$A$ person used force $(F)$,shown in the figure,to move a load with a constant velocity on a given surface. Identify the correct surface profile.

One end of a spring of force constant $150 \text{ dyne cm}^{-1}$ is connected to a block of mass $0.2 \text{ kg}$ kept on a rough horizontal surface of coefficient of friction $0.3$. The other end of the spring is connected to a rigid support as shown in the figure and the spring is initially undeformed. The maximum velocity $v$ that can be given to the block so that it travels only in one direction is . . . . . . $\text{ms}^{-1}$. (Acceleration due to gravity $= 10 \text{ ms}^{-2}$)

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

Write important guidance for solving problems in mechanics.

Explain why
$(a)$ a horse cannot pull a cart and run in empty space,
$(b)$ passengers are thrown forward from their seats when a speeding bus stops suddenly,
$(c)$ it is easier to pull a lawn mower than to push it,
$(d)$ a cricketer moves his hands backwards while holding a catch.

$A$ hosepipe directs a horizontal jet of water moving with a velocity of $20\, m/s$ onto a vertical wall. The cross-sectional area of the jet is $10^{-3}\, m^2$. If the density of water is $1000\, kg/m^3$,then the force acting on the wall is ........... $N$ [Assuming water comes to rest after striking the wall].