will mean that you will be a better technician.
Table 1.16 Useful terminology
SI units A set of standard units so we all talk the same language. SI stands for ‘Système International’. This is French for ‘International System’!
Ratio The amount of one thing compared to another, e.g. two to one is written as 2:1
Area (m 2 ) Amount of surface of anything, e.g. the surface area of a car roof would help you know how much paint would be needed to cover it
Volume (m 3 ) Capacity of an object, e.g. 1000 cc (cubic centimetres) or one litre of paint to do the job above
Mass (kg) The quantity of matter in a body. Volume does not matter, e.g. which has the greater mass, a kilogram of lead or a kilogram of feathers? They both have the same mass, but have different volumes
Density (kg/m 3 ) A full paint tin has a greater mass than an empty tin, but the volumes are the same
Energy (J) The ability to do work or the amount of work stored in something; e.g. petrol contains a lot of energy in chemical form
Force (N) When you push an object it moves (if you can apply enough force)
Work (J) Work is done when the force applied to an object makes it move. Work can also be said to be done when energy is converted from one form to another
Power (W) The rate at which work can be done, e.g. energy used per second
Torque (Nm) A turning force like a spanner turning a nut. A longer spanner needs less force
Velocity (m/s) A scientifi c name for speed; e.g. the UK national velocity limit is 70 mph (not an SI unit!) Acceleration
(m/s 2 )
The rate at which velocity changes. If positive then the car, for example, will increase in speed. If negative (or deceleration) such as when braking, the car’s speed decreases
Momentum (kg m/s)
The combination of the mass of a body and its velocity. A large goods vehicle has much greater momentum than a car at the same speed. It must have much better brakes or it will take a lot longer to stop
Friction (μ) When one surface moves over another friction tries to stop the movement. It is interesting to note that without friction a moving object such as a car would not stop!
Heat (J) This is a measure of the amount of energy in a body. Heat can only transfer from a higher to a lower temperature and this will be by conduction, convection or radiation
Temperature (°C) A measure of how hot something is, but this must not be confused with the amount of heat energy (Continued)
37
1
1.4.2 Units
When I go into a café or a bar and I ask for a pint of beer or half a litre of coke I usually get what I want ( Fig. 1.51 ). This is because I ask by using the correct units. When you blow up the tyres on a car you check the pressure in a book or on a chart and then look at the gauge. It will have the same units, and you can infl ate the tyres to the correct pressure.
The easiest units to work with are called SI units, sometimes described as the metric system. Other systems are fi ne, of course, and whatever is in common use, or whatever is stated in manufacturer’s data is what you should use. However, the basic SI units you will need to know are listed in Table 1.17 . Many other units in use are derived from the basic SI units. Some of them are combined and given new names ( Table 1.18 ).
When dealing with some of these units or derived units, we need a way of describing very large or very small quantities. For example, I would not say that I live 24 000 metres away from where I work. I would say I live 24 kilometres away, normally written as 24 km. The ‘k’ is known as a multiplier and in this case you will see it has the value of 1000.
Defi nition
SI
SI stands for ‘Système International’ (often described as the metric system).
Pressure (N/m 2
or Pa)
This is a force per area; e.g. the old tyre pressure measurement for many cars was 28 psi (pounds per square inch). The better unit to get used to is the bar: the tyre pressure would be about 1.8 bar. The SI unit is the pascal or newtons per metre squared (Pa or N/m 2 ). The pressure in this room is about 1 bar or
1 atmosphere or 100 000 Pa. It may be much more if you have been reading about science for a long time! Centrifugal
force (N)
If you swing a stone on a string round your head it tries to move outwards and you can feel the centrifugal force on the string. The faster you swing it the greater the force. When a car wheel is rotating very quickly a small imbalance in the tyre causes unequal centrifugal force and this makes the wheel wobble
Weight (N) The mass of an object acted upon by the Earth’s gravity gives it a weight. When you next go into outer space, you will fi nd that your weight is zero, or in other words you are weightless. You still have the same mass, however. The word weight is often used incorrectly, but as gravity is the same all over the Earth it doesn’t often make any difference
Centre of gravity
The point within an object at which it will balance. All the weight of an object such as a car can be said to act through the centre of gravity. If the force due to gravity and acceleration acting through this point falls outside the wheels of the car, the car will fall over!
Electricity This is the movement of electrons known as a current fl ow in a conductor or a wire. Electricity is a very convenient way of transferring energy
Strength This is hard to defi ne because different materials are strong in different ways. A material can be strong by providing opposition to bending, tension, compression or shear force
Corrosion Corrosion of materials is by a chemical process; e.g. if iron is left open to the air or water it rusts. The chemical process is that the iron reacts with oxygen in the air and turns into iron oxide (rust)
Machines A machine is something that converts one form of energy into another; e.g. an alternator converts mechanical energy from the engine into electrical energy
Hydraulics When fl uids are used to do ‘work’ this is described as hydraulics. The braking system of a car is a good example
Oscillation If you bounce a mass on a spring (a car on its suspension) it will move up and down (oscillate) until all the mechanical energy in the spring has been converted to another form (mostly heat due to friction). Dampers are used on a car to make this time as short as possible
38
1
Automobile mechanical and electrical systemsLikewise, if setting a spark plug gap I could set it at 0.001 metres, or it might be easier to say 1 millimetre, normally written as 1 mm. The ‘m’ can be thought of as a divider which in this case is 1000 or a multiplier of 0.001. Common multipliers are listed in Table 1.19 .
Figure 1.51 Mmm!
Table 1.18 Derived SI units
Unit Abbreviation Quantity
joule J Energy newton N Force watt W Power area m 2 Square metres volume m 3 Cubic metres
torque Nm Newton metres velocity m/s or m s 1 Metres per second
acceleration m/s/s or m s 2 Metres per second per second Table 1.17 SI units
Unit Abbreviation Quantity Example
metre m Length The distance from one point to another
kilogram kg Mass The quantity of matter which makes an object
second s Time About 300 s to boil an egg! ampere A Electric current The fl ow rate of electricity
through a wire
kelvin K Temperature How hot the radiator of a car is candela cd Luminous intensity How brightly a headlight shines
Table 1.19 Common multipliers
Prefi x Symbol Value Long value
mega M 10 6 1 000 000 kilo k 10 3 1000 hecto h 10 2 100 centi c 10 2 0.01 milli m 10 3 0.001 micro μ 10 6 0.000 001