + ! E thermal = Fd + W friction For simplicity, assume Q=0. ! E thermal = Fd + W friction Since Δ K=0. Since this is the real system, not the point particle system, forces do not all act at the same point, and they do not have to act over the same distance: ! E thermal = Fd " fd effective # 0 We must have F=-f, but not d=d eff ! Analysis of the Real System

Subscribe to view the full document.

Actual work done by friction forces: KEY ISSUE - The point of contact where the friction force is applied moves a shorter distance than the distance the block moves. This means friction does less work than expected. A Model for Friction
Instead of teeth stuck on each other, let them drag on opposite surfaces. Each then applies force F/2. When the top block moves by d, one tooth moves by d, while the other one just bends, meaning no displacement at the contact. Only one tooth does work across the distance d, with a force F/2, so only half of the expected amount of work is done. A More Realistic Model

Subscribe to view the full document.

Consider 2 identical blocks as shown. Lower block is fixed. Total work done on the system (2 blocks) is just Fd (top block displacement d). Divide the increased thermal energy between the two blocks: ! E th ,1 + ! E th ,2 = Fd ! E th ,1 = Fd 2 = Fd " Fd eff d eff = d / 2 Hence, In this simple model, friction only does half the expected work. The remaining work done by the external force goes into thermal energy. 2 Identical Blocks

### What students are saying

• As a current student on this bumpy collegiate pathway, I stumbled upon Course Hero, where I can find study resources for nearly all my courses, get online help from tutors 24/7, and even share my old projects, papers, and lecture notes with other students.

Kiran Temple University Fox School of Business ‘17, Course Hero Intern

• I cannot even describe how much Course Hero helped me this summer. It’s truly become something I can always rely on and help me. In the end, I was not only able to survive summer classes, but I was able to thrive thanks to Course Hero.

Dana University of Pennsylvania ‘17, Course Hero Intern

• The ability to access any university’s resources through Course Hero proved invaluable in my case. I was behind on Tulane coursework and actually used UCLA’s materials to help me move forward and get everything together on time.

Jill Tulane University ‘16, Course Hero Intern

Ask Expert Tutors You can ask 0 bonus questions You can ask 0 questions (0 expire soon) You can ask 0 questions (will expire )
Answers in as fast as 15 minutes