# max speed of circular motion

I understand that it is the static friction force that lets us turn the car around the corner. It provides the necessary centripetal acceleration. Static friction opposes the impending motion of the car moving away from the circle.

I also know that static friction has a max value. In case of a static block lying on ground, it will start to move when the force applied is greater than static friction max.

before attempt to turn, there was kinetic friction force only.

Now that we turn, we have static friction force also. What is the origin of this force? Is it by breaking down the friction force between tire and ground into two components - one still acting as kinetic friction force acting tangentially and another one as static friction force acting perpendicular to the tire?

I learnt a formula to calculate max speed of circular motion of the car possible. I don't understand what it means? What will happen if driver faster than that? would the static friction force would become too high? what are the consequences?

## 1 Answer

before attempt to turn, there was kinetic friction force only.

Where do you think you have kinetic friction? Kinetic friction is found when you have two surfaces sliding against each other. The tire-road interface is not sliding, so it is static friction, both before and during a turn.

During acceleration, the force of the tire on the road is rearward, so the reaction force of the road on the tire is forward. The only difference in the turn is that the frictional force is directed sideways instead.

If the forces between the tire and the road exceed the maximum static friction, then the tire will skid rather than roll.

• Well - because the contact area is finite in both length and width there is actually sliding during the straight rolling motion, and even more so when you try to turn the wheel (outside goes faster and inside slower). But your points are valid for "ideal" rolling motion with a point contact. – Floris Jan 23 '15 at 0:59
• Yes, the actual interaction between tire and road is complex. My description is appropriate for your average kinematics problem assuming simple coefficients of friction. – BowlOfRed Jan 23 '15 at 1:08
• With that point made I can upvote your answer. – Floris Jan 23 '15 at 1:09
• Forward friction is not fixed. It depends on the power sent to the wheel. More power, more torque, more (frictional) force. If the force exceeds that of maximum static friction, it will cause the wheel to spin. The sideways forces are more complex, but can also be varied by changing the angle of the wheel. – BowlOfRed Jan 23 '15 at 5:57
• @saurabh, Static friction is usually thought of as any force of friction less than (normal force)x(coefficient of friction). The direction of this force is either a result of direct force/torque applied (as in the case of the car accelerating) or can be deduced by looking at the direction of the acceleration of the supported object (so when car moves in circle, force points inwards). – Floris Jan 23 '15 at 8:29