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BTaylor Group TitleBest ResponseYou've already chosen the best response.0
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.
 2 years ago

imron07 Group TitleBest ResponseYou've already chosen the best response.1
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....
 2 years ago

BTaylor Group TitleBest ResponseYou've already chosen the best response.0
No, it's not. The friction always opposes motion.
 2 years ago

RaphaelFilgueiras Group TitleBest ResponseYou've already chosen the best response.1
dw:1348327921150:dw
 2 years ago

RaphaelFilgueiras Group TitleBest ResponseYou've already chosen the best response.1
dw:1348328024524:dw
 2 years ago

BTaylor Group TitleBest ResponseYou've already chosen the best response.0
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.
 2 years ago

RaphaelFilgueiras Group TitleBest ResponseYou've already chosen the best response.1
dw:1348328081329:dw
 2 years ago

imron07 Group TitleBest ResponseYou've already chosen the best response.1
@BTaylor yes, it oppose the motion, but the motion of our shoes.
 2 years ago

RaphaelFilgueiras Group TitleBest ResponseYou've already chosen the best response.1
the a point is moving in rotation direction so frictions force oppose it
 2 years ago

RaphaelFilgueiras Group TitleBest ResponseYou've already chosen the best response.1
that's why the car can rise rump
 2 years ago

RaphaelFilgueiras Group TitleBest ResponseYou've already chosen the best response.1
get @imron07 ?
 2 years ago

imron07 Group TitleBest ResponseYou've already chosen the best response.1
@RaphaelFilgueiras : What about this:
 2 years ago

RaphaelFilgueiras Group TitleBest ResponseYou've already chosen the best response.1
im talking about this
 2 years ago

RaphaelFilgueiras Group TitleBest ResponseYou've already chosen the best response.1
dw:1348328369452:dw
 2 years ago

RaphaelFilgueiras Group TitleBest ResponseYou've already chosen the best response.1
dw:1348328561760:dw
 2 years ago

RaphaelFilgueiras Group TitleBest ResponseYou've already chosen the best response.1
the friction is opposite to the moviment of weel
 2 years ago

Mikael Group TitleBest ResponseYou've already chosen the best response.1
dw:1348328592225:dw
 2 years ago

imron07 Group TitleBest ResponseYou've already chosen the best response.1
Yeah, i understand your explanation. But if we apply Newton's law, should we pick that direction too for friction to find system acceleration?
 2 years ago

Mikael Group TitleBest ResponseYou've already chosen the best response.1
the friction is opposite to the moviment of POINT OF CONTACT
 2 years ago

RaphaelFilgueiras Group TitleBest ResponseYou've already chosen the best response.1
yep
 2 years ago

Mikael Group TitleBest ResponseYou've already chosen the best response.1
Yes ma = mgsin alpha + f
 2 years ago

RaphaelFilgueiras Group TitleBest ResponseYou've already chosen the best response.1
fmgsin(alpa)=ma
 2 years ago

imron07 Group TitleBest ResponseYou've already chosen the best response.1
Shouldn't \(mg\sin{\alpha}\) is in opposite direction with friction now?
 2 years ago

RaphaelFilgueiras Group TitleBest ResponseYou've already chosen the best response.1
yes it is opposite
 2 years ago

imron07 Group TitleBest ResponseYou've already chosen the best response.1
@RaphaelFilgueiras yes that's what I mean :)
 2 years ago

Mikael Group TitleBest ResponseYou've already chosen the best response.1
NO it is coincident with mg sin alpha : here is why (let me finish pls
 2 years ago

Mikael Group TitleBest ResponseYou've already chosen the best response.1
Tdw:1348328885247:dw
 2 years ago

Mikael Group TitleBest ResponseYou've already chosen the best response.1
And when wheel is freely rolling the friction acts BACK on it
 2 years ago

RaphaelFilgueiras Group TitleBest ResponseYou've already chosen the best response.1
so your car never will rise the ramp!!!
 2 years ago

Mikael Group TitleBest ResponseYou've already chosen the best response.1
@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 !!!
 2 years ago

RaphaelFilgueiras Group TitleBest ResponseYou've already chosen the best response.1
the car is moving up
 2 years ago

Mikael Group TitleBest ResponseYou've already chosen the best response.1
The point of contact is moving a weebit slower than the surface  so the surface ACCELERATES it !
 2 years ago

imron07 Group TitleBest ResponseYou've already chosen the best response.1
@Mikael : So only when the engine turned on, friction will directed upward?
 2 years ago

Mikael Group TitleBest ResponseYou've already chosen the best response.1
YES @imron07
 2 years ago

