# Collision-conceptual doubt

#### haruspex

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I wrote about the case of non-impulsive friction which is defined that the impulse delivered can be taken zero.
That's not how I read you prior post:
If we consider the normal force between the ball and ring impulsive so should be the normal force between the ground and the ring. So the force of static friction is as big as needed: it is impulsive. The ball will roll after the collision.

If the static friction was not great enough, that would have caused slipping during the collision, but the force of friction would be finite and the change of linear momentum would be so small in the very short time of the collision that you could consider the linear momentum to be conserved.
I read that as saying that either the impulsive friction is enough to produce rolling immediately, or it can be neglected. I'm saying that there is a third possibility between the two, and you can prove this by starting with a realistic scenario in which the impact occurs over a short period of time, then letting that period tend to zero.

#### utkarshakash

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So I set up an example of how exactly the right combination can occur merely by judicious application of an impulse. Did you do the algebra?
Consider this question
"A thin spherical shell of radius R lying on a rough horizontal surface is hit sharply and horizontally by a cue. Where should it be hit so that the shell does not slip on the surface?"

#### utkarshakash

Gold Member
Pls quote the passage you wish to bring to our attention.
"That last sentence is important. The force due to friction doesn't necessarily act to oppose the motion of the body but acts to oppose slipping or sliding along the contact surface."

When an impulse is given the part of the body which is in contact with the ground has a tendency to slip. This is where frictional force comes into play by opposing the tendecy to slip along the surface. If there were no friction the tendency to slipping couldn't have been countered.

#### haruspex

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"That last sentence is important. The force due to friction doesn't necessarily act to oppose the motion of the body but acts to oppose slipping or sliding along the contact surface."
I agree entirely, and it supports my argument. In the special circumstances I've described, there is no tendency to relative motion of the two surfaces, hence no friction.
"A thin spherical shell of radius R lying on a rough horizontal surface is hit sharply and horizontally by a cue. Where should it be hit so that the shell does not slip on the surface?"

Because the impulse is purely horizontal. This means there is no impulsive downward force, so no impulsive normal reaction. The author of the question appears to be testing the reader's ability to deduce that no matter how rough the surface is it would be unable to prevent slipping if the ball were struck at just any old height. Only if it is struck at exactly the right height will the resulting horizontal impulse and angular impulse be so as to produce rolling motion straight away. In short, the surface might just as well be smooth, but it's up to the reader to figure that out.

#### ehild

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That's not how I read you prior post:
I read that as saying that either the impulsive friction is enough to produce rolling immediately, or it can be neglected. I'm saying that there is a third possibility between the two, and you can prove this by starting with a realistic scenario in which the impact occurs over a short period of time, then letting that period tend to zero.
You are right, I did not think it over. Friction means finite change of momentum even in case of non-impulsive impact.

ehild

#### sankalpmittal

You cannot give an impulse such that the ring starts rolling without slipping on a frictionless surface. I would rather like to suggest you refer to HC Verma Part 1. I have sorted out some problems that might make you think on your reasoning once again. Worked Out Examples- Q.No- 25, 29. Exercises- Q.No. - 80, 81, 82.
In each of these questions we considered rough road and took the axis of rotation to be passing through contact point of sphere, so that we could conserve angular momentum of the sphere system. This truly reveals that friction(Static) is necessary to bring pure rolling.

Pls quote the passage you wish to bring to our attention. (Mostly the article seems to be about speaking of friction as opposing motion instead of as opposing relative motion of the surfaces. Indeed, is that error at the root of your objection? You understand that once rolling is established on a horizontal surface there is no frictional force, right?)
Friction opposes skidding or slipping. Without it, what thing will prevent the object to slip? Your explanation states that during pure rolling friction does not work and it is not involved. But, that does not even mean that there can be pure rolling initiated without the aid of friction, i.e. on a frictionless road. Does it ?

