The book says that if the net of all external forces acting on a system equals zero, the system's(centre of mass of the system basically) momentum is conserved. But what if some internally stored energy in the system is converted to kinetic energy? I mean to say, for example --> Consider a toy car which runs on spring mechanism. The spring is rotated by some angle theta thus allowing elastic potential energy to be stored in it. Now if we let it(the car) free, this potential energy will be converted to the kinetic energy of the car. So this is just a case of energy conversion. No external forces are involved(think from the ponit when the car is let free), only energy is converted. Still the momentum of the system appears to change. So what's going on here? How does momentum remain conserved in this case?

See we have other examples where internally stored energy converted to kinetic energy does not affect the conservation of momentum. Think of a bomb explosion where internally stored chemical energy is converted to the kinetic energy of the disintegrated pieces. Still the centre of mass remains in position.

So what's happening in the case of the car?

  • $\begingroup$ All of the forces you talked about are internal force, momentum of the system can only be changed by external force. $\endgroup$ Jun 29, 2020 at 6:05
  • $\begingroup$ That's my question. See the spring has potential energy stored in it which is converted to the kinetic energy of the car. And the car runs. So isn't the momentum of the system changing? The car runs... with all it's components. $\endgroup$
    – user266637
    Jun 29, 2020 at 6:11
  • $\begingroup$ Can u please clarify which toy car r u talking about. Although I can explain it using block spring example.Although we did some work on rotating the spring in ur example so there is an external force. $\endgroup$ Jun 29, 2020 at 6:13
  • $\begingroup$ Well don't consider that. Think from the point when the process of storing energy is already finished $\endgroup$
    – user266637
    Jun 29, 2020 at 6:17

1 Answer 1


Try the same experiment on a very slick patch of ice, and you'll see that the wheels of the car spin but the total momentum of the car remains unchanged. It is the (external) force of friction between the tires and the ground that changes the momentum of the car.

  • $\begingroup$ Yes, you are correct!!! I got it. Thanks friend. 😊 $\endgroup$
    – user266637
    Jun 29, 2020 at 6:18

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