Can Conservative Forces Be Neglected During Collision Analysis?

[sankalpmittal]

Pure rolling is many a times shortened to "rolling" as in this case. As for initiation see haruspex's post #19.

Please do read that question again. It just asks that on what surface the sphere cannot do pure rolling. If the question would have been :

Q11 Pure rolling on sphere cannot be initiated on :
a) a smooth horizontal surface
b) a smooth inclined surface
c) a rough horizontal surface
d) a rough inclined surface

Then the answer for the same would be (a) and (b).

haruspex and consciousness,

I was NOT saying that object CANNOT DO PURE ROLLING on a SMOOTH FRICTIONLESS SURFACE. What I was saying is that the pure rolling on object CANNOT BE INITIATED on a SMOOTH FRICTIONLESS SURFACE. To BRING the condition of V_CM = Rω, rough road is necessary. Maybe we agree with each other. If not, please give me reference to back up your theory. Thanks. I already gave the references in my previous posts.

 

[haruspex]

the pure rolling on object CANNOT BE INITIATED on a SMOOTH FRICTIONLESS SURFACE.

It depends what you do to initiate it.
Uniform cylinder radius r mass m rests with axis horizontal on horizontal surface. Impulse J is delivered horizontally at height h < 2r from the surface, in the plane that perpendicularly bisects the cylinder's axis.
Cylinder moves horizontally with speed J/m.
Cylinder rotates on its axis at rate J(h-r)/(mr²/2) = 2J(h-r)/(mr²).
The part of the cylinder in contact with the surface moves horizontally (instantaneously) at J/m - 2J(h-r)/(mr) = (J/mr)(3r-2h).
If h > 3r/2, the part in contact will be moving 'backward', so any friction acts forwards.
If h < 3r/2, the part in contact will be moving 'forward', so any friction acts backwards.
If h = 3r/2 the cylinder is in rolling contact and there is no friction.

 

[utkarshakash]

It depends what you do to initiate it.
Uniform cylinder radius r mass m rests with axis horizontal on horizontal surface. Impulse J is delivered horizontally at height h < 2r from the surface, in the plane that perpendicularly bisects the cylinder's axis.
Cylinder moves horizontally with speed J/m.
Cylinder rotates on its axis at rate J(h-r)/(mr²/2) = 2J(h-r)/(mr²).
The part of the cylinder in contact with the surface moves horizontally (instantaneously) at J/m - 2J(h-r)/(mr) = (J/mr)(3r-2h).
If h > 3r/2, the part in contact will be moving 'backward', so any friction acts forwards.
If h < 3r/2, the part in contact will be moving 'forward', so any friction acts backwards.
If h = 3r/2 the cylinder is in rolling contact and there is no friction.

Yes I consulted one of my physics books and it states that friction will not act if a force is applied at a height h=I/mR, where h is the distance of application of force from the centre of mass of sphere. So haruspex you were correct. Now coming back to Tanya's doubt, she asked what would have happened if there were no friction in the question. I would like to point out here that looking at the last 2 options we can clearly make out that the situation is not frictionless because if it'd have been so then option (d) would be an obvious one and it would make no sense to ask such a question(that'd have been too easy for everyone) in which the situation is itself frictionless and we are asked if friction would act after collision(sounds really stupid! :rofl:).

 

[Tanya Sharma]

Hi utkarshakash

You must be feeling really stupid by constantly reiterating that a body cannot roll without slipping on a frictionless surface .If you had just picked a pen and a paper and written down a couple of equations all your doubts would have been cleared.Instead you chose the other path of being adamant with the false idea and asking others to prove whether this was possible.I appreciate haruspex for his patience .

Yes I consulted one of my physics books and it states that friction will not act if a force is applied at a height h=I/mR, where h is the distance of application of force from the centre of mass of sphere. So haruspex you were correct.

Despite haruspex very nicely explaining the matter,finally it took a book for you to get convinced .Had'nt you find the concept in the book,i guess you would not have accepted that indeed a body can start rolling on a frictionless surface.

I understand rotation is a difficult topic to comprehend.I myself have struggled with it .But its bad to make nonsense claims without making any effort on your part.

A piece of advice for you-If you don't agree with something ,please disagree politely and show our work to defend your claims.Afterall you yourself are in a learning phase.

Now coming back to Tanya's doubt, she asked what would have happened if there were no friction in the question. I would like to point out here that looking at the last 2 options we can clearly make out that the situation is not frictionless because if it'd have been so then option (d) would be an obvious one and it would make no sense to ask such a question(that'd have been too easy for everyone) in which the situation is itself frictionless and we are asked if friction would act after collision(sounds really stupid! :rofl:).

Regarding the doubt I put here ,see posts made by ehild ,consciousness and haruspex .They have replied sensibly .They have atleast acknowledged that yes , the surface could have been frictionless .May be the options were not right.If you look for this question ,you will find that the answer is quite debatable.

Again you make silly statements by claiming what is obvious and what isnt.Kindly reread the entire thread and especially replies by consciouness,ehild and haruspex.Things are not that obvious .When you are not clear about concepts of rolling without slipping how can things be so obvious to you :grumpy:.

In the future please don't make bogus claims and don't waste time and energy of mentors on things you yourself can find easily.Instead try to work out things mathematically and not by rote learning.

Moral of the story -Please be humble and courteous on the forum