Questions to Ask About Physics
For a student in a physics class, or a curious adult who has a physics teacher or a physicist to talk to and wants questions that get past the formula. The list runs from motion, forces and energy through light and electricity to the puzzles of daily life, like why the sky is blue, then out to space and time, down to atoms and the quantum world, and closes on experiments, measurement and what is still unsolved. The note under each question sketches the answer to expect and, where there is one, the popular wrong version, so you can tell an explanation from a recital.
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The questions
Each question, and why to ask it
Motion and Energy
Why does a moving object keep moving when nothing is pushing it?
Why ask it
Motion needs no cause; only a change in motion does, which is Newton's first law. A full answer blames friction and air resistance for the fact that everyday things slow down. If you hear that the object 'uses up its force', the oldest wrong idea in the subject is still standing.
If a heavy ball and a light ball are dropped together, why do they land at the same time?
Why ask it
Gravity pulls harder on the heavy ball, but that ball is also harder to get moving, and the two effects cancel exactly. Ask what changes for a feather and a hammer: on Earth the air decides, and on the Moon, where there is none, they hit the ground together.
What is the difference between mass and weight?
Why ask it
Mass is how hard something is to speed up or slow down and is the same anywhere; weight is the pull of gravity on that mass and changes from planet to planet. A sharp follow-up is what a bathroom scale reads in an elevator as it starts to rise, because the answer shows the scale measures a push, not a mass.
What is energy, and why can it never be created or destroyed?
Why ask it
Energy is not a substance but a number that can be added up across motion, heat, light and stored forms, and it comes out the same before and after every change. The deepest reason physicists give is that the laws themselves do not change from one day to the next. Test the answer with a bouncing ball that finally stops: 'its energy ran out' is the reply to push back on, and 'it warmed the ball and the floor' is the one you want.
Why doesn't the Moon fall down to Earth?
Why ask it
It is falling, constantly, while moving sideways so fast that it keeps missing. The same reasoning explains why astronauts float in orbit, so be wary of any answer that says there is no gravity up there: at the height of the space station it is not much weaker than on the ground.
How do speed, velocity and acceleration differ?
Why ask it
Speed is how fast, velocity is how fast and in which direction, and acceleration is how quickly the velocity is changing. The classic check is a ball thrown straight up: at the very top its velocity is zero and its acceleration is not, which is why it comes back down. An answer that treats acceleration as another word for going fast has kept the everyday meaning and dropped the physics.
Why can I feel a car speed up or turn, but not feel it cruise at a steady speed?
Why ask it
Forces show up only when velocity changes, and velocity includes direction, which is why a bend taken at constant speed still pushes you sideways. The deeper point, going back to Galileo, is that no experiment inside a smoothly moving cabin can tell you it is moving. Einstein built relativity on that sentence.
If every push comes with an equal push back, how does anything ever get moving?
Why ask it
The two forces in Newton's third law act on two different objects, so they never cancel each other. Try the horse and cart: the cart pulls back on the horse exactly as hard as the horse pulls it, and what moves them both is the ground pushing forward on the horse's hooves. 'The horse just pulls harder' misses it.
How does a rocket speed up in space with nothing to push against?
Why ask it
It pushes against its own exhaust: gas thrown backward carries momentum one way, so the rocket gains the same amount the other way. An answer that has the rocket pushing on air or on the launch pad is a mistake with a long history, and a good moment to ask what momentum is and why the total never changes.
What is the difference between heat and temperature?
Why ask it
Temperature measures how fast the particles in something are jiggling on average, and heat is energy on the move from a hotter thing to a colder one. A good check is a single spark beside a warm bath: the spark is far hotter, yet the bath holds vastly more energy and would warm a cold room much more.
Is there a coldest possible temperature?
Why ask it
Yes: absolute zero, about minus 273 degrees Celsius or minus 460 Fahrenheit, where the particles in a substance have as little motion as nature allows. The point to listen for is that cold is not a thing in itself, only less jiggling, which is why there is a floor and no matching ceiling. Ask how close a laboratory has come, and why the floor can be approached and never reached.
Why does heat flow from hot to cold and never the other way by itself?
Why ask it
This is the second law of thermodynamics, and the best answers talk about odds, not rules: there are overwhelmingly more ways for energy to be spread out than bunched up, so spreading is what you see. Follow with the refrigerator, which does move heat from cold to hot, and ask what it has to pay to do so.
