Question

Friction(conceptual)

Answer 1

To stop a moving car brakes are applied. In order to stop the car in the shortest possible distance on a horizontal road, one should apply the brakes hard enough to just prevent slipping because slipping causes kinetic friction which is less than static friction(when slipping is prevented while braking).
But what about rolling friction, isnt it the least of the three in magnitude and why does static friction act and not rolling friction above??

Answer 2

Let's imagine that the tires roll without slipping. This is defined by the fact that the point of contact of the tire with the road doesn't move relative to an observer at the center of the wheel. That being said, the tires are actually experiencing a static coefficient of friction with the road surface.

Now, if we lock the wheels up, such that we no longer have a rolling without slipping condition, the tires experience kinetic coefficient of friction. Kinetic is typically less than static.

Now, regarding rolling friction. Indeed, this will typically be less than static or kinetic friction, but rolling friction (often dubbed rolling resistance) doesn't play a huge roll in braking performance. The important variables in braking performance is the torque exerted by the brakes on the wheels and the friction between the road and tires. (Indeed, brakes operate on the principles of friction, but the friction is between the brake pads and the brake caliper or brake drum depending on the type of brake.)

Rolling resistance comes into play when we consider fuel economy. When we are rolling along the road, the tires deform. (This is why they appear bulged at the point of contact with the ground.) This deformation of the tires is the main cause of rolling resistance. This is why you are encouraged to keep your tires inflated to the proper pressure, because lowering the pressure will increase the rolling resistance of the tires, thereby decreasing fuel economy. On the other hand, if we over inflate our tires, we will see the deformation decreases but this causes the contact patch of the tires (the area that is flat and pressed against the road) decreases in size. Now, I know you've always been taught that contact area isn't a variable in friction, but tire modeling is something much more complex than Coulomb's model of friction accounts for. This decreased contact patch size decreases the friction between the road and the tires, making the car unsafe as braking performance will be reduced.

Answer 3

@shayanreloaded You probably misunderstand what is meant by rolling friction.
As Eashmore explained, it is just a consequence of the tyre's elasticity, adding a very small torque to the overall motor or braking torque applied on the axis of the wheel. It is never taken into account in classical school-type problems such as the ones you are solving.

@eashmore
The middle sentence in the first paragraph should read:
[Rolling without slipping] is defined by the fact that the point of contact of the tire with the road doesn't move relative to THE ROAD.

Answer 4

@eashmore
@Vincent-Lyon.Fr
But then why it is easier to push a block with wheels(static friction-more) as compared to a block without them(kinetic friction-less). Shouldn't it be the other way round then?

Answer 5

Friction is a term used, when two bodies rub each other. The force produced during their relative movement is called frictional force.
Say 2 bodies A & B, slides over each other and body A is moving in particular direction with particular force. The frictional force produced over there is equal in amount but opposite in direction..
There is also good amount of heat generated when friction is created, Harapans used friction principle to ignite fire with the help of lime stones.Also you generate heat in winter when you rub ur hand with each other..

Frictional force has advantages and dis-advantages as well,

The Frictional force is used as advantage in many applications..
In Automobiles, clutches and brakes works on principle of frictional force, while in fluid flows, its disadvantageous as it is considered as loss and efficiency is reduced.
The Tyre of automobile keeps their balance due to friction between road and Tyre.

I can explain more, if you want essay on that ...

Answer 6

when a block is being moved with wheels,it has contact with the surface at lesser points than when it is moving or say sliding without wheels. so it faces less friction that is rolling friction is less than sliding friction

Answer 7

@Ganpat That isn't what I wanted
@mary92 i dont think u understood my question

Answer 8

look when friction is less isn't it easier to push a box?

Answer 9

obviously when we put wheels under a box friction become less and it is easier to push it than a box without wheels:-/

Answer 10

@shayanreloaded
Because you can lubricate the axis.

The wheel changes a kinetic friction force that is doing negative work in an equally intense static friction force which is not working at all.
It sorts of splits the friction in two parts : one on the ground, intense, but workless, and another one (a torque actually) on the axis, which is reduced to zero by lubrication.

It is easier to lubricate an axis than the ground itself, and the contact force of the wheel on the ground can also be directed forward, which is not possible with slipping-friction.

Answer 11

@shayanreloaded: Sorry for unwanted description..

In above scenario, Rolling friction acts on the tyre and not static friction as there is rolling motion involved.. Its just because of Inertial Slipping takes place, Inertia is tendency of body to be in motion, after other another force is acting.. And when slipping takes place that is the only part static friction comes into scene. This is simplest what i can tell..

Hope this answers... Thank u !!!