Friction does not exert force on a Potential Energy system?

Question

In the potential energy chapter of my physics book, friction x distance = delta E-thermal

Do I ignore the Frictional Force = mass x acceleration that I learned 5 chapters ago?

In a quick skim, friction is now being treated as not a factor in decelerating the object directly. Rather, the book reads like friction is simply the "reason why mechanical energy declines by 5.5 joules" and we just solve by looking at changes in energy, rather than figuring out what the co-effecient of static friction and the Normal force are, and solving it directly.

I get the idea, that we are now supposed to think of friction and solve for it indirectly (using stuff we know about kinetic/potential energy).

Am I completely wrong? Is it safe to throw out the idea that I'll be expected to calculate the work done by friction from Newton's laws, and I'll only be asked to find work by changes in energies?

Answer 1

No, ignore, and throw out, nothing. The examples you give are typical and consistent. Friction is a force and equals ma if it is decelerating something, and the heat energy of friction and the kinetic energy loss = Fd. You may have to solve for friction on either basis, energy loss or accelerating mass. Also, expect problems where centripetal force = friction and something on a turntable starts to slide. Friction is everywhere in the real world, so expect to find it in almost all reality-based problems!