Why can no machine run forever or give back all the energy put into it?
Why ask it
Some energy always leaks away as low-grade heat, through friction, electrical resistance or sound, and that share can be shrunk but not removed. Keep the answer for later use: any gadget sold as a source of free energy is claiming to beat this rule, which is reason enough to ask how it was tested.
What are the fundamental forces, and which of them do I notice in daily life?
Why ask it
Four are known: gravity, electromagnetism, and the strong and weak forces that work inside the nucleus. The surprise in a good answer is that nearly everything you feel apart from your own weight, including friction, the floor holding you up and the snap of a rubber band, is electromagnetism between atoms.
Light and Electricity
What is light, and what makes one color different from another?
Why ask it
Light is a traveling ripple in electric and magnetic fields, and color is its wavelength: red is longer, violet shorter. The fuller answer puts radio, microwaves, infrared, ultraviolet and X-rays on the same line, with visible light as a narrow strip in the middle. If those come out as separate things, ask how they differ.
What actually travels when a wave passes, and why can light cross empty space when sound cannot?
Why ask it
A wave carries energy and a pattern while the stuff it moves through stays roughly where it was, as a stadium crowd doing the wave shows. Sound is a vibration of air, water or solid, so with no matter there is nothing to vibrate. Light needs no material at all, a fact that puzzled physicists for most of the nineteenth century.
Why does light bend when it passes from air into water or glass?
Why ask it
Light travels more slowly in water and glass, and a beam arriving at an angle swings around as one side of it slows before the other. 'Because water is denser' leaves out the speed, which is the whole mechanism. Ask next what the bending accounts for: the straw that looks broken in a glass, how a lens focuses, and why a pool looks shallower than it is.
How fast is light, and how was its speed first measured?
Why ask it
About 300,000 kilometers a second, or 186,000 miles, which is seven and a half times around the Earth in one second. The first real estimate came in the 1670s, when Ole Rømer noticed that eclipses of Jupiter's moon Io ran late when Jupiter was far from Earth and read the delay as travel time. A complete answer ends in the present, where the speed is no longer measured at all: it is fixed by definition, and the meter is derived from it.
How are electricity and magnetism connected?
Why ask it
A moving charge makes a magnetic field, and a changing magnetic field pushes charges around a wire. That pair of facts is why a motor and a generator are the same machine run in opposite directions. The strongest answers finish with Maxwell's discovery that the two together produce a wave moving at the speed of light.
What do voltage and current each measure?
Why ask it
Current is how much charge passes a point each second, and voltage is how hard each bit of charge is being pushed along. Ask why a static shock from a door handle, at thousands of volts, only stings, while a wall socket at a far lower voltage can kill: the answer has to bring in how much charge flows and for how long. If the two words are used as though they meant the same thing, ask for a battery and a bulb as the example.
What is moving inside a wire when a current flows, and how fast does it go?
Why ask it
Electrons, and they drift far slower than walking pace. The lamp still lights at once because the electric field that drives them is set up along the whole wire at close to the speed of light. Electrons racing from the switch to the bulb is the common wrong picture, and a pipe already full of water is the usual repair.
Is a field a real thing, or only a tool for doing the math?
Why ask it
Physicists treat fields as real because they carry energy and momentum across empty space: sunlight warming your face made the trip as a disturbance in the electromagnetic field. A person who says it is only bookkeeping should be asked where that energy was during the eight minutes in transit.
Why are a few materials magnetic when most are not?
Why ask it
Every electron acts as a tiny magnet, and in most materials they point every which way and cancel. In iron and a few others, neighbors line up across whole regions called domains. Honest replies concede that why iron does this is a quantum question, and a neat test is asking why heating or hammering a magnet weakens it.
Everyday
Why is the sky blue, and why does it turn red at sunset?
Why ask it
Air molecules scatter short, blue wavelengths of sunlight much more than long, red ones, so blue reaches your eye from every direction. At sunset the light crosses far more air, most of the blue is scattered out of the beam, and what is left is red. The sky reflecting the sea is the answer to correct.
How does a rainbow form, and why is it always an arc?
Why ask it
Each raindrop bends sunlight on the way in, reflects it off its back wall and bends it again on the way out, fanning the colors apart. The arc appears because only drops at one fixed angle from the point opposite the Sun send a given color to your eye. A nice consequence to ask about: nobody else sees quite the same rainbow you do.
Why does a steel ship float when a steel bolt sinks?
