# How Does Friction Function in the Movement and Turning of a Car?

• I
• PavithraSelvaraj
PavithraSelvaraj
How can friction be responsible for all of these: stopping, moving and also turning a car?
Does friction actually exist or is it something we assume because we don't know something about motion of objects?
I have read a lot of discussions about friction and now it is a cloud of mess in my brain. I would be glad if someone makes me understand it from scratch and progressively answer my query.

Huh? Friction is responsible for IMPEDING moving, not creating it.

PavithraSelvaraj
phinds said:
Huh? Friction is responsible for IMPEDING moving, not creating it.
Yes, it stops slipping, I guess?

PavithraSelvaraj said:
Yes, it stops slipping, I guess?
Well, it may or may not STOP it, but it at least slows it down. I can't see why you would say

"Friction is responsible for moving"

If there wouldn't be any friction between tires and the road the car won't move, only the wheels will spin.
This made me say that statement though it may come across differently?!

phinds and PeroK
phinds said:
Well, it may or may not STOP it, but it at least slows it down. I can't see why you would say

"Friction is responsible for moving"
Unfortunately it is! You can't walk, cycle or drive a car without friction! Friction is ultimately the motive force in most ground transportation.

russ_watters, Dale and PeterDonis
PavithraSelvaraj said:
How can friction be responsible for all of these: stopping, moving and also turning a car?
Does friction actually exist or is it something we assume because we don't know something about motion of objects?
I have read a lot of discussions about friction and now it is a cloud of mess in my brain. I would be glad if someone makes me understand it from scratch and progressively answer my query.
This is not something that is easy to do. Authors spend considerable time writing textbooks and online pdf's to present a topic from the ground up. It's better to use this forum to tackle specific issues and doubts.

The underlying principle is Newton's third law. If you push on something, then it pushes on you. This is what allows friction to become a motive force. Friction itself is complicated, but a understanding of the fine details is not necessary to understand how it operates in a mechanical system.

PavithraSelvaraj
Thank you.

PavithraSelvaraj said:
Does friction actually exist or is it something we assume because we don't know something about motion of objects?
It actually exists. Once, while walking, on an icy pavement I hit a patch of "black" ice. That is a flat transparent film of ice with a thin layer of water on top which is as close to a frictionless as can get. I slipped, fell and landed on my shoulder which broke and needed surgery to be fixed. It's static friction that propels you forward when you start walking and helps you maintain your balance as you move.

As for a car, if it hits a patch of black ice it will lose traction which means that, according to Newton's first law, it will maintain the velocity it had before hitting the ice and will not respond to the brakes or the steering wheel. That also happened to me but I managed to regain control after my car slid off the ice patch without having an accident.

A way to mimic frictionless walking is this. Put on roller-skates and try to walk normally, not skate. Be sure there is someone next to you ready to catch you when you fall because you will fall. Friction in everyday life is so common that its importance is not appreciated but its absence stands out.

PavithraSelvaraj
PeroK said:
Unfortunately it is! You can't walk, cycle or drive a car without friction! Friction is ultimately the motive force in most ground transportation.
I understand the need for friction. as you say, but I certainly would not call it the motive force. I think leg muscles, car engines, etc are the motive force and friction does not CAUSE the motion (but you make a good point that it is necessary for some kinds of motion). Some types of motion have nothing to do with friction. For example, the electrons shooting out of an electron gun, or particles being accelerated in a collider.

EDIT: oops ... I see you did specify most ground transportation, not motion in general.

bob012345 and PavithraSelvaraj
phinds said:
I understand the need for friction. as you say, but I certainly would not call it the motive force. I think leg muscles, car engines, etc are the motive force and friction does not CAUSE the motion (but you make a good point that it is necessary for some kinds of motion).
There's little point in arguing what motive force means. That said, I would say that the engine is the energy source for a car accelerating, and static friction on the road is the external motive force. Internal forces cancel by Newton's third law.

When braking the car is losing energy (probably to heat) within the braking system, and again static friction is the external force that decelerates the car.

And, the external centripetal turning force is again static friction from the road.

russ_watters and Dale
kuruman said:
Once, while walking, on an icy pavement I hit a patch of "black" ice. That is a flat transparent film of ice with a thin layer of water on top which is as close to a frictionless as can get. I slipped, fell and landed on my shoulder which broke and needed surgery to be fixed.
I am sorry to hear that!
kuruman said:
Friction in everyday life is so common that its importance is not appreciated but its absence stands out.

