# Why does friction play the role of centripetal force?

• B
Gold Member
What makes frictional force the centripetal force of a car turning along a curve?
As friction is the opposing force and acts anti-parallel so there is no component of frictional force towards the center,right? Then how can frictional force be centripetal force?

Dale
Mentor
2020 Award
As friction is the opposing force and acts anti-parallel so there is no component of frictional force towards the center,right?
No, it does act towards the center. Static friction opposes slipping. If you were to slip then you would slip outwards. Therefore the friction points inwards.

russ_watters
Mentor
Please note that for a car moving straight and at constant speed, in a vacuum or arbitrarily slow (no wind resistance) the force of friction is zero.

navneet9431
Gold Member
Please note that for a car moving straight and at constant speed, in a vacuum or arbitrarily slow (no wind resistance) the force of friction is zero.
Suppose I am moving in on straight path.To move on a circular path I need to turn my wheels.But how does Friction causes the motion inwards(I mean circular)?Please explain this!

russ_watters
Mentor
Suppose I am moving in on straight path.To move on a circular path I need to turn my wheels.But how does Friction causes the motion inwards(I mean circular)?Please explain this!
In order for the car to accelerate in a certain direction ("inwards"), *something* needs to apply a force to it.

Gold Member
Yes but how can that force be Friction.
How turning of the wheels generate friction , and that too inwards and not outwards?
In order for the car to accelerate in a certain direction ("inwards"), *something* needs to apply a force to it.

russ_watters
Mentor
Yes but how can that force be Friction.
I don't understand; how can it be anything else? Friction is the only force that can act on the car.
How turning of the wheels generate friction , and that too inwards and not outwards?
As @Dale said, if the wheels are not aligned with the direction of motion, they would have to slip in order for the car to keep going forward. They don't slip. Friction is the force that prevents the slipping.

As friction is the opposing force and acts anti-parallel...
It isn't clear to me what you are saying, but it sort of seems like you think friction always acts in the opposite direction of motion. Why don't we try straight line motion to make it clear:
1. When the car is accelerating forwards, which direction does friction act on the car?
2. When the car is accelerating backwards (braking from forward motion), which direction does friction act on the car?

Dale
Mentor
2020 Award
But how does Friction causes the motion inwards(I mean circular)?Please explain this!
I did this in post 2. What did you not understand from that post?

Static friction acts in a direction to oppose slipping. During a turn slipping is outwards, so friction is inwards.

russ_watters
Gold Member
I did not understand,how the slipping is outward?
I mean what is the reason that the car is slipping is outward?
I feel that the slipping is inward because the car seems to move on a circular track which continuously bends inward.
I did this in post 2. What did you not understand from that post?

Static friction acts in a direction to oppose slipping. During a turn slipping is outwards, so friction is inwards.

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russ_watters
Mentor
I did not understand,how the slipping is outward?
I mean what is the reason that the car is slipping is outward?
I feel that the slipping is inward because the car seems to move on a circular track which continuously bends inward.View attachment 228686
That's turning, not slipping. Slipping is what the car does if the wheels don't have friction and the car doesn't turn. E.G., when you turn the wheels on ice and the car keeps going straight. If the car moves away from the direction it is supposed to be turning, that is "outward".

Dale
Mentor
2020 Award
I did not understand,how the slipping is outward?
Imagine you are in a car and you are turning to the left, and suddenly you hit a patch of ice and lose control and start slipping. When it slips, does your car suddenly start turning inward even more sharply to the left or does the turn go to the right and become less sharp?

I feel that the slipping is inward because the car seems to move on a circular track which continuously bends inward.
I get the impression that you don’t understand what slipping is. Slipping is what happens when friction is too low, like on icy or wet roads. Under those conditions the tire slides rather than rolls. Other terms are skidding, drifting, or peeling out.

sophiecentaur
Gold Member
2020 Award
The case of an ideal tyre is just like ideal rails, with no slip or drag.
A real tyre has a friction / drag force that acts in a direction ‘behind’ the radius of the curve and that will cause slowing down, in addition to the curved path.

rcgldr
Homework Helper
Common forces only exist in Newton third law pairs. In the case of a turning car, the pavement exerts an inwards centripetal force onto the tires, coexistent with the tires exerting an outwards force onto the pavement (the outwards force is a reaction force related to centripetal acceleration). The car turns due to the centripetal force exerted by the pavement, and the earth is affected by a very tiny amount due to the outwards force exerted by the tires.

russ_watters