1
$\begingroup$

David Morin, in "Introduction to Classical Mechanics" says that friction does not exert a force on a car because the ground is fixed but that KE of the car is changing to internal kinetic energy in the form of heat and he mentions Hybrid cars , which convert kinetic energy into chemical potential energy in a battery.

But pages earlier, he mentions the famous example of friction on a sliding book: friction does no work on the cars but does work on the book. although the ground is not moving in both cases.

He says that ".. force acts over zero distance", albeit friction is so intuitively simple it contains so many subtleties that I find it hard to master it.

$\endgroup$
  • 1
    $\begingroup$ probably because these are two types of friction force. Book, sliding on a surface, experience force of dry friction. In case of car you experience rolling resistance. I would say that in latter case (car) you can imagine that at each time point wheel is trying to push away from a solid wall, if that makes sense. $\endgroup$ – aaaaaa Apr 7 '15 at 22:03
  • $\begingroup$ But they told us in statics that kinetic friction is the same as dry one but with reduced coefficient ? $\endgroup$ – Mohamed Osama Apr 7 '15 at 22:05
  • $\begingroup$ Mohamed, do you mean he said the friction does do work on a car because... $\endgroup$ – BMS Apr 8 '15 at 5:01
  • $\begingroup$ no , i mean what is the difference between dry and kinetic friction in action ? $\endgroup$ – Mohamed Osama Apr 8 '15 at 21:00
3
$\begingroup$

The contact point between the wheel and the car is stationary - there is no "rubbing" there (well there is because the contact point is really a patch but let's keep it simple). To do work you need "force times distance" - and without relative motion there is no "distance".

When you apply the brakes in a car you have sliding of the pads relative to the rotors - and work converted to heat (or, in a hybrid, when you use regenerative braking, to electrical / chemical energy)

$\endgroup$
  • $\begingroup$ Do you mean if the contact is just a point there is no work done by friction ? for example if i sliding a diamond with sharp end i do not feel friction ? or what? $\endgroup$ – Mohamed Osama Apr 7 '15 at 22:09
  • $\begingroup$ No that is not what I meant. Rolling friction (where despite having wheels you feel resistance to motion) comes about from having a finite contact surface (which of course you always have because of elasticity). Sliding friction requires the two surfaces to move relative to each other. $\endgroup$ – Floris Apr 8 '15 at 5:40
  • $\begingroup$ Do you mean because the contact point is zero velocity like the ground so no work is done ? it is very non intuitive $\endgroup$ – Mohamed Osama Apr 8 '15 at 21:01
  • 1
    $\begingroup$ Yes. The force (from the ground) is pushing against a thing that is not moving (the only bit of the wheel it touches - which is not moving). However, when you drive fast and brake hard, the wheel skids and lots of work is done. Ditto when you accelerate away hard from a stop - it can make the tires smoke. As I said earlier, in reality the contact point is non ideal and non stationary so there is friction but that is not what this simplistic problem is considering. That may however be part of your confusion. $\endgroup$ – Floris Apr 8 '15 at 21:08

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.