# Why does friction play the role of centripetal force?

• navneet9431
In summary, frictional force is the centripetal force of a car turning along a curve because it acts inwards to prevent slipping, which would cause the car to move outward. This is necessary for the car to change direction and maintain its circular motion. The direction of friction is dependent on the direction of the turn and it is the only force that can act on the car to cause acceleration.
navneet9431
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?

navneet9431 said:
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.

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
russ_watters said:
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!

navneet9431 said:
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.

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

navneet9431 said:
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?

navneet9431 said:
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
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.
Dale said:
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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navneet9431 said:
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".

navneet9431 said:
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?

navneet9431 said:
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.

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.

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

## 1. What is friction?

Friction is the force that resists motion between two surfaces that are in contact with each other. It is caused by the roughness and irregularities of the surfaces, and it acts in the opposite direction of the intended motion.

## 2. How does friction contribute to centripetal force?

When an object moves in a circular path, it experiences a centripetal force that keeps it moving towards the center of the circle. Friction plays a role in this by providing a force that acts towards the center of the circle, thus contributing to the overall centripetal force.

## 3. Can friction be the only force acting as a centripetal force?

No, friction alone cannot act as the sole centripetal force. Other forces, such as gravity or tension, are also needed to keep an object moving in a circular path. Friction only contributes to the overall centripetal force required for circular motion.

## 4. How does the coefficient of friction affect centripetal force?

The coefficient of friction is a measure of the amount of friction between two surfaces. The higher the coefficient of friction, the greater the frictional force that can contribute to the centripetal force. Therefore, a higher coefficient of friction can result in a stronger overall centripetal force.

## 5. Are there any cases where friction is not the centripetal force?

Yes, friction is not always the only or even the main force that acts as the centripetal force in circular motion. In cases such as planetary orbits, the centripetal force is provided by gravity, not friction. Similarly, when a car turns on a banked road, the normal force from the road provides the centripetal force, not friction.

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