State whether the following statement is true or false:
In order to get minimum work,the angle between force and displacement should be $90^{\circ}$.

The work done by a force on a body will be positive if the:

Justify,giving proper reasoning,whether the work done in the following case is positive or negative: Work done by friction on a body sliding down an inclined plane.

$A$ girl is carrying a school bag of $3 \, kg$ mass on her back and moves $200 \, m$ on a levelled road. The work done against the gravitational force will be $(g = 10 \, m \, s^{-2})$.

In each of the following cases,a force,$F$ is acting on an object of mass,$m$. The direction of displacement is from west to east,shown by the longer arrow. Observe the diagrams carefully and state whether the work done by the force is negative,positive,or zero.

$(a)$ How much work is done when a force of $1\, N$ moves a body through a distance of $1\, m$ in its direction?
$(b)$ Is it possible that a force is acting on a body but still the work done is zero? Explain giving one example.