The knowledge a firefighter must possess spans a surprising range of subjects, many of which may at first seem to have little to do with firefighting, such as chemistry and physics.
By: Danny Wilds
In order to understand the nature and composition of combustible materials, flammable liquids, toxic gases and the like, firefighters must possess a working knowledge of chemistry and physics. The firefighter must have knowledge of hydraulics and be able to make calculations to work out frictional loss in hoses and the flow from nozzles at stated pressures, without hesitation and in the most difficult circumstances.
To use fire pumps, hydraulic rescue equipment and other specially designed equipment to maximum efficiency, the firefighter must have a basic knowledge of hydraulics and physics.
Basic fundamentals of physics are introduced to new firefighters.
Force, work, energy and power
In studying the quantities work, energy, and power, the firefighter will discover all of them follow quantity force. Force must therefore be the starting point.
Force is the effect that changes or tends to change the condition of rest or uniform motion in a straight line of a body when supplied to it.
The unit of force is the Newton, N.
Force is equal to mass times acceleration or gravity.
F = m x a or F = m x g
Mass may be considered as a measure of quantity of matter in an object. One unit for mass is the kilogram (kg)
The earth attracts all objects that have mass.
The weight of an object is the force with which the earth attracts it and is measured in Newton (N)
Gravity is the force that pulls the objects to the centre of the earth. Gravity is proportional to the mass of the body. The value of the acceleration due to gravity in South Africa is approximately 9,8m/s². Use gravitational: g = 10m/s² for calculations.
In the pictures below, the first image is a picture of a climber on the side of a cliff. The second image shows just the climber and has vectors drawn, representing the different forces on the climber. The third image is a force diagram, the climber is simply represented by a dot, and the vectors are labelled by the type of force, the object exerting the force, and the object receiving that force.
Whenever you exert a force on an object, and the object moves in the direction of the force, you are doing work on the object.
Work is defined as the product of the force applied and the distance moved in the direction of that force.
Work = force x distance is equal to W = F x S (J)
The unit of work is N.m and is called the joule (J)
A joule is the amount of work done when a force of 1 N is applied over a distance of 1 m
Power is defined as the rate at which work is done. Stated differently, power is the amount of work done per second.
The unit of power is joules per second, called a watt (W). The power is one watt when one joule of work is done in one second.
P= Work divided by Time
Energy is the ability to do work, and is a measure of the amount of work that can be done. It follows that energy must be measured in joules.
Potential energy is the ability that an object has to do work by virtue of its position or state of compression.
Potential energy = work done = mass x gravity x weight
Kinetic energy is the ability that an object has to do work by virtue of its motion.
Principle of conservation of energy: Energy can not be created, or destroyed. It is merely changed from one form into another.
Examples for students to practise:
• Calculate the force exerted on a man of 90 kg due to gravity.
• Why will an iron ball of 10kg and an iron ball of 20kg undergo the same acceleration when falling free to the earth?
• A firefighter with a body mass of 85 kg climbs a ladder 20m high. How much work has he done?
• A force of 60N acts on a body for 12 seconds and moves it 100m parallel to the direction in which the force is operating. Calculate the power required.