Video Lecture 1: Scientific Definition of Work & Positive, Negative, Zero Work

# Video Lecture 1: Scientific Definition of Work & Positive, Negative, Zero Work
**Speaker:** Dr. Ananya Sharma | **Duration:** ~06:00 mins

### Introduction: Everyday Work vs Scientific Work
In everyday speech, a student sitting at a desk reading a textbook for five hours says, *"I have done a mountain of work today!"*
A laborer standing motionless holding a 50 kg bag of cement on his head for an hour under the sun complains of heavy work.
Yet in physics, the work done in both these cases is **strictly ZERO**!

In physics, **Work** is done if and only if two simultaneous conditions are satisfied:
1. A force must act on the object.
2. The object must undergo physical displacement along or in the direction of the force!
$$\mathbf{W = F \cdot s \cdot \cos\theta}$$

### Positive, Negative, and Zero Work
- **Positive Work ($\theta = 0^\circ$):** Force and displacement act in the same direction ($W = F \cdot s$). When you push a shopping cart forward and it rolls forward.
- **Negative Work ($\theta = 180^\circ$):** Force opposes the direction of displacement ($W = -F \cdot s$). The friction of the road acting on a braking car, or gravity acting on an apple thrown vertically upward!
- **Zero Work ($\theta = 90^\circ$ or $s = 0$):**
- A porter carrying luggage on his head walking horizontally: gravitational force acts vertically downward ($90^\circ$ to horizontal displacement), so $W_{\text{gravity}} = 0$!
- Pushing against a solid concrete wall that does not move ($s = 0$).