To calculate the work done in ploughing the length of the field, we’ll use the formula:

Work = Force x Distance x cos(θ)

Given:
Force exerted by the bullocks = 140 N
Length of the field = 15 m

Here, the force exerted and the direction of movement are assumed to be in the same line, so the angle (θ) between the force and displacement vectors is (0°). Therefore, (cos(0) = 1).

Plugging the given values into the formula:

Work = Force x Distance x cos(θ)
Work = 140N x 15 m x 1
Work = 2100 J

Therefore, the work done in ploughing the length of the field by the pair of bullocks is 2100 joules (J).