# Deciding if static or kinetic friction is acting on a system

In this recent question which discusses about why it is static friction which acts on a moving car. User Neelim answered this question with an explanation based on the condition of non accelerated rotating and hence there is no actual relative motion between road and the car wheels here

This leads me to my variant of the question: Suppose instead of a car, we had a box set to move in the circular path by action of some forces. Would static friction act still in the centripetal direction?

In other words, does the presence of motion in a tangential direction have any control over the friction which acts in the centripetal direction?

### Note:

The reason I told box is to have a flat surface w/o rolling.

The static friction in the centripetal direction is because of skidding.

You might have seen the skid marks on the turns. This skidding motion is away from the centre of the track and hence this skidding relative motion leads to the centripetal static friction force.

Indirectly we can say that the static friction force was due to the tangential velocity since this tangential motion caused skidding.

Hope it helps 🙂.

• Mhm , I do understand the cause of the static friction which is the impending skidding motion but in this case of box which has a flat surface. We notice that it is having a relative velocity with surface, yet you suggest it should be static friction which acts.. why? Dec 26, 2020 at 14:47
• @Buraian there are two types of friction in play here. One due to tangential velocities directly and other indirectly. The other one is still static since the box still skids away from the trac.. Dec 26, 2020 at 15:19
• Interesting and a bit strange at the same time :o Dec 26, 2020 at 15:44
• @Buraian was that helpful ? Dec 26, 2020 at 15:57
• Yeah, I was vaguely thinking around these lines but one thing which still bothers me is that there is a relative motion between surfaces but still it's static friction which acts Dec 27, 2020 at 4:52

Your question is a bit to broad, so I will try to break it down into simpler to analyze chunks.

In a moving car the only force doing work is friction (both while accelerating and curving). You "push" the floor, and it pushes you back in the direction you want to go (imagine walking on a piece of ice, in this case locomotion is difficult because there is little friction). So, it is always friction that makes a car move.

In the case of the box, it depends on the source of locomotion. If the box is propelling itself with wheels, it is a car. If it uses a fan, or someone is pushing it around, then these would be the sources of the force that makes it go around the curve.

In the case of a car, the reason why static friction is the responsible force is because the patch of the wheel in contact with the floor has zero velocity (relative to the floor). In the case of a box, this would be impossible (unless it has wheels and is a car).

• See edit, I was trying to capture the idea that surface is flat with the box thing Dec 26, 2020 at 14:45
• If it is flat with no rolling then: "If it uses a fan, or someone is pushing it around, then these would be the sources of the force that makes it go around the curve."
– JGBM
Dec 26, 2020 at 18:56