Define the work done by a constant force and give its SI unit. Explain, with examples, when the work done is positive, negative and zero.

1. 1 · Define. Work = component of the force along the displacement × displacement: W = Fd cos θ. SI unit: joule.
2. 2 · Draw the figure. [A block on a floor; a force arrow F at angle θ above the horizontal; a displacement arrow d along the floor; the component F cos θ marked along d]
3. 3 · Positive work. θ < 90°, so cos θ > 0. Example: the engine force on an accelerating car; gravity on a falling mango.
4. 4 · Negative work. 90° < θ ≤ 180°, so cos θ < 0. Example: brakes on a moving car; friction on a sliding box.
5. 5 · Zero work. θ = 90° or d = 0. Example: gravity on a ball rolling on a level floor; pushing a fixed wall.
6. 6 · Close in one line. So the sign of the work depends on the angle between the force and the displacement.

Scoring tip: Write W = Fd cos θ first. Then use cos 0° = 1, cos 90° = 0 and cos 180° = −1 to explain each case, with one example each.