1. They are acted on by an external force.
2. They become too cold.

Newton's First Law is summarized as the concept of inertia. In full, it says, "Objects in motion will stay in motion, and objects at rest will remain at rest, unless acted on by an external force." Common "external forces" are another moving object (a foot kicking a ball), air pressure (wind), or gravity.

1. air resistance
2. electrical resistance

Resistance is what is sounds like: a measure of the difficulty in passing an electrical current through a conductive, or not so conductive, substance. The opposite quality is conductivity, which is the ease with which current travels.

1. acceleration
2. heat

This is one of the simplest physics concepts to understand. People cock back their fist to punch because the mass of their fist doesn't change, so the way to pack a harder punch to to increase the speed at which it travels -- which requires starting from farther back.

1. true
2. false

Acceleration is rate of change in speed. Does that make "speed" the same thing as "velocity"? In everyday life, yes, but not in physics. In physics, "velocity" describes speed in a known direction. If direction is unknown, the object just has "speed. "

1. mass
2. shape

This is one of the first things students in astronomy or physics learn: that the planets revolve around the Sun because of its great mass. Here's the kicker: Physicists know this is a universal law, but still don't know why mass creates gravity.

1. volts
2. wavelengths

Volts are a measure of electric potential. Amperes are a unit of electrical current, and we didn't even get into joules or coulombs. Electricity is a complex subject, despite the fact that we use it daily. If you truly want to understand it, you'd be better off taking an electrical engineering course than a physics class.

1. convex
2. complex

You're probably used to hearing "concave" and "convex" when it comes to lenses, "planar" less often. A concave lens curved inward from the outer edges; a convex one outward. (Turn a spoon forward and backward to get an immediate visual on this).

1. buoyancy
2. resistance

Every physics student hears the story of Archimedes, who was able to determine if a crown was pure gold, or had lesser metals mixed in, by using buoyancy. Technically, Archimedes's revelation was more about displacement, but that's a concept closely related to buoyancy. Both gold and silver are too heavy to be held up by buoyancy, but the extent to which they displace water is different -- so Archimedes had a baseline for measuring how much water the crown should displace.

1. heat
2. electromagnetic fields

These structures are named for physicist Michael Faraday. Elevators work as Faraday cages (by accident, not by design), which explains why your cellphone often doesn't work in one.

1. amplitude
2. scale

When you're watching sound waves on a computer screen, (or, more likely, watched people watch sound waves on a screen in a movie with a lot of sciencey props) the amplitude is the top of the curve. Think of it like the vertex in a parabola.

1. Bohr
2. Doppler

A siren sounds higher in pitch if the emergency vehicle is approaching you, and get lower as it goes away. The same thing happens with light coming from moving sources, which moves up and down the color spectrum. This gives us the witty T-shirt with "The Doppler Effect" written in blue in front, and red in back.

1. four
2. six

The laws of thermodynamics are numbered 0 to 3, not 1 to 4. Why? Because law "zero" was added after the first three, but was so basic that the powers that be in physics decided it should come first.

1. centrilocking force
2. centripetal force

If you own a salad spinner, you're familiar with centrifugal force -- the spinning causes water to fly to the outside perimeter. Centripetal force is the opposite, causing matter to move toward the center of rotation.

1. astronomy
2. astrophysics

Don't be confused: "astronomy" is the broad study of the heavens. But when you're dealing specifically with the motion and forces of stars, planets and the like, that's astrophysics.

1. stasis
2. fission

Nuclear fission is essentially the splitting of the atom. Don't try this at home.

1. inches
2. meters

Also written as "9.8 meters per second squared," this is the rate of acceleration at which objects to fall to earth, until they are stopped by hitting another object (like the earth) or reach an absolute velocity, which will vary depending on the property of the object.

1. cold fusion
2. antimatter fission

There was a stir in the science world when two credible electrochemists claimed they'd achieved this. But the results couldn't be replicated, and the consensus of the scientific community is that it won't happen. Which is unfortunate, because the energy that cold fusion could provide the world would be incredible.

