Let's play Football


You kicked your football ( solid sphere ) as shown in the figure. You apply an impulse (J) as shown(sorry! Angle is 53 degrees ) .

What will be the Final displacement of the ball (i.e. when it's vertical velocity tends to become zero) from the initial position( in metres ).

Details and Assumptions :

  • The surface is smooth below the ball ,where it is placed. The rough surface starts at a distance of 10 metres from initial position of ball and continues to infinity .

  • Given, mass of ball M = 0.50 K g M = 0.50 Kg , Radius of ball is R = 0.20 m R = 0.20 m , J = 5 N s J = 5 N-s (at an angle of 53 degree as shown), take g = 10 m / s 2 g = 10m/s^2

  • Coefficient of restitution ( for vertical collision ) = 0.5 , and coefficient of friction of rough ground is 1.0


Try more here


The answer is 23.542.

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1 solution

Aniket Sanghi
Apr 16, 2016

First of all , due to impulse

Horizontal velocity V x = 6 m / s e c Vx = 6 m/sec

Vertical velocity V y = 8 m / s e c Vy = 8 m/sec

Angular velocity ω = 125 r a d / s e c \omega = 125 rad/sec


Before the first collision ,range travelled is R 1 = 9.6 m ( < 10 m ) R1 = 9.6m (<10m)

Since it is less than 10m , hence it is still on the smooth surface, friction is 0. (Normal collision)


In between first and the second collision , it travelled a further range of R 2 = 4.8 m R2 = 4.8 m

Now the friction is there.... It is sufficient to cause pure rolling

F d t = m ( v V x ) F dt = m ( v - Vx )

F R d t = 2 m R 2 ( 125 ω ) 5 FR dt = \frac{2mR^2 (125 - \omega )}{5}

And ... ω R = v \omega R = v

You will get v = 80 7 v = \frac{80}{7}


In between the second and the third collision, range travelled is R 3 = 32 7 R3 = \frac{32}{7}

Now , since their is no slipping therefore no frictional impulse will act for the successive collision(i.e. For the third , fourth and so on)

Range forms the series R3 + R4 + R5 + .... = 32 7 ( 1 + e + e 2 + e 3 + . . . . . ) = 9.142 \frac{32}{7} ( 1 + e + e^2 + e^3 + .....) = 9.142


Hence final displacement is R 1 + R 2 + R 3 + . . . . = 9.6 + 4.8 + 9.142 = 23.542 R1 + R2 + R3 + .... = 9.6 + 4.8 + 9.142 = \boxed{23.542}

If you like it! Then don't resist to like it! Share it! And upvote the solution

Aniket Sanghi - 5 years, 2 months ago

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My further Analysis has the following results

After Collision with rough ground if friction is sufficient for the ball to roll purely then at the time of second collision we are left with the following two possiblities

  1. Friction Do not act In third , fourth... collision with the ground as condition of pure rolling will always get satisfied if friction does not act as the frictional impulse was the only one which was responsible for change in horizontal velocity and angular speed of the ball.

  2. Friction Being a self Adjusting Force Will Adjust itself in such a way that during each of collision even after changing the horizontal velocity and angular speed of the ball the condition of pure rolling gets satisfied.

Your solution Takes into consideration only 1st case. On further Analyzing second Case I Found That this thing can also happen as i found out some velocities are fourth and 5th collision and Found friction to be sufficient. Expecting A Nice Reply :)

One more thing i want to point out is that When friction is involved in collision then Line of impact is not vertical as far as i know it will be inclined to the surface. So How can we use normal reaction in vertical direction

@Avi Ghanshani , @Aniket Sanghi , @neelesh vij

pankaj gupta - 5 years, 1 month ago

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I don't understand your second case at all , in the second collision after the condition of pure rolling is satisfied ,if the friction will act again for third collision it would make tball stop pure rolling you can see it yourself ....if v increases omega would decrease

Aniket Sanghi - 5 years, 1 month ago

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@Aniket Sanghi Is it necessary pure rolling to always take place? cant it slip in between

pankaj gupta - 5 years, 1 month ago

@Aniket Sanghi or can't it happen like friction acts in backward direction such that its velocity in horizontal direction becomes zero?

pankaj gupta - 5 years, 1 month ago

bro, great question ! +1 for both solution and question ! you rock !

A Former Brilliant Member - 4 years, 6 months ago

Bro there's a mistake (A BIG ONE) the angle should be 53 degrees not 57 degrees. You are getting the same values when the angle is 53 degrees. Please change it. I took 53 degrees as it produced very simple values as compared to 57 degrees.

