1. Unit 1
THE SCIENTIFIC WORK

2. Physics and Chemistry

What do they have in common?

Physicists and Chemists study
the same: matter.

Physicists, Chemists and other
scientists work in the same way:

SCIENTIFIC METHOD

3. Physics and Chemistry

What makes them different?

Physics studies
phenomena that don't
change the
composition of matter.

Chemistry studies
phenomena that change
the composition of
matter.

4. SCIENTIFIC METHOD

5. SCIENTIFIC METHOD
 The observation of a
phenomenon and
curiosity make
scientists ask
questions.
 Before doing anything
else, it's necessary to
look for the previous
knowledge about the
phenomenon.

6. SCIENTIFIC METHOD
 Hypotheses are
possible answers to
the questions we
asked.
 They are only
testable predictions
about the
phenomenon.

7. SCIENTIFIC METHOD
 We use experiments for
checking hypotheses.
 We reproduce a
phenomenon in
controlled conditions.
 We need measure and
collecting data in tables
or graphics

8. SCIENTIFIC METHOD
 We study the relationships
between different
variables.
 In an experiment there are
three kinds of variables
− Independent
variables: they can be
changed.
− Dependent variables:
they are measured.
− Controlled variables:
they don't change.

9. SCIENTIFIC METHOD
 After the experiment, we
analyse its results and
draw a conclusion.
 If the hypothesis is true,
we have learnt
something new and it
becomes in a law
 If the hypothesis is false.
We must look for a new
hypothesis and continue
the research.

10. Magnitudes,
measurements and units

Physical Magnitude: It refers to every
property of matter that can be measured.
− Length, mass, surface, volume, density,
velocity, force, temperature,...

Measure: It compares a quantity of a
magnitude with other that we use as a
reference (unit).

Unit: It is a quantity of a magnitude used to
measure other quantities of the same
magnitude. It's only useful if every people uses
the same unit.

11. Magnitudes,
measurements and units
Length of the classroom = 10 m
means
The length of the classroom is 10 times the
length of 1 metre.

12. The International System
of Units

The SI has:
− a small group of magnitudes whose units
are fixed directly: the fundamental
magnitudes.

E.g.: Length → meter (m); Time → second
(s)
− The units for the other magnitudes are
defined in relationship with the fundamental
units: the derivative magnitudes.

E. g.: speed → meter/second (m/s)

13. The International System
of Units
The fundamental magnitudes and their units
Length meter m
Mass kilogram kg
Time second s
Amount of substance mole mol
Temperature Kelvin K
Electric current amperes A
Luminous intensity candela cd

14. The International System
of Units
Some examples of how to build the units of derivative
magnitudes:
− Area = Length · width → m·m = m2
− Volume = Length · width · height → m·m·m =
m3
− Speed = distance / time → m/s
− Acceleration = change of speed / time →
(m/s)/s = m/s2

15. The International System
of Units
Some examples of how to build the units of derivative
magnitudes:
− Area = Length · width → m·m = m2
− Volume = Length · width · height → m·m·m =
m3
− Speed = distance / time → m/s
− Acceleration = change of speed / time →
(m/s)/s = m/s2

16. The International System
of Units
More derivative units.
Quantity Name Symbol
Area square meter
m2
Volume cubic meter m3
Force Newton N
Pressure Pascal Pa
Energy Joule J
Power Watt W
Voltage volt V
Frequency Hertz Hz
Electric charge Coulomb C

17. The International System
of Units
Prefixes: we used them when we need express quantities
much bigger or smaller than basic unit.
Power of 10 for
Prefix Symbol Meaning Scientific Notation
_______________________________________________________________________
mega- M 1,000,000 106
kilo- k 1,000 103
deci- d 0.1 10-1
centi- c 0.01 10-2
milli- m 0.001 10-3
micro- µ 0.000001 10-6
nano- n 0.000000001 10-9

18. The International System
of Units
Prefixes: the whole list
Factor Name Symbol Factor Name Symbol
10-1 decimeter dm 101 decameter dam
10-2 centimeter cm 102 hectometer hm
10-3 millimeter mm 103 kilometer km
10-6 micrometer µm 106 megameter Mm
10-9 nanometer nm 109 gigameter Gm
10-12 picometer pm 1012 terameter Tm
10-15 femtometer fm 1015 petameter Pm
10-18 attometer am 1018 exameter Em
10-21 zeptometer zm 1021 zettameter Zm
10-24 yoctometer ym 1024 yottameter Ym

19. Changing units

We can change a quantity into another unit.
Conversion factors help us to do it.

