Work done by frictional force is:

$A$ truck starting from rest moves with an acceleration of $5 \ m/s^2$ for $1 \ s$ and then moves with constant velocity. The velocity $v/s$ time graph for the block on the truck with respect to the ground is: (Assume that the block does not fall off the truck and the coefficient of friction $\mu = 0.2$,$g = 10 \ m/s^2$)

Which of the following is not a fundamental force in nature?

$A$ helicopter of mass $1000 \;kg$ rises with a vertical acceleration of $15\; m s^{-2}$. The crew and the passengers weigh $300\; kg$. Give the magnitude and direction of the
$(a)$ force on the floor by the crew and passengers,
$(b)$ action of the rotor of the helicopter on the surrounding air,
$(c)$ force on the helicopter due to the surrounding air.

Consider a system of blocks $A$, $B$, and $X$ as shown in the figure. The blocks $A$ and $B$ have equal mass $m$ and are connected by a massless string through a massless pulley. The coefficient of friction between block $A$ and $X$, and between block $B$ and $X$, is $\mu = 0.5$. If block $X$ moves on the horizontal frictionless surface, what should be its minimum acceleration $a$ such that blocks $A$ and $B$ remain stationary relative to $X$? ($g =$ acceleration due to gravity.)

$10,000$ small balls,each weighing $1 \, g$,strike $1 \, cm^2$ of area per second with a velocity of $100 \, m/s$ in a normal direction and rebound with the same velocity. The value of pressure on the surface will be: