IIT JEE , AIEEE Forum - Physics , Mechanics

Lois

Please help me with this question

A sphere cannot roll on
(a) a smooth horizontal surface
(b) a smooth inclined surface
(c) a rough horizontal surface
(d) a rough inclined surface

Thanks,
Lois

Aatish

As per the given degree of smoothness option (a) must be right

Lois

I thought so too, but the answer is (b) The q is from HCV I agree that with (b) but how can a sphere roll on a smooth horizontal surface?

uday_zingtudor

hey guyz!!!!!

u cannot roll a solid sphere on a smooth inclined surface

you may think of the case

for a bdy to roll some opposing force must be present in any form

on an incline nothing extra is found so it can't roll on a smooth incline

now i haven't used physics but common sense

vineet

both (a) & (b).

Lois

but the answer given is only (b) experts please answer

sen_holmes

both a) and b) should be right i think.coz friction helps pure rolling.but here there is no friction.

kusum

hi,
a is wrong .a body if st into pure rolling can roll on a smooth surface but on incline plane it will slip as there is nothin providin torque even if innitially it starts with pure rolling.

KAB

Mostly body can roll on a smooth horizontal surface.Consider the example of the game bowling.The surface where they roll the ball will be really smooth and very less frictional force acts.But the ball keeps rolling and does not slip.So I think this is right.

shubham_sachdeva

the answer for sure will be b)

bcz only in case of b option there will be more no. of forces acting on the sphere which will make the sphere slip.

in case of c & d option the sphere will roll.( u agree)

in case of a option only thegiven force will act on the sphere but in case of b option there will be 2 forces acting on the sphere :-

1)given force

2) mg sin theta too will be there which will make the sphere to slip ,not roll.

i hope its clear now.

if i m wrong then do correct me.

deepak_agarwal

there r plenty of opinions above...almost all of them r correct...i blv both a and b r correct and the ans given in book is wrong....as far as shubam has written...see u r absolutely right in analysing then forces but these forces act at the centre and not the point of contact of ball with the ground as a result they cant result in torques and hence rotation...

cheers

mukul1yellow9

hi ! LOIS THERE IS CONFUSION B/W (A) & (B)
THE ANS. IS ONLY (B)..YA..A BODY CAN ROLL ON SMOOTH PLANE SURFACE..BUT UNDER ONLY ONE CONDITION:a FORCE
F SHUD BE APPLIED AT DISTANCE .4 M ABOVE CENTRE...SEE HOW I GET IT..
TAKE A FORCE F ACTING AT DISTANCE 'x' above the center..apply
F+f=ma
(F-f)r=I*alpha
a=R*ALPHA ,ELIMINATE a & alpha
get relation b#w f,x & F
ON OBSERVING WE GET FOR X=0.4 f=0
so,(b) is the correct ans..
I M CORRECT ..(PLZ OFFER ME ASALUTE!)
so ENJOY...!!!

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