$A$ bullet of mass $10\, g$ moving horizontally with a velocity of $400\, m s^{-1}$ strikes a wood block of mass $2\, kg$ which is suspended by a light inextensible string of length $5\, m$. As a result,the centre of gravity of the block is found to rise a vertical distance of $10\, cm$. The speed of the bullet after it emerges out horizontally from the block will be ................... $m s^{-1}$.

$A$ ball is thrown vertically downwards from a height of $20 \, m$ with an initial velocity $v_0$. It collides with the ground,loses $50\%$ of its energy in the collision,and rebounds to the same height. The initial velocity $v_0$ is .................... $m/s$ (Take $g = 10 \, m/s^2$)

$A$ man who is running has half the kinetic energy of a boy of half his mass. The man speeds up by $1 \, m/s$ and then has the same kinetic energy as the boy. The original speed of the man was:

Which of the following statements is true?

$A$ person trying to lose weight (dieter) lifts a $10\; kg$ mass,one thousand times,to a height of $0.5\; m$ each time. Assume that the potential energy lost each time she lowers the mass is dissipated.
$(a)$ How much work does she do against the gravitational force?
$(b)$ Fat supplies $3.8 \times 10^{7} \;J$ of energy per kilogram which is converted to mechanical energy with a $20\%$ efficiency rate. How much fat will the dieter use up?

$A$ car travelling at a speed of $30 \, km/h$ is brought to a halt in $8 \, m$ by applying brakes. If the same car is travelling at $60 \, km/h$,it can be brought to a halt with the same braking force in ............... $m$.