## imron07 3 years ago At what direction is the frictional force?

1. imron07

2. BTaylor

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

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

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

5. RaphaelFilgueiras

|dw:1348327921150:dw|

6. RaphaelFilgueiras

|dw:1348328024524:dw|

7. BTaylor

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

|dw:1348328081329:dw|

9. imron07

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

10. RaphaelFilgueiras

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

11. RaphaelFilgueiras

that's why the car can rise rump

12. RaphaelFilgueiras

get @imron07 ?

13. imron07

14. RaphaelFilgueiras

15. RaphaelFilgueiras

|dw:1348328369452:dw|

16. RaphaelFilgueiras

|dw:1348328561760:dw|

17. RaphaelFilgueiras

the friction is opposite to the moviment of weel

18. Mikael

|dw:1348328592225:dw|

19. imron07

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

the friction is opposite to the moviment of POINT OF CONTACT

21. RaphaelFilgueiras

yep

22. Mikael

Yes ma = mgsin alpha + f

23. RaphaelFilgueiras

f-mgsin(alpa)=ma

24. imron07

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

25. RaphaelFilgueiras

yes it is opposite

26. imron07

@RaphaelFilgueiras yes that's what I mean :)

27. Mikael

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

28. Mikael

T|dw:1348328885247:dw|

29. Mikael

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

30. RaphaelFilgueiras

so your car never will rise the ramp!!!

31. Mikael

@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

the car is moving up

33. Mikael

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

34. imron07

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

35. Mikael

YES @imron07

36. Mikael

And pls medal (-s)

37. Mikael

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

38. Mikael

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

It is a delicate but DRAMATIC CHANGE

40. imron07

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

Of course - all is opposite

42. RaphaelFilgueiras

they are opposite but mg is greater than f

43. RaphaelFilgueiras

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

44. Mikael

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

45. Mikael

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

46. imron07

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

47. Mikael

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

48. Mikael

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

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

50. Mikael

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

51. RaphaelFilgueiras

|dw:1348329423502:dw|

52. Mikael

|dw:1348329805336:dw|

53. imron07

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

54. RaphaelFilgueiras

yw

55. Mikael

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

56. imron07

Yeah, that's what in my mind :D

57. Mikael

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

58. imron07

It was a nice discussion :)

59. Mikael

60. Mikael

Veritas in disputando gignitur.

61. imron07

What's that last phrase means?

62. Mikael
63. Mikael
64. RaphaelFilgueiras

|dw:1348331561351:dw|

65. RaphaelFilgueiras

@experimentX

66. imron07