Work and Energy. Types of Energy

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5.2Work and Energy. Types of Energy


In these revision notes for Work and Energy. Types of Energy, we cover the following key points:

  • What is Work?
  • What is Energy?
  • What are the units of Work and Energy?
  • How is Energy classified?
  • The names of all types of energy you will encounter during your study of Physics

Work and Energy. Types of Energy Revision Notes

In Physics, a system, person or object is doing work if

  1. A force is exerted on a certain object, and
  2. The object changes its place due to the action of the given force.

Mathematically, we have:

Work = Force × Displacement

In symbols,

W = F × ∆x

where:

  • F is the force exerted
  • Δx is the object's displacement.

Work is a scalar quantity. It is measured in Joules [J]. 1 J = 1 N × m.

In Thermodynamics (especially when dealing with food (chemical) energy and thermal energy, another unit known as Calorie [cal] is often used. The conversion factor between Joule and Calorie is

1cal = 4.18J

Not always, the direction of force is in the direction of motion. In this case, we have to use only the component of force that lies in the direction of motion.

In this case, only Fx = F ∙ cos α contributes to the motion. Therefore, the work done by the force F will be

W = Fx × ∆x
= F × cos α × ∆x

This is still a 1-D motion regardless the fact that the direction of force F is not in the direction of motion. You must consider only the force component that lies in the direction of motion.

When the object is moving in two or three dimensions, we use the equation

W = |F| × r
= |F| × √(xf - xi )2 + (yf - yi )2 + (zf - zi )2

To calculate the work done by the force F. Here, the expression within the square root represents the displacement r in 3 dimensions.

Geometrically, Work is represented by the area under the Force vs Position graph. This method is particularly useful when the force used is not constant.

Energy E is the ability of an object or system to do work. Therefore, Work is a kind of energy change in a system. We can write

W = ∆E
= Einitial - Efinal

Energy is also measured in Joules [J].

Energy is classified in two main groups: I. Energy of state and II. Energy of transfer. The scheme below shows the relationship between the most important types of energy with each other.

Physics Tutorials: This image shows the hierachial structure used to categorise the various disciplines in Physics

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