# Driving and Frictional Force on a Car

I learned that if an object is traveling at a constant velocity, then the external forces acting on the object must be in equilibrium with each other.

So, if the car is driving in a straight line at 30 miles per hour, and ignoring air resistance, then the driving force must equal the frictional force.

If the same car is driving in a straight line at 60 miles per hour, however, and ignoring air resistance, then the driving force must equal the frictional force.

But the frictional force is the same in both scenarios, because it only depends on the coefficient of kinetic friction (which is a constant) and the normal force of the car (which is constant).

This seems to imply that the driving force is constant in both scenarios. This is very counter-intuitive, because clearly there is some extra "oomph" when the car is driving at 60 miles per hour.

What is the cause for this extra "oomph"?

• Air resistance is equals to $kv^2$, where k is the medium resistance coefficient. You can see air resistance increases exponentially. Driving force = friction + air resistance too. – QuIcKmAtHs Jan 2 '18 at 3:25
• frictional losses in the car's transmission and differential also scale up with speed, as does flexural friction in the tires. – niels nielsen Jan 2 '18 at 3:54