Hold your ground.

A small body A is fixed to the inner side of a thin rigid hoop of radius R and mass equal to that of the body A. The hoop rolls without slipping over a horizontal plane. At the moments when the body A gets into the lower position, the centre of the hoop moves with velocity V o V_{o} . The maximum value of V o V_{o} at which the hoop moves without bouncing is x g R \sqrt{xgR} . What is the value of x 'x' ?

Try more from my set Classical Mechanics Problems .


The answer is 8.

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2 solutions

The tendency of jumping of hoop will be maximum when the body A is at top most position.

Let at this position the linear velocity of hoop is V V . So, velocity of the body is 2 V 2V .

Now for hoop to jump, Centrifugal force given by the body on hoop must be greater than weight of the combined system.

So, F c p = m V 2 R = 2 m g F_{cp}=\frac{m{V}^2}{R}=2mg (Here in F c p F_{cp} only V V is taken into account and not 2 V 2V because the velocity of body A relative to the centre of mass of hoop is V V )

or, V = 2 g R V=\sqrt{2gR}

Since there is no energy loss, so we can use conservation of energy.

So, 1 2 I c m \frac{1}{2}I_{cm} w i 2 {w_{i}}^{2} + 1 2 ( 2 m ) v c m i 2 \frac{1}{2}(2m){{v_{cm}}_{i}}^{2} = ( 2 m ) g R (2m)gR + 1 2 I c m \frac{1}{2}I_{cm} w f 2 {w_{f}}^{2} + 1 2 ( 2 m ) v c m f 2 \frac{1}{2}(2m){{v_{cm}}_{f}}^{2}

Therefore 1 2 3 m R 2 V o 2 2 R 2 \frac{1}{2}\frac{3m{R}^{2}{V_{o}}^{2}}{2{R}^{2}} + m V o 2 4 \frac{m{{V_{o}}^{2}}}{4} = 2 m g R 2mgR + 1 2 3 m R 2 V 2 2 R 2 \frac{1}{2}\frac{3m{R}^{2}{V}^{2}}{2{R}^{2}} + m ( 3 V 2 ) 2 m{(\frac{3V}{2})}^{2} [Here v c m i {v_{cm}}_{i} = V o 2 \frac{V_{o}}{2} & v c m f {v_{cm}}_{f} = 3 V 2 \frac{3V}{2} ]

or, m V o 2 m{V_{o}}^{2} = 2 m g R 2mgR + 3 m V 2 3m{V}^{2}

or, V o 2 {V_{o}}^{2} = 2 g R 2gR + 3 V 2 3{V}^{2}

Now putting the value of V = 2 g R V=\sqrt{2gR} in above equation

V o 2 {V_{o}}^{2} = 2 g R 2gR + 3 ( 2 g R ) 3(2gR)

or, V o 2 {V_{o}}^{2} = 8 g R 8gR

or, V o V_{o} = 8 g R \sqrt{8gR} .

So to prevent hoop from jumping V o V_{o} must be less than or equal to 8 g R \sqrt{8gR} .

Hi bro. If in 4th line we are observing from ground frame and want to use 2v then we cannot use radius of curvature as R. It will be 4R, I calculated it. So then our expression for centripetal force remains the same. Is the above correct? I just wanted to confirm. @Sarthak Behera

Kushagra Sahni - 3 years, 5 months ago

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Ya it will be definetly correct ,

Sanju Gupta - 2 years, 3 months ago

Hey isn't Icm supposed to be 2mrsquared

Gayathri Shrushti - 2 years, 7 months ago

Why are we not considering the normal force

Himanshu Kumar - 2 years, 7 months ago
Xyz Abc
Dec 26, 2018

The tendency of jumping of hoop will be maximum when the body A is at top most position.

Let at this position the linear velocity of hoop is . So, velocity of the body is .

Now for hoop to jump, Centrifugal force given by the body on hoop must be greater than weight of the combined system.

So, (Here in only is taken into account and not because the velocity of body A relative to the centre of mass of hoop is )

or,

Since there is no energy loss, so we can use

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