Page 54 - 05. Subyek Teknik Mesin - Automobile Mechanical and Electrical Systems Automotive Technology Vehicle Maintenance and Repair (Vehicle Maintenance Repr Nv2) by Tom Denton

1 39

1.4.3 Velocity and acceleration
Defi nition
Velocity is the speed of an object in a given direction. Velocity is a ‘vector
Velocity
quantity’, meaning that its direction is important as well as its speed. The velocity Velocity Distance travelled/Time
v of an object travelling in a fi xed direction may be calculated by dividing the taken ( v s / t )
distance s it has travelled by the time taken t . It is expressed as miles per hour
(mph) or m/s (metres per second).

Acceleration is the rate of change of velocity (how quickly speed is increasing or
decreasing). It is usually measured in metres per second per second. Newton’s
second law of motion says that a body will accelerate only if it is acted upon by
an outside force. The outside force on a car is either the accelerator to increase
Key fact
speed (accelerate) or the brakes to decrease speed (decelerate). It is usually
2
expressed as metres per second per second or ms . The force that opposes the relative
motion of two bodies in contact is
Acceleration due to gravity is the acceleration of an object falling due to the known as friction.
2
Earth’s gravity. The value used for gravitational acceleration or g is 9.806 ms
2
(10 ms is usually near enough for our calculations).
The average acceleration a of an object travelling in a straight line over a time t
may be calculated using the formula:

Acceleration Change of velocity/Time taken
Or, if v is its fi nal velocity and u its initial velocity:

a v ( u t)/
2
A negative answer (less than zero, e.g. 5 ms ) would mean that the object is
slowing down (decelerating).

1.4.4 Friction

The force that opposes the relative motion of two bodies in contact is known as Figure 1.52 Surface of a smooth material
friction. The coeffi cient of friction is the ratio of the force needed to achieve this magnifi ed thousands of times

motion to the force pressing the two bodies together.
For motor vehicle use friction is greatly reduced in some places by using
lubricants such as oil and grease. In other places friction is deliberately
increased; for example, brake shoes, pads, drive belts and tyres. Defi nition
Pressure
1.4.5 Pressure The SI unit of pressure is the pascal
(Pa), equal to a pressure of 1 newton
In a fl uid or gas, pressure is said to be the force that acts at right angles per per square metre.
unit surface area of something immersed in the fl uid or gas. The SI unit of
pressure is the pascal (Pa), equal to a pressure of 1 newton per square metre.
In the atmosphere, the pressure decreases as you go higher, from about
101 kPa at sea level to zero, where the atmosphere dwindles into space. The
other common units of pressure you will meet are the bar and psi. One bar
(100 kPa) is atmospheric pressure, which is also about 14.7 psi (pounds per
square inch). Defi nition
Absolute pressure is measured from a perfect vacuum or zero pressure ( Fig. 1.53 ). Absolute pressure
Gauge pressure is the difference between the measured pressure and atmospheric Absolute pressure is measured from
pressure. A tyre gauge works like this because it reads zero in atmospheric a perfect vacuum or zero pressure.
pressure. When we talk about a vacuum or a depression, what we really mean is
a pressure less than atmospheric. It is best to use absolute pressure fi gures for