Define work. How is work measured? When is work done by a force negative?

(N/A) Work is defined as the product of the force applied to an object and the displacement produced in the direction of the force. Mathematically,$W = F \times s \times \cos(\theta)$,where $W$ is work,$F$ is force,$s$ is displacement,and $\theta$ is the angle between force and displacement.
Work is measured as the product of the magnitude of force and the displacement in the direction of the force.
Work done by a force is negative when the force acts in the direction opposite to the direction of displacement,i.e.,the angle between force and displacement is $180^{\circ}$ (since $\cos(180^{\circ}) = -1$).