## imron07 Group Title At what direction is the frictional force? one year ago one year ago

1. imron07 Group Title

2. BTaylor Group Title

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.

3. imron07 Group Title

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....

4. BTaylor Group Title

No, it's not. The friction always opposes motion.

5. RaphaelFilgueiras Group Title

|dw:1348327921150:dw|

6. RaphaelFilgueiras Group Title

|dw:1348328024524:dw|

7. BTaylor Group Title

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.

8. RaphaelFilgueiras Group Title

|dw:1348328081329:dw|

9. imron07 Group Title

@BTaylor yes, it oppose the motion, but the motion of our shoes.

10. RaphaelFilgueiras Group Title

the a point is moving in rotation direction so frictions force oppose it

11. RaphaelFilgueiras Group Title

that's why the car can rise rump

12. RaphaelFilgueiras Group Title

get @imron07 ?

13. imron07 Group Title

14. RaphaelFilgueiras Group Title

15. RaphaelFilgueiras Group Title

|dw:1348328369452:dw|

16. RaphaelFilgueiras Group Title

|dw:1348328561760:dw|

17. RaphaelFilgueiras Group Title

the friction is opposite to the moviment of weel

18. Mikael Group Title

|dw:1348328592225:dw|

19. imron07 Group Title

Yeah, i understand your explanation. But if we apply Newton's law, should we pick that direction too for friction to find system acceleration?

20. Mikael Group Title

the friction is opposite to the moviment of POINT OF CONTACT

21. RaphaelFilgueiras Group Title

yep

22. Mikael Group Title

Yes ma = mgsin alpha + f

23. RaphaelFilgueiras Group Title

f-mgsin(alpa)=ma

24. imron07 Group Title

Shouldn't $$mg\sin{\alpha}$$ is in opposite direction with friction now?

25. RaphaelFilgueiras Group Title

yes it is opposite

26. imron07 Group Title

@RaphaelFilgueiras yes that's what I mean :)

27. Mikael Group Title

NO it is coincident with mg sin alpha : here is why (let me finish pls

28. Mikael Group Title

T|dw:1348328885247:dw|

29. Mikael Group Title

And when wheel is freely rolling the friction acts BACK on it

30. RaphaelFilgueiras Group Title

so your car never will rise the ramp!!!

31. Mikael Group Title

@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 !!!

32. RaphaelFilgueiras Group Title

the car is moving up

33. Mikael Group Title

The point of contact is moving a wee-bit slower than the surface - so the surface ACCELERATES it !

34. imron07 Group Title

@Mikael : So only when the engine turned on, friction will directed upward?

35. Mikael Group Title

YES @imron07

36. Mikael Group Title

And pls medal (-s)

37. Mikael Group Title

the vector is velocity. And please admit that I am right @RaphaelFilgueiras

38. Mikael Group Title

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

39. Mikael Group Title

It is a delicate but DRAMATIC CHANGE

40. imron07 Group Title

Well, i understand both of your opinion. But i agree with @Mikael . If the vector I draw is Force, would something change (engine on)?

41. Mikael Group Title

Of course - all is opposite

42. RaphaelFilgueiras Group Title

they are opposite but mg is greater than f

43. RaphaelFilgueiras Group Title

so it will stop,and then the friction and mg will be in same direction

44. Mikael Group Title

This time @RaphaelFilgueiras IS ok. Though he will never admit someone else IS .....

45. Mikael Group Title

The last remark of Raf is WRONG (again!)_

46. imron07 Group Title

You mean friction is downward even if the tire's working to move the car upward too?

47. Mikael Group Title

Friction is UPWARD when the tire PUSHES the surface (working) and downward when the SURFACE pushes the tire

48. Mikael Group Title

Both of you should imagine a tiny (really tiny) velocity difference betwee Contact point and surface. It has OPPOSITE directions in these opposite cases !

49. imron07 Group Title

@Mikael can you draw it please, (when engine turned on, and another force pull the car too).

50. Mikael Group Title

When wheel PUSHES it is faster downward - so frict is UPWARD

51. RaphaelFilgueiras Group Title

|dw:1348329423502:dw|

52. Mikael Group Title

|dw:1348329805336:dw|

53. imron07 Group Title

Okay, I understand now. Thanks @RaphaelFilgueiras & @Mikael !

54. RaphaelFilgueiras Group Title

yw

55. Mikael Group Title

You see the friction force is OPPOSITE to the RELATIVE VELOCITY$V_{contact} - V_{\text{surface velocity relative \to wheel's center}}$

56. imron07 Group Title

Yeah, that's what in my mind :D

57. Mikael Group Title

@imron07 and @RaphaelFilgueiras : Thank you both for educating me and all of us by your questions and thoughts.

58. imron07 Group Title

It was a nice discussion :)

59. Mikael Group Title

60. Mikael Group Title

Veritas in disputando gignitur.

61. imron07 Group Title

What's that last phrase means?

62. Mikael Group Title
63. Mikael Group Title
64. RaphaelFilgueiras Group Title

|dw:1348331561351:dw|