kuruman
PavithraSelvaraj said:
How can friction be responsible for all of these: stopping, moving and also turning a car?
Does friction actually exist or is it something we assume because we don't know something about motion of objects?
I have read a lot of discussions about friction and now it is a cloud of mess in my brain. I would be glad if someone makes me understand it from scratch and progressively answer my query.
Great topic. Why questions are open ended and sometimes lead to new discovery. Ultimatel
PavithraSelvaraj said:
How can friction be responsible for all of these: stopping, moving and also turning a car?
Does friction actually exist or is it something we assume because we don't know something about motion of objects?
I have read a lot of discussions about friction and now it is a cloud of mess in my brain. I would be glad if someone makes me understand it from scratch and progressively answer my query.
Great question. Much work has been done (pun intended) to investigate these questions. And despite numerous theories, the phenomenon ultimately is due to the forces at a very tiny scale. The interactions of molecules, their constituent atoms in an accelerating field produce the effects observed as friction here on Earth. All phases of matter have some interactions with each which can be thought of as friction. I would propose then that friction is that interaction, whether static or dynamic. Of course “static” is relative to the scale one is observing, as everything is in constant motion or vibration. So, does friction exist? Yes, a phenomenon called friction has been observed and quantified over numerous times and methods. Why? Great question. What do you think? What do you observe at the macroscopic scale? With what you know now, what can you then infer about the microscopic scale? If our knowledge to answer that is incomplete, what should we study to get that knowledge to be able to make better guesses? Anyway, food for thought to such a thoughtful question.

PavithraSelvaraj
Ken_S said:
Why questions are open ended and sometimes lead to new discovery.
Yes! Exactly why I asked this question
Most sources answer "what" rather than "why" questions
Thank you!

PavithraSelvaraj said:
How can friction be responsible for all of these: stopping, moving and also turning a car?
Friction opposes slipping. That is all it does.

We then engineer devices such that not stopping, not moving, and not turning would produce slipping.

russ_watters, A.T., TSny and 4 others
Friction, writ large:

Can you see why it's better than slick-tires-on-a-slick-road for starting, stopping and turning?

dlgoff, PavithraSelvaraj, Ibix and 2 others
PavithraSelvaraj said:
How can friction be responsible for all of these: stopping, moving and also turning a car?
You need an external force for all that, and friction is the simplest choice.

You could also build a car with pivoting rocket engines to achieve all above, but using friction is simpler and cheaper.

PavithraSelvaraj
Dale said:
Friction opposes slipping. That is all it does.

We then engineer devices such that not stopping, not moving, and not turning would produce slipping.
That explains my question in a very simple way! Thanks

Dale
When two surfaces are in contact they each exert a force on the other. It's a convention to refer to the component of those forces that is parallel to the surfaces as the friction force, and the component normal to the surfaces as the normal force.

Without this force you could, for example, pass your hand through the surface of a table. Do it hard enough and your hand will hurt. Obviously this force exists.

PavithraSelvaraj
Mister T said:
When two surfaces are in contact they each exert a force on the other. It's a convention to refer to the component of those forces that is parallel to the surfaces as the friction force, and the component normal to the surfaces as the normal force.

Without this force you could, for example, pass your hand through the surface of a table. Do it hard enough and your hand will hurt. Obviously this force exists.
Wait. This seems suss.

It seems to imply that, as the friction of a surface approaches zero, the ability to pass through the surface potentially rises from non-zero. There are not surfaces that are effectively frictionless (zero parallel resistance), yet still demonstrably solid (high normal resistance)? Or am I reading too much into your implication?

DaveC426913 said:
It seems to imply that, as the friction of a surface approaches zero, the ability to pass through the surface potentially rises from non-zero
I'm talking about the contact force between your hand and the table. The friction force is just the component parallel to the surface of the table. There is also a component normal to the surface.

PavithraSelvaraj said:
How can friction be responsible for all of these: stopping, moving and also turning a car?
Does friction actually exist or is it something we assume because we don't know something about motion of objects?
It is an interesting question. Looked at in a logical sequence, it is gravitational forces that are responsible for friction. A look at the events in the International Space Station where gravity is at a minimum, shows that objects don’t even stay put in one place, they tend to float, let alone experience friction. But, at some point electrostatic forces must also take part. The atoms in our bodies are made up of 99.9999999999 per cent empty space, similarly the atoms in a wall are also made up of 99.9999999999 per cent empty space, Yet, it is not possible to walk through walls. In fact, the reason we are able to stand on the surface of the earth is because of electrostatic forces. If not for these forces, we would sink right through the earth. So friction in the end, seems to be a combination of gravitational and electrostatic forces.

This conjecture leads to another conjecture, namely, for a person not to be able to walk through walls or stand on the surface of the earth without sinking through, means that the electrostatic forces being considered must be repulsive forces. But gravity is what keeps things anchored to the earth. If gravity is 10^40 (approx.) times weaker that the electrostatic force. How does this happen?

The massive size of the earth in comparison to everything else might have something to do with it.

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jbriggs444, weirdoguy, PavithraSelvaraj and 1 other person
jzz said:
it is gravitational forces that are responsible for friction
Not necessarily. There has to be some normal force in order for there to be a friction force, yes. But the normal force doesn't have to be provided by gravity.

jzz said:
A look at the events in the International Space Station where gravity is at a minimum, shows that objects don’t even stay put in one place, they tend to float, let alone experience friction.
An object will experience friction if you, for example, push it up against the wall of the station and then try to slide it along the wall. In other words, if you provide a normal force pushing the object into the wall.

jzz said:
at some point electrostatic forces must also take part
Of course. The forces causing atoms to repel each other at short enough distances are electrostatic.

jzz said:
gravity is 10^40 (approx.) times weaker that the electrostatic force
More precisely, gravity between two electrically charged particles like electrons is about ##10^{40}## times weaker than the electrostatic force between the same two particles.