Why ask it
Water pushes up on anything with a force equal to the weight of the water it shoves aside. A hull is mostly air, so it displaces its own weight of water long before it is fully under, and a solid bolt never can. 'Because it is hollow' is half an answer; ask for the displaced water.
How does an airplane wing produce lift?
Why ask it
The wing turns the passing air downward, and the air pushes the wing up in return; lower pressure above the wing is the same fact seen another way. Watch for the story that air over the top must speed up to rejoin the air beneath. It is widely taught and not true, since the two streams do not meet up again.
Why does a moving bicycle stay upright when a parked one topples?
Why ask it
A rider, or the bike's own geometry, steers the front wheel into each small lean, which brings the wheels back under the weight. The spinning wheels help a little, but saying it is all gyroscopes oversells them. This is a case where a careful physicist will tell you the full account is harder than it looks.
Why does metal feel colder than wood in the same room?
Why ask it
Both are at room temperature. Metal carries heat away from your hand much faster, and skin senses how quickly it is losing heat, not the temperature of what it touches. The sauna settles it: the wooden bench is bearable there, and a metal one at the same temperature would not be.
How does a microwave oven heat food?
Why ask it
Microwaves make the water molecules in food twist back and forth billions of times a second, and that agitation becomes heat. Two claims to check against the answer: the waves reach only a couple of centimeters in, so food does not cook from the inside out, and a dry ceramic plate warms mainly from the food sitting on it.
Why does a siren drop in pitch as it passes?
Why ask it
Ahead of a moving source the sound waves are bunched together and behind it they are stretched out, so you hear a higher note coming and a lower one going. Keep the answer handy, because a close cousin of this effect in the light of distant galaxies is how astronomers measure their motion away from us.
Why does a mirror swap left and right but not up and down?
Why ask it
It does neither. A mirror reverses front and back, along the line straight into the glass, and we read that as a left-right swap because we imagine turning around to face ourselves. Hold up a page of writing to test the explanation: you turned the page to face the mirror, and that turn did the swapping.
What is lightning, and why does the thunder arrive late?
Why ask it
Charge builds up inside a storm cloud until the air breaks down and a giant spark jumps, heating its channel so fast that the air expands with a bang. The flash reaches you almost instantly and the sound takes about three seconds per kilometer, or five per mile, so counting the gap tells you how far off the strike was.
Why can I see my reflection in a window at night but not during the day?
Why ask it
Glass reflects a few percent of the light that hits it at any hour. By day that faint reflection is swamped by the light pouring in from outside, and at night, with a lit room and a dark street, it is the brightest thing in view. One-way mirrors run on the same imbalance, which makes a good follow-up.
Space and Time
Why can nothing travel faster than light?
Why ask it
The faster an object with mass goes, the more of each extra push goes into its energy and the less into its speed, so reaching light speed would take an unlimited amount. Stronger answers add that the limit is really on how fast any cause can produce an effect, and light simply travels at it.
Does time really pass at different rates for different people?
Why ask it
Yes, and it has been measured: a clock moving fast, or sitting deeper in a gravity field, ticks slower than one that is not. Satellite navigation is the everyday evidence, since the clocks in orbit have to be corrected for both effects or positions would drift by kilometers within a day. Be skeptical of any reply that calls it an illusion.
Is gravity a force, or a bending of space and time?
Why ask it
In Einstein's account a falling object is not pushed at all: it follows the straightest path available through a spacetime that mass has curved, which is why you feel weightless in free fall. Ask what this explains that a simple pull does not, and expect starlight bending past the Sun, clocks running slower at sea level than on a mountain, and gravitational waves.
What does E = mc² mean in plain words?
Why ask it
That mass is a very concentrated form of energy, with the speed of light squared as the exchange rate. People often tie it only to nuclear weapons, so listen for the wider point: any object that gains energy gains mass, and a hot kettle is heavier than a cold one by an amount far too small to weigh.
What makes the Sun shine, and how long can it keep it up?
Why ask it
Hydrogen nuclei in the core fuse into helium, and the helium has slightly less mass than what went in, with the difference leaving as energy. Estimates put the remaining supply at several billion years. A good extra: ask why nineteenth-century physicists, who knew only burning and gravity as energy sources, got the Sun's age badly wrong.
What is a black hole, and what happens at its edge?
