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The models, theories, and laws we devise sometimes imply the existence of objects or phenomena that are as yet unobserved. These predictions are remarkable triumphs and tributes to the power of science. It is the underlying order in the universe that enables scientists to make such spectacular predictions. However, if experimentation does not verify our predictions, then the theory or law is wrong, no matter how elegant or convenient it is. Laws can never be known with absolute certainty because it is impossible to perform every imaginable experiment to confirm a law for every possible scenario. Physicists operate under the assumption that all scientific laws and theories are valid until a counterexample is observed. If a good-quality, verifiable experiment contradicts a well-established law or theory, then the law or theory must be modified or overthrown completely.

The study of science in general, and physics in particular, is an adventure much like the exploration of an uncharted ocean. Discoveries are made; models, theories, and laws are formulated; and the beauty of the physical universe is made more sublime for the insights gained.


  • Physics is about trying to find the simple laws that describe all natural phenomena.
  • Physics operates on a vast range of scales of length, mass, and time. Scientists use the concept of the order of magnitude of a number to track which phenomena occur on which scales. They also use orders of magnitude to compare the various scales.
  • Scientists attempt to describe the world by formulating models, theories, and laws.

Conceptual questions

What is physics?

Physics is the science concerned with describing the interactions of energy, matter, space, and time to uncover the fundamental mechanisms that underlie every phenomenon.

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Some have described physics as a “search for simplicity.” Explain why this might be an appropriate description.

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If two different theories describe experimental observations equally well, can one be said to be more valid than the other (assuming both use accepted rules of logic)?

No, neither of these two theories is more valid than the other. Experimentation is the ultimate decider. If experimental evidence does not suggest one theory over the other, then both are equally valid. A given physicist might prefer one theory over another on the grounds that one seems more simple, more natural, or more beautiful than the other, but that physicist would quickly acknowledge that he or she cannot say the other theory is invalid. Rather, he or she would be honest about the fact that more experimental evidence is needed to determine which theory is a better description of nature.

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What determines the validity of a theory?

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Certain criteria must be satisfied if a measurement or observation is to be believed. Will the criteria necessarily be as strict for an expected result as for an unexpected result?

Probably not. As the saying goes, “Extraordinary claims require extraordinary evidence.”

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Can the validity of a model be limited or must it be universally valid? How does this compare with the required validity of a theory or a law?

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Find the order of magnitude of the following physical quantities. (a) The mass of Earth’s atmosphere: 5.1 × 10 18 kg; (b) The mass of the Moon’s atmosphere: 25,000 kg; (c) The mass of Earth’s hydrosphere: 1.4 × 10 21 kg; (d) The mass of Earth: 5.97 × 10 24 kg; (e) The mass of the Moon: 7.34 × 10 22 kg; (f) The Earth–Moon distance (semimajor axis): 3.84 × 10 8 m; (g) The mean Earth–Sun distance: 1.5 × 10 11 m; (h) The equatorial radius of Earth: 6.38 × 10 6 m; (i) The mass of an electron: 9.11 × 10 −31 kg; (j) The mass of a proton: 1.67 × 10 −27 kg; (k) The mass of the Sun: 1.99 × 10 30 kg.

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Use the orders of magnitude you found in the previous problem to answer the following questions to within an order of magnitude. (a) How many electrons would it take to equal the mass of a proton? (b) How many Earths would it take to equal the mass of the Sun? (c) How many Earth–Moon distances would it take to cover the distance from Earth to the Sun? (d) How many Moon atmospheres would it take to equal the mass of Earth’s atmosphere? (e) How many moons would it take to equal the mass of Earth? (f) How many protons would it take to equal the mass of the Sun?

a. 10 3 ; b. 10 5 ; c. 10 2 ; d. 10 15 ; e. 10 2 ; f. 10 57

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For the remaining questions, you need to use [link] to obtain the necessary orders of magnitude of lengths, masses, and times.

Roughly how many heartbeats are there in a lifetime?

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A generation is about one-third of a lifetime. Approximately how many generations have passed since the year 0 AD?