But for macroscopic objects, the force balance is very different. The objects as a whole are electrically neutral, because positive and negative charges are present in equal numbers and so cancel each other out, so there is no electrostatic force between them as bulk objects; there is only the very short-range repulsion between their atoms when you try to push them together. But gravity does not cancel out; there is no gravitational counterpart to positive and negative charges, there is just positive mass. So gravity just keeps piling up the more and more mass is present.

jzz said:
The massive size of the earth in comparison to everything else might have something to do with it.
That is what makes the Earth's gravity easily perceptible to us, yes. But with very sensitive instruments we can detect the gravity of much smaller objects (look up the Cavendish experiment and more recent elaborations of it).

jzz said:
It is an interesting question. Looked at in a logical sequence, it is gravitational forces that are responsible for friction.
Not at all. Place your hands out in front of your like you're praying and them rub them vigorously. Friction without any need of gravitation. Friction is entirely electromagnetic except in the sense that gravity can force two surfaces together. That is, gravity can bring two objects together so their surfaces touch, but it is solely the electromagnetic forces of the atoms that causes friction. Or, to pet it another way, you can have friction without gravity, but you can't have friction without electromagnetism.

PavithraSelvaraj
PeterDonis said:
An object will experience friction if you, for example, push it up against the wall of the station and then try to slide it along the wall. In other words, if you provide a normal force pushing the object into the wall.
True, astronauts on the space station are able to shave using similar methods, even though bristles floating in the air might need a more complicated approach.

PavithraSelvaraj said:
How can friction be responsible for all of these: stopping, moving and also turning a car?
Yes, no propulsive force could be applied to any vehicle by driving its wheels if friction were not present between the tyre and the road; in the absence of friction the wheels would merely spin round in place.
PeterDonis said:
Not necessarily. There has to be some normal force in order for there to be a friction force, yes. But the normal force doesn't have to be provided by gravity.
An important consideration in attributing the major component involved in the generation of friction to gravity is the non-intuitive but true fact that the force of friction is independent of the area in contact. That is a large surface will experience the same frictional force as a smaller area. As an example, one could consider brakes in a car. Increasing the area over which the brake acts would not result in an increase in the braking power of the car.

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weirdoguy, PavithraSelvaraj and Motore
jzz said:
That is a large surface will experience the same frictional force as a smaller area. As an example, one could consider brakes in a car. Increasing the area over which the brake acts would not result in an increase in the braking power of the car.
If the normal force per unit of area between the two surfaces is unchanged, then that statement is nonsense.

davenn
phinds said:
If the normal force per unit of area between the two surfaces is unchanged, then that statement is nonsense.
Fortunately, or unfortunately depending upon one’s point of view, classical physics is an exact science unlike quantum mechanics which depends heavily on statistics. Therefore, there is no reason to indulge in calumny or strong language. Just look it up. And, yes, I was referring to an instance where the applied force remains the same but the area of contact increases.

renormalize, phinds and weirdoguy
Physics is not like a game of Cluedo. This thread should include a reference to Feinman's "Why?" rant.

Q. Why does the car spring forward at the traffic lights?
A. Because I need to go to the grocery store.

Friction allowed me to hold the pencil while I was writing the list. It allowed me to walk to the car and for the clutch to connect the engine to the transmission. etc. etc. We just give the lateral forces of contact the name 'friction' but they are only ever 'intermediate agencies'. They are not 'responsible' for anything.

PavithraSelvaraj
jzz said:
Just look it up.
No, you are the one who made the claim, so you provide a reference that backs it up.

jzz said:
...it is gravitational forces that are responsible for friction.
PeterDonis said:
Not necessarily. There has to be some normal force in order for there to be a friction force, yes. But the normal force doesn't have to be provided by gravity.
Drakkith said:
Not at all. Place your hands out in front of your like you're praying and them rub them vigorously. Friction without any need of gravitation.
In jzz's defense, it appears they were speciifically addressing the starting, stopping and turning of a vehicle, rasied by the OP, not generalized friction.

(My non-gravity example, before I realized this, was going to be screws, nails and knots; all of which would fall apart if not for friction.)

PeterDonis said:
No, you are the one who made the claim, so you provide a reference that backs it up.
Guillame Amonton's Second law of friction:
The force of friction is independent of the apparent area of contact.

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jzz said:
Guillame Anton's Second law of friction:
The force of friction is independent of the apparent area of contact.
This isn't a reference, since there is no link or source given.

PeterDonis said:
This isn't a reference, since there is no link or source given.
I won't argue semantics with you, he is a historically well known and documented scientist.

jzz said:
I won't argue semantics with you
I'm not "arguing semantics". I'm telling you the PF rules for references, which you signed up to when you became a member.

jzz said:
he is a historically well known and documented scientist.
So what? You still need to give a specific link or source (as in, what book or paper, published where and when). You could be quoting (or claiming to quote) Einstein and you would still need to give a specific link or source.

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