Why ask it
It is a region where gravity is strong enough that nothing inside, light included, can get back out, and the edge, the event horizon, is a boundary and not a surface. Two things mark a careful answer: black holes do not suck in distant objects any more than a star of the same mass would, and nobody knows what happens at the center.
How do we know the universe is expanding, and what is it expanding into?
Why ask it
The light from distant galaxies is stretched toward the red, and the farther the galaxy, the greater the stretch. The second half is where answers go wrong: the galaxies are not flying outward into a void, the space between them is growing, and that needs no edge and no outside. Ask where the rising raisin loaf picture stops working.
What was the Big Bang, and what came before it?
Why ask it
The name covers the hot, dense early state from which the universe has been expanding and cooling ever since, not an explosion at one spot. Evidence to listen for includes the faint microwave glow left over from that era and the measured amounts of hydrogen and helium. As for before, a straight reply is that nobody knows, or that the question may have no meaning.
What are dark matter and dark energy, and how sure are we that they exist?
Why ask it
They are names for two gaps. Galaxies spin and bend light as if they held far more mass than we can see, and the expansion of the universe is speeding up for a reason nobody has identified. Ask about the two halves separately, how good is the evidence that something is there and how much is known about what it is, because the answers are 'strong' and 'almost nothing'.
Is time travel possible?
Why ask it
Into the future, yes, in a limited way: anyone who travels fast enough comes back having aged less than those who stayed. Into the past is a different matter. The equations do not flatly forbid it, but every scheme needs ingredients nobody has found, so find out which direction the person means before you take a yes or a no.
If the laws of physics work the same forward and backward, why does time seem to run one way?
Why ask it
Almost every basic law would run just as well in reverse, yet eggs break and never unbreak. The usual explanation is that disorder increases, and it increases because the universe began in an extremely orderly state. Why it began that way is the open part, so expect this explanation to end in a question.
Quantum
What is an atom made of, and is it really mostly empty space?
Why ask it
A tiny, dense nucleus of protons and neutrons with electrons around it. 'Mostly empty' is the popular line, but the electrons are not specks circling in a void: each is smeared out as a cloud that fills the atom. The follow-up that sorts answers is why your hand does not pass through a table.
Is light a wave or a particle?
Why ask it
Neither word fits alone. Light spreads and interferes like a wave, yet it always arrives in whole lumps called photons, and electrons behave the same way. Ask for the two-slit experiment done one photon at a time. Be cautious of 'it switches between the two', which makes it sound like a choice.
What is the uncertainty principle, and is it only a limit on our instruments?
Why ask it
The more sharply a particle's position is defined, the less sharply its momentum is, and the reverse. Better instruments would not help: in the standard account the particle does not have both values at once to be found. 'Measuring it disturbs it' is the version to question, because it is the one most often repeated.
What was Schrödinger's cat meant to show?
Why ask it
Schrödinger offered it as an objection, not a prediction: if an atom can be in two states at once, tying a cat's fate to that atom seems to put the cat in two states as well, which he thought absurd. Where and why the fuzziness ends is called the measurement problem, and physicists still disagree about it.
What is quantum entanglement, and can it send a signal faster than light?
Why ask it
Two particles can share one quantum state, so that measurements on them match up more strongly than any pre-agreed plan could arrange, however far apart they are. No message can ride on it, because each person alone sees only random results. Treat talk of particles 'communicating instantly' as a red flag.
Why does each element glow with its own set of colors?
Why ask it
Electrons in an atom can hold only certain energies, and a jump between two of them emits light of one exact color. Every element has its own ladder of levels, so its pattern of lines works like a fingerprint. This is the answer to a question worth asking next: how anyone knows what a star is made of.
What is radioactivity, and what does half-life mean?
Why ask it
Some nuclei are unstable and shed energy by throwing out particles or radiation. The half-life is the time in which half of a large sample decays, and no one can say when any single atom will go. A quick check of understanding: after two half-lives a quarter is left, not none.
How is nuclear fusion different from fission, and which one powers what?
Why ask it
Fission splits a very heavy nucleus such as uranium into lighter pieces, and fusion joins very light ones such as hydrogen. Both release energy because the pieces end up more tightly bound than they started. Nuclear power stations run on fission and stars run on fusion, so the follow-up is why fusion is so much harder on Earth: the nuclei repel one another and have to be made extremely hot and held together long enough to collide.
What are the smallest things we know of, and could there be anything smaller?
