The story of a mercilessly thrown sphere...

A spherical ball of mass M and radius R is thrown along a rough horizontal surface so that initially , it slides with a linear speed υ 0 \displaystyle{\upsilon_0} ,but does not rotate . As it slides, it begins to spin and eventually rolls without slipping. The time taken to start rolling can be expressed as a b × υ 0 μ k g \dfrac{a}{b} \times \dfrac{\upsilon_0}{\mu_k g} Where μ k \displaystyle{\mu_k} is the coefficient of kinetic friction between the surface and the ball .What is a + b \displaystyle{a+b} ?


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The answer is 9.

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5 solutions

Abhishek Singh
Oct 30, 2014

Taking the center of the sphere at the origin ,equation of motion of the sphere are: F x = M a x = f = μ k M g ( 1 ) \sum F_x = Ma_x = -f = \mu_k Mg …………(1) F y = N M g = 0 ( 2 ) \sum F_y = N - Mg = 0 …………(2) From the equations (1) & (2) we get a = μ k g . . ( 3 ) a = -\mu_k g ………..(3) The velocity of the centre of mass at time t is υ = υ 0 + a x t . \upsilon=\upsilon_0 + a_x t. or υ = υ 0 μ k g t . . . . . . . . . . . . . . . . ( 4 ) \upsilon=\upsilon_0 - \mu_k g t ................(4) Equation of motion for rotation is τ = μ k M g R = ( 2 5 M R 2 ) α . . . . . . . . . . . . . . . . . ( 5 ) \sum \tau=\mu_k MgR= \Big( \dfrac{2}{5} MR^2\Big) \alpha .................(5) or α = 5 2 × μ k g R . . . . . . . . . . . . . . ( 6 ) \alpha =\dfrac{5}{2} \times \dfrac{\mu_k g}{R} ..............(6) Then the angular velocity at time t is ω = ω 0 + α t = 5 2 × μ k g t R . . . . . . . . . . . . . . . . . . . ( 7 ) \omega=\omega_0 + \alpha t =\dfrac{5}{2} \times \dfrac{\mu_k gt}{R} ...................(7) condition for pure rolling is υ = ω R \displaystyle{ \upsilon = \omega R} Now by using (4) & (7) we get υ 0 μ k g t = 5 2 × μ k g t R \upsilon_0 - \mu_k g t = \dfrac{5}{2} \times \dfrac{\mu_k gt}{R} or t = 2 7 × υ 0 μ k g \boxed{ t= \dfrac{2}{7} \times \dfrac{\upsilon_0}{\mu_k g} }

one can also used angular momentum conservation about the bottom most point . To get final velocity , m v o R = m v R + 2 5 m R 2 ( v R ) v = 5 v o 7 m{ \quad v }_{ o }R\quad =\quad m\quad { v }^{ ' }R\quad \quad \quad +\quad \cfrac { 2 }{ 5 } m{ R }^{ 2 }(\cfrac { { v }^{ ' } }{ R } )\quad \\ \\ v\quad =\quad \cfrac { 5{ \quad v }_{ o } }{ 7 } .

Deepanshu Gupta - 6 years, 7 months ago

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I never thought of that..! Brilliant..!

Abhishek Sharma - 6 years, 7 months ago

This is exactly what I did!!!

Rohit Nair - 5 years, 2 months ago

Exactly what I did....

Kushal Patankar - 6 years, 6 months ago

ONE SHOULD MENTION THAT THE BALL IS SOLID NOT A SHELL AS I DID THE SAME METHOD BUT I TOOK MOMENT OF INERTIA OF SHELL

Utkarsh Tiwari - 6 years ago

Shouldn’t the torque be negative?

Justin De Los Santos - 1 year, 11 months ago

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We can take it to be positive without changing the math since w_0 is zero.

Nathan Klassen - 2 months, 4 weeks ago

I think we should multiply by R on the RHS in the final equality.

N. Aadhaar Murty - 9 months, 1 week ago
Soumen Goswami
Nov 6, 2014

Conserving angular momentum about any point in the horizontal plane, we get

m v 0 R = m v R + 2 5 m R 2 ( v R ) 2 v = 5 7 v 0 mv_{0}R=mvR+\frac{2}{5}mR^{2}\left(\frac{v}{R}\right)^{2}\Rightarrow v=\frac{5}{7}v_{0}

and

5 7 v 0 = v 0 μ g t t = 2 v 0 9 μ g \frac{5}{7}v_{0}=v_{0}-\mu gt\Rightarrow t=\frac{2v_{0}}{9\mu g}

OR

we can solve for t t from the following equations

μ m g = m a \mu mg=ma

μ m g R = 2 5 m R 2 α \mu mgR=\frac{2}{5}mR^{2}\alpha

v 0 a t = α v_{0}-at=\alpha

This gives the same answer for t t

Usama Khidir
Feb 5, 2015

Please be so kind to mention the next time whether the ball is hollow or filled it was certainly a bit confusing

you should give in question that the ball is solid sphere not hollow

Jaivir Singh
Nov 4, 2014

FIRST BY USING CONSERVATION OF ANGULAR MOMENTUM ABUT POINT OF CONTACT FIND THE VELOCITY OF CENTER OF SPHERE time is 2/7 times yhe given exp

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