# (INTRODUCTION) PHYSICAL QUANTITIES AND UNITS ```Colegio Ntra. Sra. del Prado - Marianistas
UNIT 0
2&ordm; E.S.O. Natural Science - Physics
(INTRODUCTION)
PHYSICAL QUANTITIES AND UNITS
SUMMARY
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Physical quantities are a characteristic or property of an object that can be measured or
calculated from other measurements.
Units are standards for expressing and comparing the measurement of physical
quantities. All units can be expressed as combinations of fundamental units.
There are six fundamental quantities. The three fundamental units we will use this year
are the metre (for length), the kilogram (for mass), the second (for time). These units
are part of the metric system, which uses powers of 10 to relate quantities over the vast
ranges encountered in nature.
The three fundamental units are abbreviated as follows: metre, m; kilogram, kg and
second, s. The metric system also uses a standard set of prefixes to denote each order of
magnitude greater than or lesser than the fundamental unit itself.
Unit conversions involve changing a value expressed in one type of unit to another type
of unit. This is done by using conversion factors, which are ratios relating equal
quantities of different units.
Fundamental S.I. Quantities:
LENGTH (metre, m.)
MASS (kilogram, kg.)
TIME (second, s.)
TEMPERATURE (Kelvin, K.)
…
Derived S.I. Quantities:
SPEED, FORCE, DENSITY, POWER, …
Colegio Ntra. Sra. del Prado - Marianistas
UNIT 1
2&ordm; E.S.O. Natural Science - Physics
MOTION AND SPEED.
WHAT IS MOTION?
A body is in motion when it changes its position compared to a fixed point of reference.
A body in motion is called a moving object.
Motion can be classified in the following ways:
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Uniform motion: a body has uniform motion when its speed is constant. It travels the
same distance in each unit of time, for example in each second.
Variable motion: a body has variable motion when its speed isn’t constant. It travels a
different distance in each unit of time.
If a body moves at a constant speed in a straight line, we say it has uniform rectilinear
motion (URM).
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SPEED
Speed is the quantity that tells us how quickly bodies change their position: it refers to
the distance travelled in a specific time.
Speed is measured in metres/second (m/s), we say “metres per second”. Speed is also
measured in kilometres/hour (km/h), we say “kilometres per hour”.
Speed from a moving object in a certain instant is called instantaneous speed. When a
car travels from one place to another it doesn’t always travel at the same speed. We can
calculate the car’s average speed for the whole journey. Average speed is the quotient between
distance travelled and time taken:
av
=
In uniform motion, instantaneous speed is always the same; we say that it is constant.
In variable motion, instantaneous speed isn’t constant. That’s why we use average speed
concept.
→ACTIVITIES 1, 2 (Physics activities (II))
Colegio Ntra. Sra. del Prado - Marianistas

2&ordm; E.S.O. Natural Science - Physics
ACCELERATION
Acceleration is the change of speed per unit of time. To calculate the value of acceleration:
An object has acceleration when it starts from its initial position with a speed and
finishes at a different speed. The object may move more and more quickly or more and more
slowly.
When a body moves in a straight line and has a constant acceleration, we say it has
uniformly accelerated rectilinear motion (UARM). These objects don’t travel the same distance
in each unit of time.
→ACTIVITIES 3, 4, 5, 6 (Physics activities (II))
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