$A$ block of mass $m = 0.1\,kg$ is connected to a spring of unknown spring constant $k.$ It is compressed to a distance $x$ from its equilibrium position and released from rest. After approaching half the distance $(\frac{x}{2})$ from the equilibrium position,it hits another block and comes to rest momentarily,while the other block moves with a velocity $3\,m/s.$ The total initial energy of the spring is ................ $J$.

$A$ smooth sphere of mass $m$ moving with velocity $u$ collides with another smooth sphere of mass $2m$ at rest. What is the range of the velocity $v$ of the second sphere after the collision?

$A$ small disk can slide in a circular path on a frictionless inclined plane inclined at an angle of $30^\circ$ with the help of a thread as shown. The mass of the disk is $m$ and the acceleration due to gravity is $g$. If the disk is released when the thread is horizontal,the expression for the tension in the thread at the lowest point is: ................. $mg$

From a building, two balls $A$ and $B$ are thrown such that $A$ is thrown upwards and $B$ is thrown downwards (both vertically) with the same speed. If $v_A$ and $v_B$ are their respective velocities on reaching the ground, then:

$A$ ball is projected vertically upwards. Air resistance and variation in $g$ may be neglected. The ball rises to its maximum height $H$ in a time $T$,the height being $h$ after a time $t$.
[$1$] The graph of kinetic energy $E_k$ of the ball against height $h$ is shown in figure $1$.
[$2$] The graph of height $h$ against time $t$ is shown in figure $2$.
[$3$] The graph of gravitational potential energy $E_g$ of the ball against height $h$ is shown in figure $3$.
Which of the following shows the correct answers?

$A$ student skates up a ramp that makes an angle $30^{\circ}$ with the horizontal. He/she starts (as shown in the figure) at the bottom of the ramp with speed $v_0$ and wants to turn around over a semicircular path $xyz$ of radius $R$ during which he/she reaches a maximum height $h$ (at point $y$) from the ground as shown in the figure. Assume that the energy loss is negligible and the force required for this turn at the highest point is provided by his/her weight only. Then ($g$ is the acceleration due to gravity):
$(A)$ $v_0^2 - 2gh = \frac{1}{2} gR$
$(B)$ $v_0^2 - 2gh = \frac{\sqrt{3}}{2} gR$
$(C)$ The centripetal force required at points $x$ and $z$ is zero.
$(D)$ The centripetal force required is maximum at points $x$ and $z$.