If a body is in equilibrium under a set of non-collinear forces, then the minimum number of forces has to be

Three blocks of masses $2 \,kg, 3 \,kg$ and $5\, kg$ are connected to each other with light string and are then placed on a frictionless surface as shown in the figure. The system is pulled by a force $F = 10N,$ then tension ${T_1} = $  .......... $N$ 

Give the magnitude and direction of the net force acting on a stone of mass $0.1\; kg$,

$(a)$ just after it is dropped from the window of a stationary train,

$(b)$ just after it is dropped from the window of a train running at a constant velocity of $36 \;km/h$,

$(c)$ just after it is dropped from the window of a train accelerating with $1\; m s^{-2}$,

$(d)$ lying on the floor of a train which is accelerating with $1\; m s^{-2}$, the stone being at rest relative to the train.

Neglect air resistance throughout.

The restoring force of a spring with a block attached to the free end of the spring is represented by

If net force on a system is zero then

Which force act on body first action or reaction ?