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I was reading this chapter on conservation of energy and I couldn't think of a suitable example for the argument given in the image attached. Now for friction to increase the mechanical energy of a system it has to be an external force as internal forces can't change the total mechanical energy. But every example that I could think of, Friction was only decreasing the Mechanical energy of the system.

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    $\begingroup$ Related: physics.stackexchange.com/q/190412/2451 , physics.stackexchange.com/q/514347/2451 , physics.stackexchange.com/q/425509/2451 $\endgroup$
    – Qmechanic ♦
    Dec 11, 2021 at 13:09
  • $\begingroup$ Can you think of situations where the speed (kinetic energy) or height (potential energy) of an object is increased as a result of friction ? $\endgroup$
    – gandalf61
    Dec 11, 2021 at 13:23
  • $\begingroup$ Friction cannot create kinetic or potential energy. However it can change types of energy. Consider a car engine that is production rotational energy that is transferred to the axel again rotational energy. If the tires are not slipping, it is the friction between the tires and the road that converts the rotational energy into liner kinetic energy. Again this is not creation of mechanical just the conversion of rotational kinetic energy into linear kinetic energy. $\endgroup$
    – Michael
    Dec 11, 2021 at 13:49
  • $\begingroup$ @Michael I agree. But if we consider the whole car to be a system, then the mechanical energy is conserved and not increased. Hence friction is not increasing the mechanical energy rather changing the form of energy as your rightly mentioned. But what will be the scenario in which it increases the mechanical energy? Or you are saying that there will be no such scenario? $\endgroup$
    – VMnM7
    Dec 11, 2021 at 14:18
  • $\begingroup$ @gandalf61 Nope. It either came out to be negative or zero. $\endgroup$
    – VMnM7
    Dec 11, 2021 at 14:21

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