Q.Explain how the definition of work in physics is different from general perception.

Step 1. In general (non-physics) usage, "work" refers to almost any physical or mental effort -- studying for an exam, or pushing hard against an immovable wall until exhausted, would both be called "work" in daily speech.

Step 2. Physics defines work precisely as $W=\vec F\cdot d\vec r = F\,dr\cos\theta$: work is done by a force only if that force actually produces a displacement of the point it acts on, and only the component of the force along the direction of that displacement contributes.

Step 3. This produces results that contradict everyday intuition: pushing against an immovable wall (zero displacement) does zero work in physics, however tiring it feels; carrying a heavy bag while walking horizontally does zero work against gravity, since gravity acts vertically, perpendicular to the horizontal displacement.

Step 4. Physics work is also a signed scalar -- it can be positive (force helps the motion), negative (force opposes the motion, e.g. friction decelerating a sliding block), or zero -- whereas everyday "work" is never thought of as negative.

Contrast the formal definition W = F.dr with the loose everyday usage of the word 'work', using the zero-work and negative-work cases as concrete illustrations.

• Saying physics work is 'the same as' everyday work with just a formula attached, without noting the crucial displacement and angle requirements.
• Forgetting to mention that work can be negative in physics, a case with no everyday-language analogue.