Frictional Force: Reaction Force & Turning Cars

[chocofingers]

1.is frictional force a reaction force.?

e.g when we walk there is force of friction that allow us to walk.if there isn't any friction we can't walk.
my question is that if there is no friction there will be reaction from the ground when someone will walk,why doesn't he walk then.
if friction is a reaction force then why it is less than the driving force very often.


2.how frictional force provides a centripetal force when turning a car.
plx help me in this. thnxz in advance

 

[K^2]

It's the direction. If there is no friction, reaction force from ground is perpendicular to surface. On a flat surface, that's straight up. Useful for not falling towards the center of the planet, not so much when you try to move. If you ever lost footing on ice, you should have some idea of what it's like. Except, on ice you can get a grip if you are careful, because there is still some friction.

As far as friction itself, which one you call action and which you call reaction is matter of preference, really. But yes, the ground always pushes on you as much as you push on it. That's true for vertical and horizontal components. No getting around that.When you make a turn, the car would "like to" continue traveling in the straight line, but that would require tires to slip relative to ground. The force preventing the slipping is the static friction force. Since the wheel rolls along direction of motion, there is no static friction in that direction. You get a force that's perpendicular to direction of travel and points into the turn. That's your centripetal force which keeps the car following the curve.

 

[chocofingers]

How do I thankyou K^2 :P

Thankyou so much!

 

[player1_1_1]

what is friction? i read article about friction in wikipedia but there is not explained what it is, is it something like fluid resistance?

 

[chocofingers]

yeah friction is an opposing force to every moving object plus an object that is at rest. A friction causes the book to stay on the table and yet on other occasions it is not necessary like when reducing friction b/w moving parts of machine
And fluid resistance has another term called drag force :)

 

[player1_1_1]

aha:D of course i know this but i didnt associated this with "tarcie" in polish, thanks:)

 

[chocofingers]

aww welcome :P

ANything else? what don't U understand ? :)

 

[chocofingers]

aww welcome :P

ANything else? what don't U understand ? :)

 

[K^2]

what is friction? i read article about friction in wikipedia but there is not explained what it is, is it something like fluid resistance?

There isn't a terribly good quantitative model that perfectly explains friction in all situations, but qualitative explanation for static friction force can be given.

An object supported by horizontal surface experiences some pressure distributed over the contact surface. If we look at it closer, we see that the surfaces are not perfectly smooth, and so instead, you have a whole bunch of contact points between object and support, and surface at points of support isn't always level. Since the normal force is perpendicular to the surface, you end up with support forces not all pointing straight up, but rather point at different angles. The net force is still pointing straight up.

Now, suppose you start pushing an object. You are now shifting the weight off the support points where support force points in the direction of your push and onto the support surfaces that point against it. You are effectively trying to push an object uphill. So in order to get the object to move you are going to apply a force that is some fraction of the object's weight. The fraction is the static friction coefficient, and will depend on smoothness and hardness of the two surfaces.

Once an object is in motion, however, this picture changes. Kinetic friction is much more difficult to explain on microscopic level, but it still has to do with object effectively always going "up-hill".