<< Chapter < Page Chapter >> Page >

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.

Got questions? Get instant answers now!

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

Got questions? Get instant answers now!

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.

Got questions? Get instant answers now!

What determines the validity of a theory?

Got questions? Get instant answers now!

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.”

Got questions? Get instant answers now!

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?

Got questions? Get instant answers now!


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.

Got questions? Get instant answers now!

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

Got questions? Get instant answers now!

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?

Got questions? Get instant answers now!

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

10 2 generations

Got questions? Get instant answers now!

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

Got questions? Get instant answers now!

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

Got questions? Get instant answers now!

(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?

Got questions? Get instant answers now!

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

Got questions? Get instant answers now!

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

Got questions? Get instant answers now!

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

10 26 floating-point operations per human lifetime

Got questions? Get instant answers now!

Questions & Answers

who is Newton?
John Reply
a scientist
that discovered law of motion
but who is Isaac newton?
a postmodernist would say that he did not discover them, he made them up and they're not actually a reality in itself, but a mere construct by which we decided to observe the word around us
what is a scalar quantity
Peter Reply
scalar: are quantity have numerical value
is that a better way in defining scalar quantity
upward force and downward force lift
adegboye Reply
upward force and downward force on lift
what's the answer? I can't get it
Rachel Reply
what is the question again?
What's this conversation?
what is catenation? and give examples
what's the si unit of impulse
Iguh Reply
The Newton second (N•s)
what is the s. I unit of current
Roland Reply
thanks man
u r welcome
the velocity of a boat related to water is 3i+4j and that of water related to earth is i-3j. what is the velocity of the boat relative to earth.If unit vector i and j represent 1km/hour east and north respectively
Pallavi Reply
what is head to tail rule?
kinza Reply
Explain Head to tail rule?
what is the guess theorem
Monu Reply
viva question and answer on practical youngs modulus by streching
Akash Reply
send me vvi que
a car can cover a distance of 522km on 36 Liter's of petrol, how far can it travel on 14 liter of petrol.
yoo the ans is 193
whats a two dimensional force
Jimoh Reply
what are two dimensional force?
Where is Fourier Theorem?
Atul Reply
what is Boyle's law
Amoo Reply
Boyle's law state that the volume of a given mass of gas is inversely proportion to its pressure provided that temperature remains constant
how do I turn off push notifications on this crap app?
what is the meaning of in.
In means natural logarithm
is dea graph for cancer caliper experiment using glass block?
identity of vectors?
Choudhry Reply
Practice Key Terms 5

Get the best University physics vol... course in your pocket!

Source:  OpenStax, University physics volume 1. OpenStax CNX. Sep 19, 2016 Download for free at http://cnx.org/content/col12031/1.5
Google Play and the Google Play logo are trademarks of Google Inc.

Notification Switch

Would you like to follow the 'University physics volume 1' conversation and receive update notifications?