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

1. anonymous

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. anonymous

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. anonymous

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6. anonymous

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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. anonymous

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9. anonymous

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

10. anonymous

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

11. anonymous

that's why the car can rise rump

12. anonymous

get @imron07 ?

13. anonymous

14. anonymous

15. anonymous

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16. anonymous

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17. anonymous

the friction is opposite to the moviment of weel

18. anonymous

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19. anonymous

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. anonymous

the friction is opposite to the moviment of POINT OF CONTACT

21. anonymous

yep

22. anonymous

Yes ma = mgsin alpha + f

23. anonymous

f-mgsin(alpa)=ma

24. anonymous

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

25. anonymous

yes it is opposite

26. anonymous

@RaphaelFilgueiras yes that's what I mean :)

27. anonymous

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

28. anonymous

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29. anonymous

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

30. anonymous

so your car never will rise the ramp!!!

31. anonymous

@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. anonymous

the car is moving up

33. anonymous

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

34. anonymous

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

35. anonymous

YES @imron07

36. anonymous

And pls medal (-s)

37. anonymous

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

38. anonymous

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. anonymous

It is a delicate but DRAMATIC CHANGE

40. anonymous

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. anonymous

Of course - all is opposite

42. anonymous

they are opposite but mg is greater than f

43. anonymous

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

44. anonymous

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

45. anonymous

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

46. anonymous

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

47. anonymous

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

48. anonymous

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. anonymous

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

50. anonymous

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

51. anonymous

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52. anonymous

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53. anonymous

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

54. anonymous

yw

55. anonymous

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

56. anonymous

Yeah, that's what in my mind :D

57. anonymous

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

58. anonymous

It was a nice discussion :)

59. anonymous

60. anonymous

Veritas in disputando gignitur.

61. anonymous

What's that last phrase means?

62. anonymous
63. anonymous
64. anonymous

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65. anonymous

@experimentX

66. anonymous