As ehild pointed out, you could have rolling occurring on a frictionless surface in consequence of the ring first rolling on a frictional surface then transiting to a smooth one. I suspected that would not satisfy you because friction was still involved in establishing the rolling motion, but it does demonstrate that if the linear velocity just happens to be rω, by whatever means, then you have frictionless rolling. So I set up an example of how exactly the right combination can occur merely by judicious application of an impulse. Did you do the algebra?
Yes, Vcm=Rω, the condition of pure rolling once established, does not need friction, but again this condition cannot be without the help of friction. I did the algebra in the questions posted by utkarshakash of H.C. Verma.

haruspex:
Because the impulse is purely horizontal. This means there is no impulsive downward force, so no impulsive normal reaction. The author of the question appears to be testing the reader's ability to deduce that no matter how rough the surface is it would be unable to prevent slipping if the ball were struck at just any old height. Only if it is struck at exactly the right height will the resulting horizontal impulse and angular impulse be so as to produce rolling motion straight away. In short, the surface might just as well be smooth, but it's up to the reader to figure that out.
Friction does not act for a very short time. It keeps the relative motion opposed(in case of kinetic) and it endeavours to bring the condition of relative momentary rest(in case of static). Maybe I am overlooking your post, but I cannot fathom what you said. Can you please explain ?

#### haruspex

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I did the algebra in the questions posted by utkarshakash of H.C. Verma.
I don't have a copy of those, but most likely friction is necessary to establish rolling in those cases. My argument is that there are special cases where it is not. I provided an example.
Friction does not act for a very short time.?
It can be impulsive. If the impact has a vertical component, resulting in an impulsive normal reaction (i.e. a very large force for a very short time) then the kinetic friction will be correspondingly impulsive. However, that's all irrelevant to the question of whether it is possible to initiate rolling motion in the absence of friction.

#### consciousness

You cannot give an impulse such that the ring starts rolling without slipping on a frictionless surface. I would rather like to suggest you refer to HC Verma Part 1. I have sorted out some problems that might make you think on your reasoning once again. Worked Out Examples- Q.No- 25, 29. Exercises- Q.No. - 80, 81, 82.

HCV Rotational Mechanics OBJECTIVE II (multiple correct options)

Q11 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

Answer is (b), not (a)(b) the only required condition for pure rolling is v=rω.

I think that students were supposed to ignore the possibility that Tanya mentioned in her post (case three). As far as I can tell this was an instance of mismanagement and the question was wrong.

See this link for IIT JEE 2011 mistakes-http://www.hindustantimes.com/India-news/NewDelhi/IITs-correct-JEE-error-sparks-more-confusion/Article1-697995.aspx

It is mentioned that one 4 marks physics question was wrong from paper 2.

#### utkarshakash

Gold Member
HCV Rotational Mechanics OBJECTIVE II (multiple correct options)

Q11 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

Answer is (b), not (a)(b) the only required condition for pure rolling is v=rω.

I think that students were supposed to ignore the possibility that Tanya mentioned in her post (case three). As far as I can tell this was an instance of mismanagement and the question was wrong.

See this link for IIT JEE 2011 mistakes-http://www.hindustantimes.com/India-news/NewDelhi/IITs-correct-JEE-error-sparks-more-confusion/Article1-697995.aspx

It is mentioned that one 4 marks physics question was wrong from paper 2.
Please read the question again. It simply asks whether a sphere can roll or not. Rolling does not always mean "pure rolling". Also it mentions nowhere whether it can be initiated or not.

Last edited:

#### consciousness

Please read the question again. It simply asks whether a sphere can roll or not. Rolling does not always mean "pure rolling". Also it mentions nowhere whether it can be initiated or not.
Pure rolling is many a times shortened to "rolling" as in this case. As for initiation see haruspex's post #19.

#### 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 VCM = 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

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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

Gold Member
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 dont 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 dont make bogus claims and dont 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

"Collision-conceptual doubt"

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