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

Work is said to be done when a force produces a displacement in the body and acts in any direction except perpendicular to the direction of displacement. 

Work = Force×Displacement×Cos θ

where, θ is the angle between directions of force and displacement. 

The SI unit of work is joules (J).

Work done is said to be negative, if the displacement of the given object occurs in the opposite direction (180°) of the applied force.

 Examples: 

1) When we throw a stone upwards, the gravity acting on the stone is downwards. But the displacement of the stone is in the upward direction. So the work done by the gravitational force is negative. 

2) If we drag an object on a rough surface, the frictional force will be acting in the direction opposite to the displacement of the object. The angle between the displacement and the frictional force is 180°. Thus the work done by the frictional force is negative. 

The physical significance of negative sign is that work needs to be done on the body to displace it.