I know that as a car accelerates on earth, for the car-earth system angular momentum is conserved.

Attached is a nice animation for simplistic proposes. https://www.animations.physics.unsw.edu.au/jw/momentum.html

However, the front wheel experiences a torque due to friction. The front non-driving wheel also has angular momentum about it axle. Where is it conserved in the system? Does it get its angular momentum from earth or the car?

I can see how the forces on earth and on the car as a whole keep momentum conserved, but I am conceptually wondering how angular momentum is conserved if one includes the wheel.


1 Answer 1


The initially stationary wheel gets its angular velocity from the ground, as the car itself exterts no torque on the front wheel. The angular momentum here obviously isn't conserved, since there is a net torque on the wheel. When the car is moving with a constant velocity, the frictional torques from the ground and from the axle of the wheel balance out, thus producing no net torque.

  • $\begingroup$ That makes sense, but again i am wondering how if you now included the ground or earth as your system, how would ang. Momentum be conserved? The overall car gains AM from earth, and the tire from the ground/earth. But arent these also at different axis of rotation so how is it all conserved? $\endgroup$ Oct 1, 2020 at 17:49
  • $\begingroup$ In order to conserve angular momentum, the earth will gain a really, really small angular momentum in the opposite direction, but considering earth's mass, it makes no difference $\endgroup$
    – dnaik
    Oct 2, 2020 at 2:33
  • $\begingroup$ I don't think you're understanding my question. Car moves forward relative to Earth's axis, THerfore the car has AM. The earth rotates opposite and has equal but opposite AM. However, the front tires are spinning independently. Where is this AM accounted for ? If I look at the torque on earth and torque on car, everything is equal, but what about torque that makes front tires spin? $\endgroup$ Oct 2, 2020 at 3:10

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