Work done by friction when car is breaking

[Dale]

1)Is Work Energy theorem a consequence of Newtons' II Law but not Law of conservation of energy ?

The work energy theorem is a consequence of Newton's 2nd law and the assumption that the body under consideration is rigid. So in Newtonian mechanics the conservation of energy is derived from Newton's laws. However, in Lagrangian mechanics Newton's laws are derived from the conservation of energy. The two formulations are equivalent, but take different starting points.

2) Are law of conservation of energy and Newtons II law completely independent from each other?

Not completely independent. Essentially, if you assume one (and some other supporting stuff) you can derive the other.

3) Suppose the car starts on a frictionless surface . In this case it does not move W_net = 0 and ΔKE = 0 as well . What does internal chemical energy convert into ?

Thermal energy. In this case there will not be any thermal energy generated at the frictionless wheel, but only at all of the internal parts of the car.

4) Can we compare the movement of a car rolling without slipping on a surface to walking of a person ? In walking also W= 0 ,but ΔKE ≠ 0 ?

Yes.

5)If we try to under the physics of walking from force perspective then we can say that friction produces acceleration . And from energy perspective we can say that work done by friction is zero and internal energy in the human body converts into kinetic energy .Is that correct ? But again as I stated above I am a not clear how external force on the system(friction in this case) is in no way related to energy conversions within a system (human body in this case ).

I think that "in no way related" is a little too strong. It certainly is related because without it you will have a very hard time changing your internal chemical energy into kinetic energy. But nonetheless, from an energy transfer standpoint the energy is not transferred but only changed from chemical to kinetic.

 

[Vibhor]

I think that "in no way related" is a little too strong. It certainly is related because without it you will have a very hard time changing your internal chemical energy into kinetic energy.

Can you explain how the external force despite not doing any work is responsible/related for changing our internal chemical energy into kinetic energy ?

 

[Dale]

This kind of thing happens all the time. For example, if you roll a ball up a ramp the ball's KE is converted to gravitational PE. The ramp does no work but is required for the ball's change of energy.

The ramp/ground does provide the force that is responsible for the acceleration, but work is about the transfer of energy, not merely the presence of a force. Many forces exist which do not transfer energy.