Page 52 - 05. Subyek Teknik Mesin - Automobile Mechanical and Electrical Systems Automotive Technology Vehicle Maintenance and Repair (Vehicle Maintenance Repr Nv2) by Tom Denton

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Table 1.16 (Continued)
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Pressure (N/m This is a force per area; e.g. the old tyre pressure measurement for many cars was 28 psi (pounds per
or Pa) square inch). The better unit to get used to is the bar: the tyre pressure would be about 1.8 bar. The SI unit
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is the pascal or newtons per metre squared (Pa or N/m ). 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 If you swing a stone on a string round your head it tries to move outwards and you can feel the centrifugal
force (N) 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 The point within an object at which it will balance. All the weight of an object such as a car can be said to
gravity 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

1.4.2 Units
Defi nition
When I go into a café or a bar and I ask for a pint of beer or half a litre of coke SI
I usually get what I want ( Fig. 1.51 ). This is because I ask by using the correct SI stands for ‘Système International’
units. When you blow up the tyres on a car you check the pressure in a book or (often described as the metric
on a chart and then look at the gauge. It will have the same units, and you can system).
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.