versión en español (argentino)


Lessons from Teacher Ciruela
(Las lecciones del Maestro Ciruela)

Suppose a car is traveling with constant speed along a rectilinear path in normal and real conditions, without neglecting any interaction. Is there a frictional force between the tires and the pavement? (Net force, ie, overall forces of all four wheels). And if there is, what other force restores balance? If not, why must the engine continue making force? And the big question... If your answer is yes, there is a frictional force between the tires and the pavement... then... where does it point to?


Yes, there is a net frictional force between the tires and the pavement... and it points forward! Of course... and that against all expectations of students and a number of teachers and graduates used to repeat without thinking "Friction opposes motion". The force that balances out this forward friction is another friction, the one with the air, and it points backwards.

The force that the engine makes is to drive the wheels so that they push the pavement backwards. The pavement is too hard and supportive to the solid Earth, but it also follows the Third Law of Dynamics, also called Principle of Action and Reaction, so if the car pushes the pavement backwards, the pavement pushes the car forward, through friction.

The engine spends a considerable share of energy to resist one more friction in this case an internal type of friction which results from spinning the wheels on their axes and other engine parts around themselves... to the delight of the oil dealers.

Translated by Patricia Pilgrim. This article was published in the magazine EXACTAmente. Some rights reserved. Reproduction is allowed if the source is mentioned. Last updated on Nov-06. Buenos Aires, Argentina.