## DLS 2 years ago 1)A disc has pure rotation.It has angular velocity w.It then comes into contact with a surface which has static friction us,kinetic friction uk and rolling friction ur,When will the disc stop? 2)A disc is placed on the ground.The co-efficient of friction is u,and the co-efficient of rolling friction is u_r.What is the minimum force required to move the disc?The force is applied at the bottom most point

1. DLS

@Mashy

2. DLS

@Mashy

3. yrelhan4

@Mashy @shubhamsrg

4. DLS

@Mashy

5. DLS

@yrelhan4 @RnR :P

6. yrelhan4

:/

7. Mashy

should we consider all the types of friction in question 1? :P.. we can only consider rotational rigth? :D..

8. DLS

you will require all 3,when you start the question you will realise :)

9. Mashy

but at a time only one friction comes into play.. ! :-/

10. DLS

11. Mashy

but it ll only act for a particular small amount of time.. which i dunno how we could calculate :P

12. DLS

:/ okay

13. Mashy

and besides i really don't think the static and kinetic have to be considered.. i mean how can you consider static? the wheel is already in rotation .. so no question of static.. and the moment it would come in contact it would start rolling OR slipping.. so only one would come into picture!

14. DLS

proceed with whatever u want :/

15. Mashy

wait.. rolling friction wouldn't make it stop :D.. it should be the kinetic friction :D

16. DLS

arrre kar to :/

17. DLS

@Mashy ?

18. Mashy

so as i was saying it should be rotational friction itself anyways wat about mass and the radius?!

19. DLS

M,R

20. Mashy

lol just like tat you grant wishes huh? :D lol

21. DLS

assume anything duh

22. Mashy

so now there would be a force acting creating a torque that would eventually sue the rolling.. so i guess all you have to do is finde the torque.. then you can find the deceleration!

23. DLS

in the end you will get F=-f

24. DLS

after applying torque equation n stuff

25. DLS

now?

26. Mashy

what?!? f = (mur)N/R.... thats the formula for rolling friction right?!

27. DLS

/R? upon R?no simply u(r)N

28. Mashy

it depends upon the dimensions of mur .. rolling friction can be expressed in both the ways :P

29. DLS

abe yr ata hai to kar kya ghante me reply kar rha h :/

30. yrelhan4

^ Best response. :P

31. Mashy

arre sorry.. i was wtching dbz abridged :D..

32. wio

Can you draw a picture?

33. wio

When it comes to static and kinetic friction, my intuition, and it could be wrong, is that static friction is only meant as a thresh hold to find out whether the object will be moved. Kinetic friction on the other hand is something which applied a resting force over time.

34. Vincent-Lyon.Fr

What is your definition of µr (rolling friction coefficient)? It is important, because it is only if rolling friction exists that the disc will eventually come to a halt. Otherwise, it will roll for ever.

35. DLS

Your answer is right. I don't have the exact definition of µr (rolling friction coefficient) or even rolling friction. I just know that rolling friction is 1/100th of sliding friction and it depends on hardness of surface. Sorry,ill be glad to get some details about it!

36. Vincent-Lyon.Fr

The first phase of the motion is sliding. It is not so straightforward to find the time it will last. The answer is $$\tau_1=\Large \frac {R\omega_0}{3\mu _kg}$$ The remaining rotation velocity is $$\omega_1=\Large \frac {\omega_0}{3}$$ Velocity of centre of mass is $$v_1=\Large \frac {R\omega_0}{3}$$