# Friction questions, How does a car turn?

#### ocdocd

Q1. Is static friction between a non-moving object and surface? Or is it specific to velocity components?

For example, there's a box sliding along a table. It's moving east at some velocity, and it's not moving at all in the north/west direction. If I push it in the north direction, and it doesn't move, does it resist that push with kinetic friction or static friction? I mean, since it has no velocity component in the north/west direction...

Q2. How does a car turn? I know that static friction is the centripetal force, causing the centripetal acceleration, but where does the friction come from? It has to resist motion... What motion is it resisting? How?

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#### chroot

Staff Emeritus
Gold Member
A1. The two forces applied together act as a single force pushing in a different direction. (If you're pushing both north and east, it's exactly as if you were just pushing northeast.) If the box is moving, then the friction experienced it that of kinetic friction in its new direction of motion.

A2. When a car turns left, it pushes the pavement to the right, and the lateral movement is resisted by friction. The force of the friction results in the centripetal acceleration.

- Warren

#### rcgldr

Homework Helper
Is static friction between a non-moving object and surface?
By definition, static friction is the friction between non-moving (with respect to each other) surfaces. Dynamic friction is the friction when the surfaces are moving with respect to each other.

For example, there's a box sliding along a table.
That would be dynamic friction, not static friction.

It's moving east at some velocity, and it's not moving at all in the north/south direction. If I push it in the north direction, and it doesn't move, does it resist that push with kinetic friction or static friction?
I would expect the box to move in response to the northwards force.

How does a car turn? I know that static friction is the centripetal force, causing the centripetal acceleration, but where does the friction come from? It has to resist motion... What motion is it resisting? How?
The steering driven tires are angled, so that the direction of their "flow" of the contact patch is different than the direction the tires are actually traveling. This difference is called the slip angle, but at low forces, it's mostly due to flexing of the contact patch, not actually slippage in the contact patch.

The net result is that the slip angle causes the tires to extert an outwards force onto the pavement, which responds with an equal and opposite force inwards, which causes the car to turn.

This happens even if the tire is skidding, as the force doesn't drop to zero even when the entire contact patch is sliding.

For an example of slip angle, imagine a car with the tires all pointed straight ahead, at rest on pavement that is sloped left to right. There's a component of gravity that pulls the car to the right. If the car is moving forwards, then flex in the contact patches will allow the car move to the right with the tires pointed straight. The car will need to be steered slightly to the left in order to go straight ahead.

#### eaboujaoudeh

just a small note, the car turns because the differential gear distributes motion unequally, so that u have 1 wheel turning quicker than the other, thus in the same amount of time 1 wheel turns more than the other, as if a plane hasthe right engine on and the left engine off u'll expect it to turn. same happens with a car, the wheels are twisted so that they don't break.

#### brewnog

Gold Member
just a small note, the car turns because the differential gear distributes motion unequally, so that u have 1 wheel turning quicker than the other, thus in the same amount of time 1 wheel turns more than the other.
This isn't why a car turns. This is designed in as a consequence of the car needing to turn.

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#### AlephZero

Homework Helper
Q1. Is static friction between a non-moving object and surface? Or is it specific to velocity components?

For example, there's a box sliding along a table. It's moving east at some velocity, and it's not moving at all in the north/west direction. If I push it in the north direction, and it doesn't move, does it resist that push with kinetic friction or static friction? I mean, since it has no velocity component in the north/west direction...
There is a difference between static and dynamic friction.

Static friction is between a non-moving object and a surface (thats what "static" means). The friction force can be in any direction on the surface. The magnitude of the friction force must be less than some limit otherwise the object will start to move. The simplest description of the limiting force is the Coulomb law: friction force <= mu_s times the normal force between the object and the plane where mu_s is the static coefficient of friction.

Dynamic friction is between a moving object and a surface. The Coulomb law says that the friction force is equal to mu_d times the normal force, and acts in the opposite direction to the motion. mu_d is the coefficient of dynamic friction.

So far as the friction is concerned, you can't split up the motion by saying something is moving east-west but not moving north-south. Either it's moving (dynamic friction applies) or it isn't (static friction applies).

If your box is moving east, the dynamic friction force is towards the west. If you suddenly apply a force towards the north at time t=0, the dynamic friction force at t=0 will still be towards the west. The applied force will accelerate the box towards the north, so it will move in a curve. The friction force is always along the tangent to that curve and opposite to the direction the box is moving. So the component of friction force in the south direction will increase and the component in the west direction will decrease.

#### rbj

Q2. How does a car turn? I know that static friction is the centripetal force, causing the centripetal acceleration, but where does the friction come from? It has to resist motion... What motion is it resisting? How?
your front wheels are not spherically-shaped rolling ball bearings. they are wheeled bearings. that means the frictional resistance of rolling in the direction at a right angle of the axis is much lower than the resistance of sliding along at a direction closer to that which is in line with the (turned) wheel axel. so the resistance force required to roll straight-ahead is larger (because straight-ahead has a non-zero component that is parallel to the axis) than the resistance to roll at a right angle to the axel.

essentially, if you considered your wheel bearing to be perfectly lubricated, then the road cannot push on your car in the direction perpendicular to the axel. so any force acting on the car has to be inline with the (turned) axel. so if you're turing left, there is a force from ahead pushing back as well as a force from the right pushing left. and these two forces have to add to a net force that points inline with the axel. so what happens is your velocity component that is in the direction of what used to be straight ahead is being reduced and the velocity from right to left is increasing due to these two forces (that add to the net force inline with the axel).

#### Red_CCF

A2. When a car turns left, it pushes the pavement to the right, and the lateral movement is resisted by friction. The force of the friction results in the centripetal acceleration.
Hi

I was wondering if someone can explain how the car wheel push the pavement?

Thanks