Author: Lori Stearns

When you place a hot cooking pot in a cooler place, does the pot get hotter as it stands? No, it will always get cooler. This illustrates a scientific law that states that warm objects always become cooler when they are placed in cooler surroundings. A scientific law describes a process in nature that can be tested by repeated experiments. A law allows predictions to be made about how a system will behave under a wide range of conditions. However, a law does not explain how a process takes place.
In the example of the cooking pot, nothing in the law tells why hot objects become cooler in cooler surroundings. Such an explanation of how a natural process works must be provided by a scientific theory. Scientific theories are always being questioned and examined. To be valid, a theory must continue to pass several tests.
A theory must explain observations clearly and consistently.
Experiments that illustrate the theory must be repeatable.
You must be able to predict from the theory.
Theories and laws are always being tested. Sometimes theories have to be changed or replaced completely when new discoveries are made.
Over 200 years ago, scientists used the caloric theory to explain how objects become hotter and cooler. Heat was thought to be an invisible fluid, called caloric, that could flow from a warm object to a cool one. People thought that fires were fountains of caloric, which flowed into surrounding objects, making them warmer. The caloric theory explained everything that they knew about heat. But, the caloric theory could not explain why rubbing two rough surfaces together made them warmer. After many experiments in the 1800s, scientists suggested a new theory based on the idea that heat was a result of the motion of particles. The new theory was that heat is really a form of energy that is transferred when fast-moving particles hit others. Because this theory, the kinetic theory, explained the old observations as well as the new ones, it was kept and the caloric theory was discarded.

Scientific Laws and Theories

Distinguishing Between Laws and Theories

Introduction to Science

Special Points of Interest:
Scientific Laws
Scientific Theories
Scientific Method
Hypothesis
Controlled experiments
Variables
Units of Measurement
Derived units
Converting measurements
Presenting Data
Line graph
Independent variable
Dependent variable
Bar graph
Pie chart
Scientific Notation
Significant Figures
Precision
Accuracy

Mrs. Wachal’s Physical Science Class

Chapter 1

Volume 1, Issue 1

In the scientific method, critical thinking is used to solve scientific problems. The scientific method is a series of steps followed to solve problems including collecting data, formulating a hypothesis, testing the hypothesis, and stating conclusions. It is important to note that the scientific method can be more of a cyclic pattern than a straight line path.
Most scientific questions begin with observations. The observations led you to a question, “Why does..” You may gather data that will lead to possible answers to your questions. This possible answer that you can test is known as a hypothesis. Scientists test hypothesis by performing controlled experiments. In a controlled experiment, all variables that can affect the outcome of the experiment are kept constant, or controlled, except for one. Only the results of changing one given variable are observed. When you change more than one thing at a time, it’s harder to make reasonable conclusions.
No experiment is a failure. Experiments may not give the results you expected, but they are all observations of events in the natural world. These results are used to revise a hypothesis and to plan tests of a different variable.

Mathematics is the language of science. But if your scientific measurements are in inches and gallons, some scientists may not understand because they do not use these units. For this reason scientists use the International System of Units, or SI, which stands for the French phrase le Systeme Internationale d’Unites.
When all scientists use the same system of measurement, sharing data and results is easier. SI is based on the metric system and uses the seven SI base units seen in Figure 1.

The Scientific Method

UNITS OF MEASUREMENT

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Introduction to Science

“No experiment is a failure.”

Perhaps you noticed that the base units do not include area, volume, pressure, weight, force, speed, and other familiar quantities. Combinations of the base units, called derived units, are used for these measurements.
For example, suppose you want to order carpet for a floor that measures 8.0 m long and 6.0 m wide. You know that the area of a rectangle is A = l x w. The area of the floor can be calculated: A = 8.0 m x 6.0 m =
48.0 m2 .
The SI unit of area, m2 , is a derived unit.
Another benefit of the SI system is the use of prefixes to express very small or very large numbers. These prefixes help scientists to avoid writing a lot of decimal places and zeros and are shown in Figure 2.
Notice that the prefixes in Figure 2. are all multiples of 10. This makes it easy to convert SI units to smaller or larger units.
If you are converting to a smaller unit, multiply the measurement to get a bigger number:
1.85 m x 100 cm/ m =
185 cm.
If you are converting to a larger unit, divide the measurement to get a smaller number:
185 cm x 1 m/100 cm =
1.85 m.

