# Crazy ball is really Crazy !

Let an crazy ball has to be projected on Rough horizontal surface such that after striking surface it comes back to initial position then itself repeat such cycle again and again without loss of energy then Find all Required conditions For This Event ?

NOTE:

$\bullet$ Here in this situation i take ideal conditions .So try this it is really funny and interesting !

But This is not practically possible because it's loses energy because of air resistance and many other factor.

See practical example of this :

Note by Deepanshu Gupta
6 years ago

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@Deepanshu Gupta @Ronak Agarwal @Mvs Saketh @Krishna Sharma @megh choksi @satvik pandey @jatin yadav @Sudeep Salgia @Karthik Kannan @Pratik Shastri what do you think about this question ??

I think that we should project it in that way such that it follows the angle $\theta$ and then 90-\(\theta) and crazy ball should repeat this condition So that Range is same , Am I correct ?

- 5 years, 11 months ago

I would like to see what other guys think about that. I think I don't have sufficient knowledge to comment on this question. :(

- 5 years, 11 months ago

I explored about it and I came up with this.

I think the ball retraces its path after each collision. For that, after each collision the magnitude of X and Y components of velocity should be equal.

Let the initial X and Y component of the velocity of the ball be Vx and Vy. So at the time of 1st collision X and Y component of velocity will be -Vy and Vx.

As Relative velocity of separation =e(Relative velocity of approach)

So ${ V }_{ y' }=e{ V }_{ y }$ but in order to retrace its path magnitude of ${ V }_{ y' }$ and ${ V }_{ y }$ should be equal. So $e=1$

Let ${ J }_{ N }$ be impulse due to Normal force and ${ J }_{ f }$ be impulse due to frictional force.

So ${ J }_{ f }=\mu { J }_{ N }$

and ${ J }_{ N }=2m{ V }_{ y }$

also ${ J }_{ f }=-2m{ V }_{ x }$ where (-)ve sign indicates the direction of $J_{f}$

So $\mu =\frac { { V }_{ x } }{ { V }_{ y } }$

Now Angular impulse about CoM= $-RJ_{f}$ (taking anti clockwise direction +ve)

So $-RJ_{f}=I({ \omega }_{ f }-{ \omega }_{ i })$ I think that ${ \omega }_{ f }=-{ \omega }_{ i }$

So $\omega =\frac { 5{ V }_{ x } }{ 2R }$.

What do think guys. Please reply. @Deepanshu Gupta @Mvs Saketh @Ronak Agarwal @Mardokay Mosazghi @KARAN SHEKHAWAT @Karan Siwach @Nathanael Case @Pratik Shastri .

- 5 years, 11 months ago

By writing This equation :

${ J }_{ F }\quad =\quad \mu { J }_{ N }$.

$\bullet$ You Have Taken an Assumption Try To Find what was it ?

$\bullet$ Also By doing all calculation comment on the Nature of Ball , i.e is it Possible That this Phenomena Can occur if we Throw an Rigid Ball ( Say Cock Ball) ?

$\bullet$ Also Find what Should Be The Minimum and Maximum required $\mu$ For this Phenomena ?

Try To Answer These question So that You Understand Many things from This question !

And Your Mathematical Calculation are Correct !

- 5 years, 11 months ago

1) I think the normal force that acts on the ball for an infinitely small time period over which the collision occurs should be constant .

2) The given conditions will take place only if the ball does not loose any kinetic energy after each collision. So I think it should be Rigid.

3) It's minimum value can't be zero. :D I don't know how to find minimum and maximum value of $\mu$. Please give some hint. :)

- 5 years, 11 months ago

No Ball must be elastic in nature( So Only Crazy ball show This ) , Rigid Ball Doesn't Show This Phenomena , Think why ? (If You Have Cock ball and a Crazy ball then You Can Do experiment at home , for better understanding)

And Minimum Coffecient of friction is ${ \mu }_{ min }\quad =\quad \cot { \theta }$. where $\theta$ is inclination angle( angle of projection ) , Think why or why not?

Maximum $\mu$ can be infinitely Large , Think why or why not ?

- 5 years, 11 months ago

Thanks Deepanshu. I got your explanation for values of $\mu$. I have found $\mu =\frac{Vx}{Vy}=cot \theta$ and maximum value can be infinite because the value of $\mu$ can vary according to given condition.

- 5 years, 11 months ago

I believe that only rigid ball can show this in perfectly ideal conditions, Let us not forget that what we mean by rigidity, is that it is like made up of a material which is identical to several stiff extremely stiff springs, which means or implies that not only does it bounce back in minimal time of contact (which is needed because the lesser the time of contact, the stiffer the Normal impulse and the corresponding frictional impulse ), also rigidity is not opposite to elasticity, it is opposite to plasticity,,(example- steel is more elastic than rubber) the ball must be both rigid and elastic, plasticity will cause faster damping by energy loss, and lack of rigidity will cause weak unsharp impulses, (not to mention, larger the energy loss in a realistic scene due to longer time of contact)

- 5 years, 11 months ago

No I still Says, Rigid Ball Can't Show This , And Sorry I word it wrongly That ball must be Elastic ( which is loose Talk) Instead I Should Say That Ball must be Flexible , Because when The Collision b/w ball and surface occurs then it takes some time ( may be fraction of second) But during this fraction of Time Friction does work and due to which if body is not rigid Then there should be some energy Loss (may be in too small amount )Takes Place So This Phenomena Can't Happens ! And If we consider ball as perfectly Flexible ( I don't want say that elastic , which is loose talk) then due to it's Flexibility it Can Stored Potential energy in it during The time interval in which work is done by friction , So that when Ball get's detached from surface it regains it's original Energy and also regain it's original Shape and Size (In ideal Conditions , which is already Mentioned above in Note ) ( even the small amount of energy which is lost due to work done by friction is regained by it)

Hence I Still Says That Ball must be Flexible ! (and of course it must be elastic also)

- 5 years, 11 months ago

Well yes, you are correct in absolute terms, rigid ball because it cant restore its energy through spring action will attenuate and stop at first collision, but also note that i have said rigidity as being made of very stiff springs,, not infinitely stiff but very stiff :)

- 5 years, 11 months ago

Yes But , did you Please Explain more Precisely That what is 'Rigidity' means ? From My Views Cock ball or an Lather Type Ball is Rigid , According to me , I mean that Rigidity means Very Hard mass which Can't be compressed and Expand by applying a force i.e we can say and Solid Ball , Solid Rigid Rod etc.

I'am gotta Confused with this Term ' Rigidity '
what is Precise Meaning of this Term ?

thanks ! My good Friend :)

- 5 years, 11 months ago

Well,,, Rigidity basically means no particle can have relative motion with another particle of the same system which ofcourse as you rightly said can never perform spring action to jump up , but i meant that a too much flexibility means longer time of contact so maybe more attenuation as well

- 5 years, 11 months ago