Work and Energy

Work

In physics, work is defined as done when a force is applied to an object and the object moves in the direction of the applied force. It is not enough to just apply a force; the object must be displaced for work to be done. The displacement must have a component in the direction of the force. Mathematically, work (W) done by a force (F) is given by the product of the component of the force in the direction of displacement and the magnitude of displacement (d). If θ is the angle between the force and displacement vectors, then work done is W = F × d × cosθ. The SI unit of work is the joule (J), where 1 joule is the work done when a force of 1 newton displaces an object by 1 meter in the direction of the force. Work can be positive, negative, or zero depending on the direction of force and displacement. Positive work is done when the force and displacement are in the same direction, negative work when they are opposite, and zero work if there is no displacement or the force is perpendicular to the displacement. For example, when you push a box and it moves forward, you do positive work. If you hold a box stationary, you apply force but do no work since there is no displacement. If friction acts opposite to the motion, friction does negative work. Work is a scalar quantity as it has magnitude but no direction. Understanding work is fundamental to studying energy and power, which are closely related concepts. **Table on page 14 (4×3)** | State | Substance/Medium | Approximate speed | | --- | --- | --- | | Solid | Steel | 5000 m s-1 | | Liquid | Water | 1500 m s-1 | | Gas | Air | 340 m s-1 |

• Work is done when a force causes displacement in the direction of the force.
• Mathematical formula: Work W = F × d × cosθ, where θ is the angle between force and displacement.
• SI unit of work is joule (J).
• Work can be positive, negative, or zero depending on force and displacement directions.
• Work is a scalar quantity.
• No displacement means no work done even if force is applied.
• 📌 Work: Product of force component in displacement direction and displacement.
• 📌 Displacement: The distance moved by an object in a specific direction.
• 📌 Joule (J): SI unit of work.

Energy

Energy is defined as the capacity to do work. It is a scalar quantity measured in joules (J) in the SI system. Energy exists in various forms such as kinetic energy, potential energy, thermal energy, chemical energy, electrical energy, and more. The concept of energy helps us understand how work is done and how physical systems change. Energy can be transformed from one form to another but cannot be created or destroyed, which is the basis of the law of conservation of energy. For example, chemical energy in food is transformed into kinetic energy when we move. The two most common forms of mechanical energy are kinetic energy and potential energy. Kinetic energy is energy due to motion, while potential energy is energy due to position or configuration. Other forms of energy like thermal energy relate to the temperature of a system, and chemical energy is stored in chemical bonds. Energy is essential in all natural phenomena and human activities. It is involved whenever work is done, whether lifting an object, heating water, or running a machine. Understanding energy allows us to analyze and predict the behavior of physical systems.

• Energy is the capacity to do work.
• Measured in joules (J) in the SI system.
• Energy exists in various forms: kinetic, potential, thermal, chemical, electrical, etc.
• Energy can be transformed from one form to another.
• Energy cannot be created or destroyed (law of conservation of energy).
• Mechanical energy includes kinetic and potential energy.
• 📌 Energy: Capacity to do work.
• 📌 Law of Conservation of Energy: Energy can neither be created nor destroyed, only transformed.
• 📌 Kinetic Energy: Energy due to motion.