In which of the following cases the net force is not zero?

Two masses $M_1$ and $M_2$ are attached to the ends of a light string which passes over a massless pulley attached to the top of a double inclined smooth plane of angles of inclination $\alpha$ and $\beta$. If $M_2>M_1$ and $\beta >\alpha$, then the acceleration of block $M_2$ down the inclined will be:

The pulley arrangements shown in figure are identical, the mass of the rope being negligible. In case I, the mass $m$ is lifted by attaching a mass $2m$ to the other end of the rope. In case II, the mass $m$ is lifted by pulling the other end of the rope with a constant downward force $F=2\mathrm{mg}$, where $g$ is acceleration due to gravity. The acceleration of mass in case I is:

A body is moving with a speed of $1 \mathrm{m} {\mathrm{s}}^{-1}$ and a constant force $F$ is needed to stop it after a distance $x$. If the speed of the body is $3 \mathrm{m} {\mathrm{s}}^{-1}$, the force needed to stop it in the same distance $x$ will be,

Two blocks, each having mass $M$, rest on a frictionless surface as shown in the figure. If the pulleys are light and frictionless and $M$ on the incline is allowed to move down, then the tension in the string will be,