Mikael Group TitleBest ResponseYou've already chosen the best response.1
And pls medal (s)
 2 years ago

Mikael Group TitleBest ResponseYou've already chosen the best response.1
the vector is velocity. And please admit that I am right @RaphaelFilgueiras
 2 years ago

Mikael Group TitleBest ResponseYou've already chosen the best response.1
Again I clarify : When the wheel is free it serves as a frictionconduit from the surface to cart. When the motor is working the opposite: the wheel transfers torque from cart to surface
 2 years ago

Mikael Group TitleBest ResponseYou've already chosen the best response.1
It is a delicate but DRAMATIC CHANGE
 2 years ago

imron07 Group TitleBest ResponseYou've already chosen the best response.1
Well, i understand both of your opinion. But i agree with @Mikael . If the vector I draw is Force, would something change (engine on)?
 2 years ago

Mikael Group TitleBest ResponseYou've already chosen the best response.1
Of course  all is opposite
 2 years ago

RaphaelFilgueiras Group TitleBest ResponseYou've already chosen the best response.1
they are opposite but mg is greater than f
 2 years ago

RaphaelFilgueiras Group TitleBest ResponseYou've already chosen the best response.1
so it will stop,and then the friction and mg will be in same direction
 2 years ago

Mikael Group TitleBest ResponseYou've already chosen the best response.1
This time @RaphaelFilgueiras IS ok. Though he will never admit someone else IS .....
 2 years ago

Mikael Group TitleBest ResponseYou've already chosen the best response.1
The last remark of Raf is WRONG (again!)_
 2 years ago

imron07 Group TitleBest ResponseYou've already chosen the best response.1
You mean friction is downward even if the tire's working to move the car upward too?
 2 years ago

Mikael Group TitleBest ResponseYou've already chosen the best response.1
Friction is UPWARD when the tire PUSHES the surface (working) and downward when the SURFACE pushes the tire
 2 years ago

Mikael Group TitleBest ResponseYou've already chosen the best response.1
Both of you should imagine a tiny (really tiny) velocity difference betwee Contact point and surface. It has OPPOSITE directions in these opposite cases !
 2 years ago

imron07 Group TitleBest ResponseYou've already chosen the best response.1
@Mikael can you draw it please, (when engine turned on, and another force pull the car too).
 2 years ago

Mikael Group TitleBest ResponseYou've already chosen the best response.1
When wheel PUSHES it is faster downward  so frict is UPWARD
 2 years ago

RaphaelFilgueiras Group TitleBest ResponseYou've already chosen the best response.1
dw:1348329423502:dw
 2 years ago

Mikael Group TitleBest ResponseYou've already chosen the best response.1
dw:1348329805336:dw
 2 years ago

imron07 Group TitleBest ResponseYou've already chosen the best response.1
Okay, I understand now. Thanks @RaphaelFilgueiras & @Mikael !
 2 years ago

RaphaelFilgueiras Group TitleBest ResponseYou've already chosen the best response.1
yw
 2 years ago

Mikael Group TitleBest ResponseYou've already chosen the best response.1
You see the friction force is OPPOSITE to the RELATIVE VELOCITY\[V_{contact}  V_{\text{surface velocity relative \to wheel's center}}\]
 2 years ago

imron07 Group TitleBest ResponseYou've already chosen the best response.1
Yeah, that's what in my mind :D
 2 years ago

Mikael Group TitleBest ResponseYou've already chosen the best response.1
@imron07 and @RaphaelFilgueiras : Thank you both for educating me and all of us by your questions and thoughts.
 2 years ago

imron07 Group TitleBest ResponseYou've already chosen the best response.1
It was a nice discussion :)
 2 years ago

Mikael Group TitleBest ResponseYou've already chosen the best response.1
DUBITANDO AD VERITATEM PERVENIMUS !
 2 years ago

Mikael Group TitleBest ResponseYou've already chosen the best response.1
Veritas in disputando gignitur.
 2 years ago

imron07 Group TitleBest ResponseYou've already chosen the best response.1
What's that last phrase means?
 2 years ago

Mikael Group TitleBest ResponseYou've already chosen the best response.1
http://translate.google.com/#la/en/Veritas%20in%20disputando%20gignitur.
 2 years ago

Mikael Group TitleBest ResponseYou've already chosen the best response.1
http://translate.google.com/#la/en/PERVENIMUS%20DUBITANDO%20AD%20VERITATEM
 2 years ago

RaphaelFilgueiras Group TitleBest ResponseYou've already chosen the best response.1
dw:1348331561351:dw
 2 years ago