Q.A box is pulled with a force of 25 N to produce a displacement of 15 m. If the angle between the force and displacement is 30 degrees, find the work done by the force.

When a force $F$ acting on a body remains constant in magnitude throughout the displacement (with a fixed angle $\theta$ between the force and the direction of motion), the small work done for a small displacement $dr$ is $dW=(F\cos\theta)\,dr$. Because $F\cos\theta$ does not change with position, integrating from an initial position $r_i$ to a final position $r_f$ gives simply

$W = (F\cos\theta)(r_f - r_i)$ …