Two forces of magnitudes $4 \mathrm{N}$ and $8 \mathrm{N}$ are acting on a box when the box moves rightward across frictionless horizontal support. The speed of the box at time $t$ is $1{ \mathrm{m} \mathrm{s}}^{-1}$. The change in kinetic energy of the box is

A bullet of mass $20 \mathrm{g}$ moving with a speed of $100 \mathrm{m} {\mathrm{s}}^{-1}$ enters a heavy wooden block and stops after a distance of $50 \mathrm{cm}$. The average resistive force exerted by the block on the bullet is

Two wooden blocks are moving on a smooth horizontal surface such that the block having mass $m$ remains stationary with respect to block of mass $M$ as shown in the figure below. The magnitude of the applied force $P$ is,