Work done by frictional force is:

An object of mass $10 \,kg$ is projected from the ground with a speed of $40 \,m/s$ at an angle of $60^{\circ}$ with the horizontal. The rate of change of momentum of the object one second after projection in $SI$ units is ........ [Take $g = 9.8 \,m/s^2$]

$A$ mass of $1 \text{ kg}$ is kept on an inclined plane with $30^\circ$ inclination with respect to the horizontal plane and it is at rest initially. Then,the whole assembly is moved up with a constant velocity of $4 \text{ m/s}$. The work done by the frictional force in time $2 \text{ s}$ is . . . . . . $\text{J}$. (Take $g = 10 \text{ m/s}^2$)

Can the work done by a frictional force be positive or negative?

$A$ car of mass $500\, kg$ is driven with acceleration $1\, m/s^2$ along a straight level road against a constant external resistance of $1000\, N$. When the velocity is $5\, m/s$,the rate at which the engine is working is .............. $kW$.

In the figure,two blocks are connected by a uniform strut attached to each block with frictionless pins. Block $A$ weighs $400 \, N$,block $B$ weighs $300 \, N$,and the strut $AB$ weighs $200 \, N$. If $\mu = 0.25$ under block $B$,determine the minimum coefficient of friction under block $A$ to prevent motion.