Great question btw. A similar question was posted by ronak also.

neelesh vij - 5 years, 1 month ago

Did it the same way......I want to ask can there be some value of e and coefficient of friction for which pure rolling never begins.

Avi Ghanshani - 5 years, 1 month ago

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Nice question!

If you think logically it is not possible if you allow ball to go to infinity, 2 cases may arise either a complete halt or pure rolling because even if the coefficient of friction is too less to stop relative motion but it's motion to infinity (infinite collision)will certainly give it a large value that is net effect is large ..........

I have not proved it mathematically , If I could find something interesting I will soon post a q related to this........

Aniket Sanghi - 5 years, 1 month ago

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Well, logically it seems correct but, as the impulse due to normal force is decreasing, so when I sumed the GP to infinity and imposed the condition of pure rolling , I was getting a relation between e and k(coefficient of friction) . And I wanted to say that it will not start pure rolling after some finite collisions. In this question please analyse the case when coefficient of friction is less than 19/42.

Avi Ghanshani - 5 years, 1 month ago

Got the conclusion! It is not possible at all if we take values very small then too due to infinite collisions it will stop slipping in infinity

Do you like the question? Then reshare to give others the chance to attempt it

Aniket Sanghi - 5 years, 1 month ago

I Cannot understand why frictional impulse won't act for successive collisions @Aniket Sanghi I Solved Till R3 = 32/7 And got struck

pankaj gupta - 5 years, 1 month ago

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Because it wont

Aditi Ekbote - 7 months, 3 weeks ago

I Got my mistake But here are some points i am not still clear about

  1. If i assume frictional impulse to act at third collision , i found that friction is not sufficient for pure rolling. Hence Slipping can take place at third collision where in impulse due to friction = mu(impulse due to normal reaction)

so in this way the ball can slow down @Aniket Sanghi @aryan goyat @Prakhar Bindal

pankaj gupta - 5 years, 1 month ago

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Actually friction is sufficient in second collision for pure rolling and when pure rolling starts frictional force don't do any work on it so horizontal velocity will not change after third and successive collisions

Avi Ghanshani - 5 years, 1 month ago

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My point is that Why cannot Slipping START AT The THIRD Collision.

pankaj gupta - 5 years, 1 month ago

This is one of the best problems i have solved on brilliant!. I Revised rolling concepts from my notes then i was able to solve it after 1 wrong attempt. Keep Going!

Prakhar Bindal - 5 years, 1 month ago

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Its not only rolling. Its Also Collision!! This question thrilled me :)

Md Zuhair - 3 years, 6 months ago

I have a lot of doubts. First one being,how do you know friction is sufficient for pure rolling?

Adarsh Kumar - 5 years, 1 month ago

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You can check it !! :)

Aniket Sanghi - 5 years, 1 month ago

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Ohk,you mean by putting values?

Adarsh Kumar - 5 years, 1 month ago

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@Adarsh Kumar By calculating mu for this case

Aniket Sanghi - 5 years, 1 month ago

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@Aniket Sanghi Ohk!So that is one doubt cleared,now my next doubt is,why is frictional impulse 0?I get that work done by f is 0 but f has a magnitude,then why is f dt =0?

Adarsh Kumar - 5 years, 1 month ago

The required friction was as I can recall 19/21 (mu)

Aniket Sanghi - 5 years, 1 month ago

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no bro, it was 19/7 :) !

A Former Brilliant Member - 4 years, 6 months ago

exactly same as your solution !

A Former Brilliant Member - 4 years, 6 months ago

Can u please tell me how is friction sufficient for pure rolling u required comes out to be 19/21 but u max is only 0.1

Soumyadeep Basu - 3 years, 5 months ago

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Read it again , μ m = 1.0 \mu_m = 1.0

Aniket Sanghi - 3 years, 5 months ago

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oh sorry my bad!!

Soumyadeep Basu - 3 years, 4 months ago

Good question!!! Nice ..

Ankit Kumar Jain - 3 years, 3 months ago

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True, Awesome question

Md Zuhair - 3 years, 3 months ago

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The level of his Mechanics problems reflects why he is in IIT Kanpur today. BRILLIANT

Ankit Kumar Jain - 3 years, 3 months ago

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@Ankit Kumar Jain Hmm... Dont worry, IIT's are also waiting for you

Md Zuhair - 3 years, 3 months ago

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@Md Zuhair You are the one for whom IIT is waiting for... @Md Zuhair

Ankit Kumar Jain - 3 years, 3 months ago

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@Ankit Kumar Jain That must be a joke

Md Zuhair - 3 years, 3 months ago

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