Car Traveling in Circle: Centripetal Force = Force of Friction?

[MissionStanford]

I can't understand this conceptually/visually at all. Why is the frictional force providing the centripetal force? When I think friction, the immediate image that comes to my mind is this: When applying a force towards the right on a block, across a surface, the force of friction acts towards the left. That's simple. The force of friction simply resists the applied force and is thus in the opposite direction.

What about when a car's turning? There's the force of gravity, the normal force and the force of friction but no applied force (as far as I know) that is being resisted by the force of friction, so how does this work out? I'm also aware that this force of friction is static friction, but I still don't get it or why that makes a difference.

Please explain very clearly/simply. People who are good at physics tend to give explanations that might seem simple to themselves but just completely go over my head.

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[sciencebooks]

I hope somebody can help you (read: us) on this, 🙂.

 

[circulus vitios]

I'm having trouble understanding what you mean? Are you confused about the free body diagram, thinking there should be another vector opposing the centripetal force to create some sort of equilibrium? If you are, this doesn't happen because objects in uniform circular motion are always accelerating. The centripetal acceleration and centripetal force are constantly directed radially inward. The frictional force provides the centripetal force to keep the car from veering off the road, just as the tension in a mass-on-a-string keeps the mass and string from flying out of your hands. Without the frictional force there would be no centripetal force and the car would veer off the road, tangentially.

 

[MissionStanford]

I'm having trouble understanding what you mean? Are you confused about the free body diagram, thinking there should be another vector opposing the centripetal force to create some sort of equilibrium? If you are, this doesn't happen because objects in uniform circular motion are always accelerating. The centripetal acceleration and centripetal force are constantly directed radially inward. The frictional force provides the centripetal force to keep the car from veering off the road, just as the tension in a mass-on-a-string keeps the mass and string from flying out of your hands. Without the frictional force there would be no centripetal force and the car would veer off the road, tangentially.

Right, I get all of that. My question is, why would the force of friction be pointed radially inwards? Now, I realize the obvious answer is that there must be a centripetal force pointing radially inwards for an object to travel in circular motion. However, what I'm not getting is, what exactly would cause the force of friction to point in that direction?

To clarify my question, let me give you an example. If you're pushing a box towards the left, the force of friction points right, and the reason it points right is to resist the direction that the applied force pushes the object in. It is clear in this scenario as to what actually causes the force of friction to point to the right (The direction of the applied force, since the direction of the applied force is leftwards).

However, it is unclear to me as to what is causing the force of friction to point radially inwards during circular motion. Once again, I get that it needs to point inwards to keep the object in a circle. However, that doesn't tell me what is actually causing the force to point radially inwards. Maybe I'm just over-thinking. I just want to understand this concept like the back of my hand.

 

[V5RED]

Right, I get all of that. My question is, why would the force of friction be pointed radially inwards? Now, I realize the obvious answer is that there must be a centripetal force pointing radially inwards for an object to travel in circular motion. However, what I'm not getting is, what exactly would cause the force of friction to point in that direction?

To clarify my question, let me give you an example. If you're pushing a box towards the left, the force of friction points right, and the reason it points right is to resist the direction that the applied force pushes the object in. It is clear in this scenario as to what actually causes the force of friction to point to the right (The direction of the applied force, since the direction of the applied force is leftwards).

However, it is unclear to me as to what is causing the force of friction to point radially inwards during circular motion. Once again, I get that it needs to point inwards to keep the object in a circle. However, that doesn't tell me what is actually causing the force to point radially inwards. Maybe I'm just over-thinking. I just want to understand this concept like the back of my hand.

If the car's wheels were pointing forward, there would be no centripetal motion.
If the car's wheels were turned to the right, it would have centripetal motion in a clockwise circle.
If the car's wheels were turned to the left, it would have centripetal motion in a counterclockwise circle.

The direction of the motion is due to aiming the wheels in a desired direction and there being some force to allow the motion.

In all three cases, there is static friction that allows the tires to power the vehicle by preventing them from slipping as they turn. This points toward the front of the car because friction always opposes sliding, and a force toward the front of the car will oppose the sliding of the wheels.

In the case of straight motion, that is the only friction that is relevant to causing the motion.

In the case of the arcs, the momentum of the car must change constantly as the car turns, so another force is needed. This force needs to prevent the car from continuing in its current direction. You already knew this.

What you seem to be asking is why this force is friction and why it points directly toward the center of the circle.

The force is static friction because it is preventing the wheels from sliding on the road. The wheels are not pointing in the same direction as the car is moving. They are tilted at an angle.

This is static friction because it is preventing sliding, not slowing down an already sliding object and the wheels are not turning such that they can use the static friction to push off of. As such, it can neither slow down nor speed up the car. This means it needs to point in a direction perpendicular to the velocity of the car.

We already knew that the friction had to oppose the sliding of the tires, and we knew that the tires were aimed to turn the car in a circle and thus at an angle to the direction of the car.

Knowing that the friction must prevent the tires from sliding, it must be perpendicular to the motion of the car, and that the wheels are aimed at a tilt to the direction of the car, we know the direction of the force. It must face the center of the arc because no other direction fits the required conditions.

 

[sciencebooks]

I'm having trouble understanding what you mean? Are you confused about the free body diagram, thinking there should be another vector opposing the centripetal force to create some sort of equilibrium? If you are, this doesn't happen because objects in uniform circular motion are always accelerating. The centripetal acceleration and centripetal force are constantly directed radially inward. The frictional force provides the centripetal force to keep the car from veering off the road, just as the tension in a mass-on-a-string keeps the mass and string from flying out of your hands. Without the frictional force there would be no centripetal force and the car would veer off the road, tangentially.

I'm so ridiculous. The whole time I considered these problems, I was thinking of a car "sliding" into the middle when it didn't make the curve. WTH? Obviously it means, it'd just continue on it's tangential path forward instead of "turning". (...Right?)

 

[MissionStanford]

I'm so ridiculous. The whole time I considered these problems, I was thinking of a car "sliding" into the middle when it didn't make the curve. WTH? Obviously it means, it'd just continue on it's tangential path forward instead of "turning". (...Right?)

Correct. It's Newton's first law. An object in motion wants to stay in motion. A force is needed to change the path. The car's motion is always in a straight line at any instant (tangent to the path). It wants to keep going straight. The reason it doesn't is that a force changes its direction.

@V5RED - Thanks! Nice explanation. I think I'll have to look over it a couple of times to really get it.