﻿ General Definitions

### Mass

Unit - Kilogram (kg) - ‘The quantity of matter in a body.’ The mass of a body is a measure of how difficult it is to start or stop. (a 'body', in this context, means a substance. Any substance: a gas, a liquid or a solid.)

• The larger the mass, the greater the FORCE required to start or stop it in the same distance.
• Mass has a big influence on the time and/or distance required to change the direction of a body.
• ### Force

Unit - newton (N) - ‘A push or a pull’. That which causes or tends to cause a change in motion of a body.

There are four forces acting on an aircraft in flight - pushing or pulling in different directions.

### Weight

Unit - newton (N) - ‘The force due to gravity’. ( F = m × g ) where (m) is the mass of the object and (g) is the acceleration due to the gravity constant, which has the value of 9.81 m/s2. (A 1 kg mass ‘weighs’ 9.81 newtons)

If the mass of a A320 is 65000 kg and F = m × g it is necessary to generate: [65000 kg × 9.81 m/s2] 637650 N of lift force.

### Centre of Gravity (CG)

The point through which the weight of an aircraft acts.

• An aircraft in flight rotates around its CG.
• The CG of an aircraft must remain within certain forward and aft limits, for reasons of both stability and control.
• ### Work

Unit - Joule (J) - A force is said to do work on a body when it moves the body in the direction in which the force is acting. The amount of work done on a body is the product of the force applied to the body and the distance moved by that force in the direction in which it is acting.

If a force is exerted and no movement takes place, no work has been done.

• Work = Force × Distance (through which the force is applied)
• If a force of 10 newtons moves a body 2 metres along its line of action, it does 20 newton metres (Nm) of work. [10 N × 2 m = 20 Nm]
• A newton metre, the unit of work, is called a joule (J).
• ### Power

Unit - Watt (W) - Power is simply the rate of doing work (the time taken to do work).

• Power (W) = Force (N) × Distance (m) / Time (s)
• If a force of 10 N moves a mass 2 metres in 5 seconds, then the power is 4 joules per second.
• A joule per second (J/s) is called a watt (W), the unit of power. So the power used in this example is 4 watts.

### Energy

Unit - Joule (J) - Mass has energy if it has the ability to do work. The amount of energy a body possesses is measured by the amount of work it can do. The unit of energy will, therefore, be the same as those of work, joules.

### Kinetic Energy

Unit - Joule (J) - ’The energy possessed by mass because of its motion’. ’A mass that is moving can do work in coming to rest’.

KE = ½ m V2 joules

The kinetic energy of a 1 kg mass of air moving at 52 m/s (100 knots) is 1352 joules; it possesses 1352 joules of kinetic energy. [ 0.5 × 1 × 52 × 52 = 1352 J ]

From the above example it can be seen that doubling the velocity will have a greater impact on the kinetic energy than doubling the mass (velocity is squared).

### Newton’s First Law of Motion

’A body will remain at rest or in uniform motion in a straight line unless acted on by an external force’.

To move a stationary object or to make a moving object change its direction, a force must be applied.

### Inertia

‘The opposition which a body offers to a change in motion’. A property of all bodies, inertia is a quality, but it is measured in terms of mass, which is a quantity.

• The larger the mass, the greater the force required for the same result.
• A large mass has a lot of inertia.
• Inertia refers to both stationary and moving masses.
• ### Newton’s Second Law of Motion

’The acceleration of a body from a state of rest, or uniform motion in a straight line, is proportional to the applied force and inversely proportional to the mass’.

### Velocity

Unit - Metres per second (m/s). - ‘Rate of change of displacement’

### Acceleration

Unit - Metres per second per second (m/s2) - ‘Rate of change of velocity’.

A force of 1 newton acting on a mass of 1 kg will produce an acceleration of 1 m/s2

Acceleration = Force / Mass

• For the same mass; the bigger the force, the greater the acceleration.
• For the same force; the larger the mass, the slower the acceleration.
• ### Momentum

Unit - Mass × Velocity (kg-m/s) - ‘The quantity of motion possessed by a body’. The tendency of a body to continue in motion after being placed in motion.

• A body of 10 kg mass moving at 2 m/s has 20 kg-m/s of momentum.
• At the same velocity, a large mass has more momentum than a small mass.
• ### Newton’s Third Law

‘Every action has an equal and opposite reaction’

• If a force accelerates a mass in one direction, the body supplying the force will be subject to the same force in the opposite direction.
• Principles of Flight