10 2 generations

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Roughly how many times longer than the mean life of an extremely unstable atomic nucleus is the lifetime of a human?

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Calculate the approximate number of atoms in a bacterium. Assume the average mass of an atom in the bacterium is 10 times the mass of a proton.

10 11 atoms

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(a) Calculate the number of cells in a hummingbird assuming the mass of an average cell is 10 times the mass of a bacterium. (b) Making the same assumption, how many cells are there in a human?

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Assuming one nerve impulse must end before another can begin, what is the maximum firing rate of a nerve in impulses per second?

10 3 nerve impulses/s

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About how many floating-point operations can a supercomputer perform each year?

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Roughly how many floating-point operations can a supercomputer perform in a human lifetime?

10 26 floating-point operations per human lifetime

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Questions & Answers

What is th formular for force
Joseph Reply
F = m x a
State newton's second law of motion
Seth Reply
can u tell me I cant remember
force is equal to mass times acceleration
The uniform seesaw shown below is balanced on a fulcrum located 3.0 m from the left end. The smaller boy on the right has a mass of 40 kg and the bigger boy on the left has a mass 80 kg. What is the mass of the board?
Asad Reply
Consider a wave produced on a stretched spring by holding one end and shaking it up and down. Does the wavelength depend on the distance you move your hand up and down?
Sohail Reply
how can one calculate the value of a given quantity
Helen Reply
To determine the exact value of a percent of a given quantity we need to express the given percent as fraction and multiply it by the given number.
briefly discuss rocket in physics
Ibrahim Reply
ok let's discuss
What is physics
Nura Reply
physics is the study of natural phenomena with concern with matter and energy and relationships between them
a potential difference of 10.0v is connected across a 1.0AuF in an LC circuit. calculate the inductance of the inductor that should be connected to the capacitor for the circuit to oscillate at 1125Hza potential difference of 10.0v is connected across a 1.0AuF in an LC circuit. calculate the inducta
Royalty Reply
L= 0.002H
how did you get it?
is the magnetic field of earth changing
tibebeab Reply
what is thought to be the energy density of multiverse and is the space between universes really space
can you explain it
Energy can not either created nor destroyed .therefore who created? and how did it come to existence?
Suzana Reply
this greatly depend on the kind of energy. for gravitational energy, it is result of the shattering effect violent collision of two black holes on the space-time which caused space time to be disturbed. this is according to recent study on gravitons and gravitational ripple. and many other studies
and not every thing have to pop into existence. and it could have always been there . and some scientists think that energy might have been the only entity in the euclidean(imaginary time T=it) which is time undergone wick rotation.
What is projectile?
Nana Reply
An object that is launched from a device
2 dimensional motion under constant acceleration due to gravity
Not always 2D Awais
no comments
why not? a bullet is a projectile, so is a rock I throw
bullet travel in x and y comment same as rock which is 2 dimensional
no all pf you are wrong. projectile is any object propelled through space by excretion of a force which cease after launch
for awais, there is no such thing as constant acceleration due to gravity, because gravity change from place to place and from different height
it is the object not the motion or its components
where are body center of mass on present.
Balwant Reply
on the mid point
is the magnetic field of the earth changing?
does shock waves come to effect when in earth's inner atmosphere or can it have an effect on the thermosphere or ionosphere?
and for the question from bal want do you mean human body or just any object in space
A stone is dropped into a well of 19.6m deep and the impact of sound heared after 2.056 second ,find the velocity of sound in air.
Sisco Reply
9.53 m/s ?
In this case, the velocity of sound is 350 m/s.
some calculations is need. then you will get exact result.
i mean how? isn't it just a d over t?
calculate the time it takes the stone to hit the ground then minus the stone's time to the total time... then divide the total distance by the difference of the time
awit lenard. Hahahah ari ga to!
Why do we use mochromatic light? If white light is used, how would the pattern change
Dominic Reply
what is the dimension of force
Omolola Reply
Practice Key Terms 5

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Source:  OpenStax, University physics volume 1. OpenStax CNX. Sep 19, 2016 Download for free at http://cnx.org/content/col12031/1.5
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