Define force. What is contact force ? Write two example of field force.

Two masses $m$ and $M$ are attached to the strings as shown in the figure. If the system is in equilibrium, then 

Two masses $M$ and $m$ are connected by a weightless string. They are pulled by a force $F$ on a frictionless horizontal surface. The tension in the string will be

Inside a horizontally moving box, an experimenter finds that when an object is placed on a smooth horizontal table and is released, it moves with an acceleration of $10\,m / s ^2$. In this box if $1\,kg$ body is suspended with a light string, the tension in the string in equilibrium position (w.r.t. experimenter) will be$.........\,N$ (Take $g =10\,m / s ^2$ )

A body of mass $60\,kg$ is suspended by means of three strings $P, Q$ and $R$ as shown in the figure is in equilibrium. The tension in the string $P$ is $..........g\,N$

A block of mass $M$ is pulled along a horizontal frictionless surface by a rope of mass $M / 2$. If a force $2 Mg$ is applied at one end of the rope, the force which the rope exerts on the block is -