Suppose you are trying to determine the speed of a chemical reaction that produces a gas. You can let the gas displace water in a graduated cylinder. You read the volume of gas in the cylinder every 10 seconds from the start of the reaction until there is no change in volume. Because you did the experiment, you saw how the volume changed over time. But how can someone who reads your report see it? To show the results, you can make a graph.
Many types of graphs can be drawn: a bar graph, a pie chart, and a line graph. But which one should you use?
A bar graph is useful when you want to compare similar data for several individual items or events. A bar graph often makes clearer how large or small the differences in individual values are.
A pie chart is ideal for displaying data that are parts of a whole. You can draw a pie chart to show percentages as a portion of the whole pie.
A line graph is best for displaying data that change. Your experiment has two variables, time and volume. Time is the independent variable because you chose the time intervals to take the measurements. The volume of the gas is the dependent variable because its value depends on what happens in the experiment. Line graphs are usually made with the x-axis showing the independent variable and the y-axis showing the dependent variable.
A person who never saw your experiment can look at the line graph and know what took place.

MORE UNITS OF MEASUREMENT

Presenting Data

Volume 1, Issue 1

1.85 m
x
100 cm/ m =
185 cm
Remember that to make a measurement, it takes more of a small unit or less of a large unit.
185 cm
x
1 m/100 cm = 1.85 m

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Scientists sometimes need to express measurements using numbers that are very large or very small. For example, the speed of light through space is about 300 000 000 m/s. Suppose you want to calculate the time required for light to travel from Neptune to Earth when Earth and Neptune are 4 500 000 000 000 m apart. To find out how long it takes, you would divide the distance between Earth and Neptune by the distance light travels in 1 s.
t = distance from Earth to Neptune (m) ÷ distance light travels in 1 s (m/s)
t = 4 500 000 000 000 m ÷ 300 000 000 m/s
This is a lot of zeros to keep track of!
To reduce the number of zeros, you can express values as a simple number multiplied by a power of 10. This is called scientific notation. In scientific notation, 4 500 000 000 000m can be written as 4.5 x 10 12 m. The speed of light in space is 3.0 x 10 8 m/s.
When you use scientific notation in calculations, you follow the math rules for powers of 10. When you multiply two values in scientific notation, you add the powers of 10. When you divide, you subtract the powers of 10.
So the problem about Earth and Neptune can be solved more easily:
t = 4.5 x 10 12 m ÷ 3.0 x 10 8 m/s OR t = 1.5 x 104 s.

Scientific Notation

Quantity
Unit
Abbreviation
Length
meter
m
Mass
kilogram
kg
Time
second
s
Temperature
kelvin
K
Electric current
ampere
A
Amount of
substance
mole
mol
Luminous
intensity
candela
cd
FIGURE 1. SI Base Units

Prefix
Symbol
Meaning
Multiple of base unit
Kilo-
k
Thousand
1000
Mega-
M
Million
1 000 000
Giga-
G
Billion
1 000 000 000
FIGURE 2. SI Prefixes
Deci-
d
Tenth
0.1
Centi-
c
Hundredth
0.01
Milli-
m
Thousandth
0.001
Micro-
µ
Millionth
0.000 001
Nano-
n
Billionth
0.000 000 001

Using Significant Figures

Suppose you measure a length of wire with two tape measures. One tape is marked every 0.001 m, and the other is marked every 0.1 m. The tape marked every 0.001 m gives you more precision, because with it you can report a length of 1.638 m. The other tape is only precise to 1.6 m. The exactness of a measurement is precision. To show the precision of a measured quantity, scientists use significant figures. Significant figures are a prescribed decimal place that determines the amount of rounding off to be done based on the precision of the measurement. The length of 1.638 m has four significant figures because the digits 1638 are known for sure. The measurement of 1.6 m has two significant figures.
If the tip of your tape measure has broken off, you can read 1.638 m precisely, but that number is not accurate. A measured quantity is only as accurate as the tool used to make the measurement. Accuracy is the description of how close a measurement is to the true value of the quantity measured.

Precise and Accurate

Precise and Inaccurate

Imprecise and Accurate

Imprecise and Inaccurate

EXAMPLE
THEORY OR LAW?
REASONS
Plate Tectonics
Theory
The key principle of plate tectonics is that the lithosphere exists as separate and distinct tectonic plates, which ride on the fluid-like asthenosphere. Tectonic plates are able to move because of the relative density of oceanic lithosphere and the relative weakness of the asthenosphere.
Conservation of Energy
Law
The total amount of energy in any isolated system remains constant but cannot be recreated, although it may change forms.