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Does the friction between the tires and the pavement of a turning car "cause" the Centripetal Acceleration that is pushing the car towards the center of the turn arc ?


ANSWER: According to Newton's First Law an object in motion continues in motion at a constant speed and direction until an outside, unbalanced force acts upon that object so as to change that motion. In your car you have three controls, the gas pedal, the brake and the steering wheel, all three of which can change the velocity of the car by way of the frictional force. When you "step on the gas" you apply an unbalanced frictional force forward on the car causing the car to increase in speed. when you "step on the brake" you apply an unbalanced frictional force opposite the direction of the car's motion and the car slows down (commonly referred to a "deceleration" but more correctly as a "negative" acceleration. Finally, when you turn the steering wheel you generate an unbalanced frictional force perpendicular to the direction of motion of the car ca causing the direction of the motion to change while leaving the rate of the motion, the "speed" to remain constant. All three controls you will notice affect the motion of the car, the velocity, by either increasing the velocity (the gas pedal), decreasing the velocity (the brake) or the direction of the velocity (the steering wheel). All three are changing the velocity and, therefore, all three qualify as "accelerators" where acceleration is any change in the velocity! The thing that is unique about the steering wheel is that the steering wheel changes the direction of the velocity without affecting the speed. Any acceleration that results in a change in only the direction of the velocity is called a "centripetal" acceleration and the force that causes the centripetal acceleration is known as the "centripetal force". In this case a frictional force applied by the steering wheel perpendicular to the velocity causes a change in the direction of that velocity. Since the velocity of an object traveling along a curved path is always tangent to that path and since the centripetal acceleration must be perpendicular to the velocity if it is to NOT change the speed, the only direction for the centripetal acceleration is towards the center of curvature of the curved path! If the centripetal acceleration is in any other direction a component of that acceleration will generate a change in speed rather than a change in direction.

---------- FOLLOW-UP ----------

QUESTION: Wow !  Thanks James, you're the Best !

But due to my ignorance I'm still not sure of the either Yes or No answer;

Does the friction between the tires and the pavement of a turning car "cause" the Centripetal Acceleration ?

Could you please help me with a Yes or No or why it's not either Yes or No.

I truly appreciate your help.


Yes! It is the frictional force between the tires of the car and the road bed that generates the centripetal acceleration enabling the car to turn. Without that friction the car would continue forward in a straight line in accordance with the first law. It is the frictional force that enables to car to accelerate following the second law.


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James J. Kovalcin


I am teaching or have taught AP physics B and C [calculus based mechanics & electricity and magnetism] as well as Lab Physics for college bound students. I have a BS in Physics from the University of Pittsburgh and a Master of Arts in Teaching from same. I have been teaching physics for 34 years. I am constantly updating my skills and have a particular interest in modern physics topics.

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