If the resultant force on a system of particles is non-zero, then

Two small spheres of equal mass are heading towards each other with equal speeds, undergo a head-on collision (no external force acts on the system of two spheres). Then which of the following statement is correct?

Two masses $\mathrm{m}$ and $2\mathrm{m}$ are placed in a fixed horizontal circular smooth hollow tube as shown. The mass $\mathrm{m}$ is moving with speed $\mathrm{u}$ and the mass $2\mathrm{m}$ is stationary. After their first collision, the time elapsed for next collision is (Coefficient of restitution is, $\mathrm{e}=\frac{1}{2}$)

A particle $A$ moves with a speed of $10 \mathrm{m} {\mathrm{s}}^{-1}$ in a frictionless circular fixed horizontal pipe of radius $5 \mathrm{m}$ and strikes the particle $B$ of double the mass that of $A$. The coefficient of restitution is $\frac{1}{2}$ and the particle $A$ starts its journey at $t=0$. The time at which the second collision occurs is

Three small bodies of identical masses can move along a straight line. The central body $2$ is initially at rest and bodies $1$ and $3$ are at a distance $l$ and $2l$ from the central body, respectively. Bodies $1$ and $3$ move towards body $2$ with speeds $v_0$ each. The collision between $1$ and $2$ is perfectly elastic and the collision between bodies $2$ and $3$ is perfectly inelastic. After all the collisions are over,

In the figure shown, a particle $P$ strikes the inclined smooth plane horizontally and rebounds vertically. If the angle $\text{θ}$ is $60°$ then, the coefficient of restitution is,