Let us imagine a car (or just a block) moving in uniform circular motion with radius $R$ with velocity $v$, on a flat surface. It is obvious that friction is the only force capable of providing the centripetal acceleration $\frac{mv^2}{R}$. However, I have the following doubts:

  1. Is this friction static or kinetic? I feel that as there is relative motion occurring between the road and the car, it should be kinetic. However, my book and common experience says that is static as if it would be kinetic we could only move at a particular $v$.
  2. What would be the direction of the frictional force? I understand that the car would have a tendency to skid outward, and friction would try to oppose it, thus having a centripetal component. But why doesn't it have a tangential component, as the velocity of the cycle is tangential and friction acts so to oppose relative motion, which I interpret as opposing the direction of change of motion?
  3. I suppose when driving a car in circular motion we will need to put on the accelerator, but I do not see why we would need to keep applying a force to move circularly?

1 Answer 1


Static if there is no relative motion between the ground and the tyres at the point of contact.
If it was a block then as there was relative movement between the block and the ground then it would be kinetic friction.

You need to produce a centripetal acceleration and so need to provide a force towards the centre of the circular trajectory.
Although the cycle is moving forward if there is no slipping at the point of contact between the ground and the tyres (and also there are no other frictional forces eg air resistance) no tangential force should be needed to maintain a constant speed.
If it was a block you would need a tangential force to maintain a constant speed and a radial force to make the block move in a circle so the net foce would be at some angle between the radial and the tangential forces.

You only need the car engine to do work if there are frictional etc forces acting on the car and you want it to maintain a constant speed. In a lot of Physics problems the frictional forces are assumed to be zero.

  • $\begingroup$ I don't think you've actually answered my doubts. Why is there no relative motion between the ground and a block in circular motion? $\endgroup$
    – Sawarnik
    Jun 30, 2016 at 20:00
  • $\begingroup$ Sorry. I wrote about a car and then about a cycle. It was not clear to me that you wanted to know about a block. I have now added that. $\endgroup$
    – Farcher
    Jun 30, 2016 at 20:47
  • $\begingroup$ @Sawarnik All 3 of your questions ask about a car or cycle not a block. $\endgroup$ Apr 5, 2020 at 20:16

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.