A conversion factor is a fraction with the
same quantity in its denominator and in its
numerator but expressed in different units.
1h 1 km
=1 =1
60 min 1000 m
60 min 1000 m
=1 =1
1h 1 km

20. Changing units
Let's see a few examples of how to use them
1 km 2570 km ·1
2570 m · = =2,570 km
1000 m 1000
1 h 1 min 3500 h
3500 s · · = =0,972 h
60 min 60 s 3600
2

500 cm² ·
1m
100 cm 
=500 cm² · 
1m²
10000 cm²
= 
500 m²
10000
=0,05 m²
m m 1 km 3600 s 30 · 3600 km km
30 =30 · · = =108
s s 1000 m 1 h 1000 h h

21. Significant figures
•
They indicate precision of a measurement.
•
Sig Figs in a measurement are the really
known digits.
2.3 cm

22. Significant figures

Counting Sig Figs:
− Which are sig figs?

All nonzero digits.

Zeros between nonzero digits
− Which aren't sig figs?

Leading zeros – 0,0025

Final zeros without
a decimal point – 250

Examples:
− 0,00120 → 3 sig figs; 15000 → 2 sig figs
− 15000, → 5 sig figs; 13,04 → 4 sig

23. Significant figures

Calculating with sig figs
− Multiplicate or divide: the factor with the
fewer number of sig figs determines the
number of sig figs of the result:

2,345 m · 4,55 m = 10,66975 m 2 = 10,7 m2

(4 sig figs) (3 sig figs) → (3 sig figs)
− Add or substract: the number with the
fewer number of decimal places
determines the number of decimal places
of the result:

3,456 m + 2,35 m = 5,806 m = 5,81 m

(3 decimal places) (2 decimal places) → (2 decimal places)

24. Significant figures

Calculating with sig figs
− Exact number have no limit of sig fig:

Example: Area = ½ · Base · height.

½ isn't taken into account to round the
result.
− Rounding the result:

If the first figure is 5, 6, 7, 8 or 9, the last
figure taken into account is increased in 1

If not, it doesn't change.

25. Scientific notation

Is used to write very large or very small quantities:
− 385 000 000 Km = 3.85·108 Km
− 0,000 000 000 157 m = 1,57·10-10 m

Changing a number to scientific notation:
− We move the decimal point until there is an only
number in its left side.
− The exponent of 10 is the number of places we
moved the decimal point:

The exponent is positive if we move it to the left side

It's negative if we move it to the right side.

26. Measurement errors

It's impossible to measure a quantity with
total precision.

When we measure, we'll never know the
real value of the quantity.

Every measurement has an error because:
− The measurement instrument can only see
a few sig figs.
− It may not be well built or calibrated.
− We are using it in the wrong way.

27. Measurement errors

There are two ways for expressing the error
of a measurement:
− Absolute error: it is the difference
between the value of the measurement and
the value accepted as exact.
− Relative error: it is the absolute error in
relationship with the quantity.

28. Measurement errors

How to calculate the error. EXAMPLE 1:
− We have measured several times the mass of a
ball:

20,17 g, 20,21 g, 20,25 g, 20,15 g, 20,28 g
− It's supposed that the real value of the ball of the
mass is the average value of all the
measurements:

Vr = (20,17 g + 20,21 g + 20,25 g + 20,15 g + 20,27 g )/5 = 20,21 g
− The absolute error of the first measurement is:

Er = |20,17 g – 20,21 g| = 0,04 g
− The relative error is calculate dividing the absolute
error by the value of quantity.

29. Measurement error

How to calculate the error. EXAMPLE 2:
− We have measured once the length of a
piece of paper using a ruler that is
graduated in millimetres: 29,7 cm
− We suppose that the real value is the
measured value.
− The absolute error is the precision of the
rule:
 Ea = 0,1 cm
− Relative error:

Er = 0,1 cm / 29,7 cm = 0,0034 = 0,34 %