Explain how friction affects the mechanical energy or a system.?

Question

[just a short explanation to help me understand]

Answer 1

A system has total energy TE = PE + KE + WE; where PE is potential energy, KE is kinetic (mechanical) energy, and WE is work energy (e.g., from working against friction, gravity, or air drag).

Typically, PE comes from things like a wound up spring, chemical energy (like gasoline or Diesel fuel), or location relative to the ground. Like the name implies, there is a potential for converting to other kinds of energy, like KE and WE.

Without WE, all that potential PE would be converted to kinetic energy (mechnaical). For example, if a block slides down a frictionless ramp, its PE = mgh = 1/2 mV^2 = KE which means all its PE at height h, before release, will be converted to KE with velocity V at the bottom of the ramp.

But that's for a frictionless block, which is impossible. Now let's throw in friction WE = kN = kmg cos(theta), where theta is the slope of the ramp, m is the mass, k is the coefficient of friction, and g = acceleration due to gravity. [There are other kinds of friction; for example, wind drag is a friction force created by air.]

So now PE = mgh = 1/2 mv^2 + kmg cos(theta) = ke + WE, which means some of the potential energy is coverted into kinetic energy and some of it is converted into work energy from friction. Since not all of the PE is converted into KE when there is friction, this ke = 1/2 mv^2 < 1/2 mV^2 = KE. That is, because of friction, the mechanical energy ke is less than the mechanical energy KE without friction.

And mechanical energy is the useful energy because it can be put to work. WE on the other hand is useless energy, it usually is lost as heat. We can define something as efficiency in that E = (ke/KE) X 100%. And there you have it. Because ke < KE and ke/KE < 1.00 when there is friction, the system's efficiency is less than 100%.