# Does static friction affect the motion of a wheel moving at uniform velocity?

Imagine a car moving in a circular track with constant speed. I am aware that kinetic friction is responsible for the centripetal acceleration of the car. But is it right to say that the car, or more specifically the wheel of the car, is also subject to static friction? Can I conclude that static friction will be tangential in nature? Also, I am not able to visualise how exactly static friction acts when the wheel is turning.

In continuation with the previous question, will it be correct to say that static friction does not really affect the motion of the wheel (or any other rolling object for that matter) if it is moving with constant speed, since it is neither accelerating nor decelerating it? One last question that I have is that when the wheel is turning, is it subject to kinetic friction? or is it static friction? Also in which direction?

I am aware that kinetic friction is responsible for the centripetal acceleration of the car. But is it right to say that the car, or more specifically the wheel of the car, is also subject to static friction?

Static friction, not kinetic friction, is responsible for the centripetal acceleration of the car. Kinetic friction occurs if the wheels skid (lose traction) on the road.

Can I conclude that static friction will be tangential in nature?

Static friction responsible for centripetal acceleration is not tangential to the motion of the car. Static friction responsible for tangential acceleration is tangential in nature. See the figures below.

Also, I am not able to visualise how exactly static friction acts when the wheel is turning.

See the figures below.

In continuation with the previous question, will it be correct to say that static friction does not really affect the motion of the wheel (or any other rolling object for that matter) if it is moving with constant speed, since it is neither accelerating nor decelerating it?

Static friction is needed for circular motion even though the speed of the car is constant. Static friction would not be needed for linear motion at constant speed if there were no dissipative friction forces (primarily air drag) acting on the car. In other words, if the car could coast at constant speed.

One last question that I have is that when the wheel is turning, is it subject to kinetic friction? or is it static friction? Also in which direction?

For pure rotation (turning without skidding) during acceleration or braking only static friction is involved. It acts in the direction of motion for acceleration and opposite the direction of motion for braking. If the wheels skid (lose traction) when accelerating or braking, then there is kinetic friction which always acts opposite to the motion of the car.

If my car is moving at uniform speed, along a straight line, static friction is still being applied on its wheels right?

In order to move at constant speed, the net force acting on the car must be zero. So if there is an air drag force opposing the motion of the car, an equal static friction must act forward on the car in order for it to move at constant speed. Think about what happens when you take your foot off the gas. The car slows down. This is due to air drag as well as rolling resistance and other frictional forces.

And also just to confirm, static friction will be needed in circular motion to change the direction right? It won't change the speed right?

That is correct.

Hope this helps.

• If my car is moving at uniform speed, along a straight line, static friction is still being applied on its wheels right? Commented Oct 14, 2022 at 16:03
• And also just to confirm, static friction will be needed in circular motion to change the direction right? It won't change the speed right? Commented Oct 14, 2022 at 16:16
• @Obinna I have updated my answer to respond to your follow up questions. Commented Oct 14, 2022 at 16:26
• Thanks a lot! And if there is no air drag, there still will be static friction right? Commented Oct 14, 2022 at 16:29
• @Obinna If there are no forces opposing the motion of the car, which includes air drag, rolling resistance, axle friction, etc., then there will be no static friction. Static friction only exists in opposition to another force. Commented Oct 14, 2022 at 16:33

It's actually quite the opposite - static friction causes the motion of the wheel. If the wheels were not subject to any frictional forces, they would simply slide across the ground without turning - think of a car skidding across the pavement after braking suddenly. However, in normal rotational motion ("rolling without slipping"), the static friction applies a force tangent to the rim of the wheel at the point where the wheel contacts the ground, which is static friction as long as the wheel is not sliding. This force, in turn, creates a torque on the wheel which allows it to turn and move the car forward. Below is an FBD of the situation:

So I guess the simplest way to think about it is that kinetic friction creates centripital force that keeps the car moving in a loop, while static friction provides the "equal and opposite" reaction counterpart to the torque generated by the engine of the car, ultimately causing the wheels to carry the car forward. Hope that helps!

• I think there are a couple of subtleties here. First of all, the static frictional force is in general lower or equal to $\mu N$. Furthermore, I am not sure that the fricitonal force is acting when the wheel rolls with constant velocity, but for sure it acts when the wheel is accelerated (as it is always the case in real situations). Do you agree? Commented Oct 14, 2022 at 14:53
• I'm pretty sure that the static friction is acting when the wheels roll with constant or changing velocity - that's the force that causes the wheels to roll over the ground. If there was no static friction, the wheels would just spin endlessly without ever going anywhere (imagine trying to drive a car on a buttered floor or something like that). Commented Oct 14, 2022 at 15:05
• Yes I understand your point: if the wheel is at rest, and there is no friction (for example if the road is covered by ice), then the wheel spins endlessly and never translate, just as you said. However, if the wheel is moving at constant linear velocity $v$ and spinning with angular velocity $\omega = v/R$ (R is the radius), then there is no acceleration and the static friction should be zero. I can see that this is theoretical, as in practice accelerations are always involved, but I think it's important to understand Commented Oct 14, 2022 at 15:12
• thanks a lot! But what I actually wanted to know was that how does friction act on the wheel when say, a car moving towards north suddenly takes a right towards east. I guess like what happens when the wheel turns 90 degree in the above scenario? I hope you got what I meant Commented Oct 14, 2022 at 16:11