Question

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

Answer 1

@Mashy

Answer 2

@Mashy

Answer 3

@Mashy @shubhamsrg

Answer 4

@Mashy

Answer 5

@yrelhan4 @RnR :P

Answer 6

:/

Answer 7

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

Answer 8

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

Answer 9

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

Answer 10

yes..start with static one

Answer 11

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

Answer 12

:/ okay

Answer 13

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!

Answer 14

proceed with whatever u want :/

Answer 15

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

Answer 16

arrre kar to :/

Answer 17

@Mashy ?

Answer 18

so as i was saying it should be rotational friction itself

anyways wat about mass and the radius?!

Answer 19

M,R

Answer 20

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

Answer 21

assume anything duh

Answer 22

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!

Answer 23

in the end you will get
F=-f

Answer 24

after applying torque equation n stuff

Answer 25

now?

Answer 26

what?!?

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

Answer 27

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

Answer 28

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

Answer 29

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

Answer 30

^ Best response. :P

Answer 31

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

Answer 32

Can you draw a picture?

Answer 33

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.

Answer 34

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.

Answer 35

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!

Answer 36

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 \(v_1=\Large \frac {R\omega_0}{3}\)