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At what direction is the frictional force?

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The frictional force is parallel to the surface, opposing the motion. So, it would be pointing down the slope, if the drawing in the arrow represents the motion.
Yes, the arrow represent the motion. But ummm, i'm not sure the frictional force is parallel to the surface, opposing the motion. Just like when we walk, the frictional force is in the same direction as our motion....

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No, it's not. The friction always opposes motion.
the friction force reduces the acceleration of the object. When you are walking, the friction between your shoe/foot and the ground keeps your foot from sliding forward indefinitely.
@BTaylor yes, it oppose the motion, but the motion of our shoes.
the a point is moving in rotation direction so frictions force oppose it
that's why the car can rise rump
get @imron07 ?
@RaphaelFilgueiras : What about this:
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im talking about this
the friction is opposite to the moviment of weel
Yeah, i understand your explanation. But if we apply Newton's law, should we pick that direction too for friction to find system acceleration?
the friction is opposite to the moviment of POINT OF CONTACT
Yes ma = mgsin alpha + f
Shouldn't \(mg\sin{\alpha}\) is in opposite direction with friction now?
yes it is opposite
@RaphaelFilgueiras yes that's what I mean :)
NO it is coincident with mg sin alpha : here is why (let me finish pls
And when wheel is freely rolling the friction acts BACK on it
so your car never will rise the ramp!!!
@RaphaelFilgueiras you are wrong : in his picture there are NO OTHER forces. The cart is only rolling up BY INERTIA and of course slows eventually. This is different from motorized climbing up !!!
the car is moving up
The point of contact is moving a wee-bit slower than the surface - so the surface ACCELERATES it !
@Mikael : So only when the engine turned on, friction will directed upward?
And pls medal (-s)
the vector is velocity. And please admit that I am right @RaphaelFilgueiras
Again I clarify : When the wheel is free it serves as a friction-conduit from the surface to cart. When the motor is working the opposite: the wheel transfers torque from cart to surface
It is a delicate but DRAMATIC CHANGE
Well, i understand both of your opinion. But i agree with @Mikael . If the vector I draw is Force, would something change (engine on)?
Of course - all is opposite
they are opposite but mg is greater than f
so it will stop,and then the friction and mg will be in same direction
This time @RaphaelFilgueiras IS ok. Though he will never admit someone else IS .....
The last remark of Raf is WRONG (again!)_
You mean friction is downward even if the tire's working to move the car upward too?
Friction is UPWARD when the tire PUSHES the surface (working) and downward when the SURFACE pushes the tire
Both of you should imagine a tiny (really tiny) velocity difference betwee Contact point and surface. It has OPPOSITE directions in these opposite cases !
@Mikael can you draw it please, (when engine turned on, and another force pull the car too).
When wheel PUSHES it is faster downward - so frict is UPWARD
Okay, I understand now. Thanks @RaphaelFilgueiras & @Mikael !
You see the friction force is OPPOSITE to the RELATIVE VELOCITY\[V_{contact} - V_{\text{surface velocity relative \to wheel's center}}\]
Yeah, that's what in my mind :D
@imron07 and @RaphaelFilgueiras : Thank you both for educating me and all of us by your questions and thoughts.
It was a nice discussion :)
Veritas in disputando gignitur.
What's that last phrase means?
@RaphaelFilgueiras : can you explain more about your pic please?
forget the plane|dw:1348332081044:dw|
Okay, agree, then
the direction of friction is always in the moviment direction
in this case
Yes, please continue
so in your picture f is in direction of moviment
You mean my first, or second picture (or both)?
get it?
just matter the direction of moviment
Mmm, still thinking. For the first picture, it makes sense. But for the seecond (where another force pull the car)?
no matter the friction is always in direction of moviment
What disturb me: when we calculate the car's acceleration, ma=F+f-mg*sin(theta) It seems friction 'helps' F to push the car up.
the second car is moving?
but friction help,imagine if there is no friction a car will not move when you accelarate it,the wheel will slip
The force of friction is the only force that actually pushes a motor-vehicle that produces work. Yes it "helps" motion of vehicle by OPPOSING the backward motion OF THE WHEEL CONTACT POINT. For visual clarity - look at this wheeled vehicle . What pushes it forward on land or in water ? Well the friction force of course - friction of wheel contact point with the surface !
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@imron07 read.
by the way I have a more measurement oriented proof for you. You can measure the DIRECTION of friction force with your hands and feet DIRECTLY. Experiment 1: CLIMB A CHILDRENS Kindergarten's slide with your palms. What do you feel - does the friction help you climb ? Well of course ! But now Experiment 2: Slide on your derriere DOWN the slide. Does the friction obstruct your motion ? or Better - try rolling a small coin on flat plane. Frictions opposes of course. See I gave the correct description
kinetic Frictional force always opposes the moving body... therefore its always is in the opposite direction :B|dw:1348390006451:dw|
@Alice_Dump : Thanks for contribution, but I think you should read all the comments from beginning :D #Nice drawing btw ;)