If the radius of the earth is reduced to three-fourth of its present value without any change in its mass,then the duration of the day on earth will be . . . . . . hours $30$ minutes.

If the Earth were to suddenly contract to $\frac{1}{n}$ th of its present radius without any change in its mass,the duration of the new day will be nearly

$A$ stone of mass $m$,tied to the end of a string,is whirled around in a circle on a horizontal frictionless table. The length of the string is reduced gradually,keeping the angular momentum of the stone about the centre of the circle constant. Then,the tension in the string is given by $T = Ar^n$,where $A$ is a constant and $r$ is the instantaneous radius of the circle. The value of $n$ is equal to

$A$ uniform disc of mass $M$ is rotating with a constant angular velocity $\omega$ about an axis passing through its center and perpendicular to its plane. Let its angular momentum be $L$. $A$ drop of molten plastic falls onto the disc and sticks to it. Which of the following remains constant?

$A$ small object is attached to a light string which passes through a hollow tube. The tube is held by one hand and the string by the other. The object is set into rotation in a circle of radius $r_1$. The string is then pulled down,shortening the radius of the circle to $r_2$. The ratio of the new kinetic energy to the original kinetic energy is

Suppose all the people in the world line up at the equator and all start running at speed $v_{rel}$ relative to the surface of the Earth along the equatorial circle. The initial angular velocity of the Earth is $\omega_0$. Let the moment of inertia of the Earth be $I_E$,the moment of inertia of all people be $I_P$,and the radius of the Earth be $R$. Which of the following statements is correct?