Why We Use Cosθ (Cosine Theta) In Formula Of Work Done?

We know that, work done is defined as the product of  force(F) and displacement(s) (shortest distance).

i.e. Work done = Force × Displacement,

or, W= F×s

For simplicity, let us assume a work is done against friction by sliding an object along X-axis.  So the object doesn't make any angle with X-axis (Angle 0°).  then formula of work done is, 

W = F× s× cosθ

or, W = F× s × cos0°

or, W = F× s × 1          [ since, Cos0° =1]

Therefore, W = F×s


From the below figureit is easier to understand the formula,

Here, F is the resultant force which makes angle θ with horizontal X-axis (displacement during work against gravity).