1. diffraction
2. refraction

You've seen visible evidence of refraction if you've seen a "rainbow maker," a crystal carved so that it creates a small rainbow when light hits it. This might be the only piece of physics equipment you can buy at your local New Age store.

1. quantum
2. vacuum

What we tend to think of as the absence of "anything" is actually air -- which is matter; it moves, and it exerts about 14 psi of pressure at sea level. It's space which is a true vacuum.

1. elasticity
2. integrity

This is another physics concept which is important to engineering. Not all concepts in physics have such real-life applications.

1. Boyle's
2. Cooper's

This is an easier concept to understand than it at first appears. If you have air in a closed container, and squeeze the container so that the interior space gets smaller, the pressure of the air has to rise proportionally. This is why, under too much pressure, the lid of the container might blow off.

1. a type of telescope
2. a space shuttle

Newton is also the namesake of an early Apple computer and a mathematics institute at Cambridge. But we're partial to the Newtonian telescope, a long-bodied but inexpensive kind that uses a mirror, not a lens, to enlarge images.

1. Fahrenheit
2. Kelvin

You might have learned in school that this is the lowest temperature matter can reach, but that's not entirely true. At absolute zero, matter still retains zero-point quantum-mechanical energy. Which we'd explain, if we understood. Which we don't.

1. British thermal unit
2. millijoule

The use of a Fahrenheit degree should have tipped you off, since the British, like the US, have stubbornly clung to the standard-measure system instead of centigrade. Perhaps that's why the BTU is rarely used compared to the joule.

1. spectral physics
2. optics

We tend to think of the prefix "opt-" as having to do with vision, thanks to "optometry" and "opticians." However, all vision is a function of light, and at its core, optics is the study of how light behaves, including its creation of color.

1. Isaac Newton, Michael Faraday, James Clerk Maxwell, Albert Einstein
2. Albert Einstein, Michael Faraday, Isaac Newton, James Clerk Maxwell

Probably the first and last physicists on this list are the most famous. But James Clerk Maxwell and Michael Faraday were huge in the field as well.

1. expansion of gases
2. the Krebs cycle

To an extent, every other science is dependent on the laws of physics. That's why it's sometimes called "the fundamental science." But the Krebs cycle, about how nitrogen goes from the earth's soil to the atmosphere and back again, is usually studied in biology classes.

1. Sure is!
2. You're joking, right?

Though it sounds very informal, "stiffness" is indeed a term from physics. It describes a material's ability to resist changing shape in response to an external force, which is important in engineering.

1. no
2. Depends what you mean by "visible"

Physicists do refer to "visible light," but they're referring to the fact that we can see the objects that light illuminates, not light itself. They use the term "visible light," though, because in a larger sense, light is electromagnetic radiation, and there are spectrums of it that are completely invisible to humans -- they don't affect what we can see. Clear now?

1. Entropy
2. Infinity

Newton's laws are also called the laws of motion. They explain energy, inertia and entropy.

1. red
2. violet

Red has the longest wavelength, and violet the shortest. The wavelengths in between, in order, are orange, yellow, green, blue and indigo.

1. true
2. false

Ferromagnetism is named for iron, of course. But other materials like nickel and cobalt are ferromagnetic. "Ferromagnetism" is simply the term for magnetism of metals, which is easily observed compared to other electromagnetic phenomena.

1. airplanes
2. cell phones

The Bernoulli principle has to do with how fast molecules of air travel over an airfoil -- a surface which is curved on top, in order to have greater area than it does on the bottom. The slower motion of air under the airfoil creates upward pressure. In this way, wings allow flight. It's really that simple!

1. a quarter pound (102 grams)
2. 2.2 pounds

A newton is 102 grams, or about one-fourth of a pound. Why is this amount special enough to be named after Sir Isaac Newton? It represents the force that would cause a 1 kilogram object to accelerate 1 meter per second, squared.

1. atom
2. liquid

It's common for us to say that matter can be a solid, liquid or a gas. However, plasma is the fourth state, which is very rare and usually has to be created under lab conditions.