Friction forces on the tires of a moving car

[titasdasplus]

When a car is moving the velocity of the connection point between the car and the ground is zero. So static friction must be act here ,mustn't it ? But my mentor said ,it is kinetic friction. Which is correct? If kinetic friction, why?

 

[PeroK]

When a car is moving the velocity of the connection point between the car and the ground is zero. So static friction must be act here ,mustn't it ? But my mentor said ,it is kinetic friction. Which is correct? If kinetic friction, why?

Your mentor is completely wrong. There is no active friction force on a car moving at constant velocity. A car could move normally at constant velocity on a frictionless surface.

When the car accelerates, turns or brakes, static friction acts to provide the external force. Only if the wheels lock and the car skids does kinetic friction apply.

Note that tires are made of rubber with a high coefficient of friction. This allows greater acceleration and grip on the road. This would be an absurd design if kinetic friction were acting while driving.

A car moving on a flat surface will gradually lose speed through air resistance and rolling resistance, which is caused by the interaction of the tires deforming slightly against the surface. I.e. the tires cannot remain perfectly circular. But not by kinetic friction.

 

[A.T.]

When a car is moving the velocity of the connection point between the car and the ground is zero. So static friction must be act here ,mustn't it ? But my mentor said ,it is kinetic friction. Which is correct? If kinetic friction, why?

Cars are designed to ideally use static friction with the road to accelerate or brake. If you accelerate or brake too hard, the tires will slip and you have kinetic friction. You also have rolling resistance, which is not road friction but a combined effect of deformations and internal frictions.

 

[Arjan82]

Your mentor is completely wrong. There is no active friction force on a car moving at constant velocity. A car could move normally at constant velocity on a frictionless surface.

Although I agree that the mentor is completely wrong, I don't understand the part after that. There is aerodynamic drag acting on the car which means that a constant force is necessary to maintain constant velocity. This means a car cannot maintain constant velocity at a frictionless surface, you need the friction between the tire and road to overcome the aerodynamic drag. This is of course still static friction.

 

[PeroK]

Although I agree that the mentor is completely wrong, I don't understand the part after that. There is aerodynamic drag acting on the car which means that a constant force is necessary to maintain constant velocity. This means a car cannot maintain constant velocity at a frictionless surface, you need the friction between the tire and road to overcome the aerodynamic drag. This is of course still static friction.

Well, I did say "could" rather than "would". We'd also need no air resistance for the already hypothetical scenario to apply.

 

[Arjan82]

Well, I did say "could" rather than "would".

That's just too subtle for me to pick up

 

[titasdasplus]

Although I agree that the mentor is completely wrong, I don't understand the part after that. There is aerodynamic drag acting on the car which means that a constant force is necessary to maintain constant velocity. This means a car cannot maintain constant velocity at a frictionless surface, you need the friction between the tire and road to overcome the aerodynamic drag. This is of course still static friction.

Can you tell me about aerodynamic drag?

 

[jbriggs444]

Can you tell me about aerodynamic drag?

That is a pretty open ended question. What do you want to know?

Aerodymamic drag is a force that opposes the motion of an object through a gas. The faster the object moves relative to the gas, the larger the drag force becomes.

At low speeds where not much energy is imparted to the gas, aerodynamic drag scales linearly with relative velocity. The drag in this regime depends on the viscosity of the gas.

At higher speeds, turbulence comes into play and aerodymic drag scales quadratically with relative velocity. The drag in this regime depends on the density of the gas. Cars on the highway typically operate in the quadratic regime.