A boy holds a pendulum in his hand while standing at the edge of a circular platform of radius r rotating at an angular speed $\omega$. The pendulum will hang ai an angle $\theta$ with the vertical so that : 
(Neglect length of pendulum)

As shown in the figure, to light springs of force constant $K_1$, and $K_2$, oscillate a block of mass $M$. Its effective force constant will be :

Two springs of force constant $k$ and $2k$ are connected to a mass as shown below. The frequency of oscillation of the mass is:

A $0.5 \mathrm{kg}$ ball moving with a speed of $12 \mathrm{m} {\mathrm{s}}^{-1}$ strikes a hard wall at an angle of $30°$ with the wall. It is reflected with the same speed and at the same angle. If the ball is in contact with the wall for $0.25 \mathrm{seconds}$, the average force acting on the wall is:-