Concept of ‘work’ in physics is different from the usual meanings of work, when we use the word work in day-to-day language. When a body under the influence of a constant force $\overline{\text{F}}$  moves a distance $\overline{\text{d}}$ , work done  $=\overline{\text{F}}.\overline{\text{d}}$. Work is not a vector but it can be positive or negative. If the body moves in the direction of force applied, positive work is done on the body. If the moves in the opposite direction of the force applied, we say negative work is done. The body on which work is done gains or loses energy depending on whether positive or negative work is done.

Force doing the work can be conservative or non-conservative. If from state A to state B a body is taken through different paths and work done is same, then the forces are conservative. An example is gravitational force. On the other hand frictional force is non-conservative and the work done depends on the path taken.
If a body is in motion under a system of conservative forces, the sum of kinetic and potential energies is constant. Now consider a system shown in the figure. A body of 150kg mass is sliding down the inclined plane having a smooth surface, with velocity $v=2m{s}^{-1}.$ A force of F = 1000N is applied as shown by switching on a uniform electric field $\overline{\text{E}}$ on the body ‘B’, with charge q at time $t = 0.$ The body stops after time t.