Why ask it
Today's list has quarks, which make up protons and neutrons, the electron and its relatives, and the particles that carry the forces, with the Higgs alongside. None has shown any inner parts in experiments so far. The list leaves out gravity, though, which is why few physicists expect it to be the last word.
Is antimatter real, and where is it all?
Why ask it
It is real and routinely made: for each kind of matter particle there is a twin with the same mass and the opposite charge, and the two vanish into energy when they meet. Hospital PET scanners depend on positrons, the electron's antiparticle. Why the universe ended up built from matter with almost no antimatter is one of the open problems, so 'we do not know' belongs in the answer.
Where does quantum physics show up in things I own?
Why ask it
In nearly every piece of electronics: the transistors in a phone, the laser in a barcode scanner and the LEDs in a lamp were all designed with quantum theory. It is a useful corrective to the idea that the subject is only strange thought experiments. It also helps to hear that 'quantum' on a wellness product or a detergent means nothing.
What is a quantum computer, and what would it be good for?
Why ask it
Its bits can be in combinations of 0 and 1 and can be entangled with one another, which suits certain problems, such as simulating molecules or factoring very large numbers. 'It tries every answer at once' is the sign of a borrowed explanation, and so is any promise that it will speed up everything.
How We Know
Which experiment convinced physicists of this, and could it be repeated in a school lab?
Why ask it
Physics is unusual in how many of its founding experiments fit on a bench: Galileo's ramps, Young's two slits, Ørsted's compass beside a wire. Asking for the experiment after any claim on this page gets you apparatus and a result in place of a statement. Better still, ask what the doubters at the time said was wrong with it.
Why does physics lean so heavily on math, and how far can I get without it?
Why ask it
Math lets a claim be exact enough to be checked against a measurement and found wrong, which words alone rarely allow. The ideas travel a long way in plain language, though. Ask for one equation read aloud as a sentence, such as force equals mass times acceleration, and judge the answer by whether it then makes sense.
Why do physics problems ignore friction and air resistance, and when does that stop being fair?
Why ask it
Stripping a situation down to a point mass on a frictionless slope shows the main effect cleanly, and the leftovers are added back one at a time. The skill worth asking about is knowing when a leftover matters: air resistance is nothing for a dropped coin and everything for a parachute.
Why does every measurement come with an uncertainty?
Why ask it
No instrument reads perfectly, and a number without its margin cannot be compared with a prediction or with someone else's result. Listen for two different kinds: scatter that averages out over many readings, and a bias, like a miscalibrated ruler, that repeating will never reveal.
How can physicists be sure about things nobody can see, such as quarks or the center of the Sun?
Why ask it
Confidence comes from several independent methods agreeing. The Sun's core is known through the particles called neutrinos that stream out of it and through vibrations of its surface, and quarks through how other particles bounce off protons. An appeal to authority in place of evidence like this is the weak answer.
When a new theory takes over, was the old one simply wrong?
Why ask it
Rarely. Newton's laws are what Einstein's reduce to at ordinary speeds and in ordinary gravity, and engineers still build bridges and steer spacecraft with them. A new theory has to reproduce every success of the one it replaces, which is a higher bar than most people expect.
What would it take to overturn an accepted theory?
Why ask it
One solid result that other groups can reproduce. A fair answer also explains why the first reaction to such a result is to hunt for a mistake: in 2011 an experiment appeared to clock neutrinos beating light, and the cause turned out to be a loose cable. That caution is the system working, not closing ranks.
Which picture in the textbook do physicists themselves no longer believe?
Why ask it
Electrons circling the nucleus like planets is the usual first example, with the stretched rubber sheet for gravity close behind. Each survives because it is easy to draw and partly right. Ask what the picture gets right, where it misleads, and what a physicist imagines in its place.
What physics can I measure myself with a phone, a ruler and a stopwatch?
Why ask it
Plenty: the strength of gravity from the swing of a pendulum, the speed of sound from an echo off a distant wall, your own acceleration in an elevator from a phone's motion sensors. The numbers will be a little off, and working out why they are off teaches more than getting them right.
Are string theory and the multiverse science if they cannot yet be tested?
Why ask it
String theory is a candidate for joining gravity with quantum theory, and a multiverse is something certain theories imply; neither has yet made a prediction that an experiment has confirmed. Physicists disagree, sometimes heatedly, about what that makes them, so expect an opinion and ask for the opposing one as well.
What are the biggest unsolved problems in physics today?
Why ask it
The usual list has how gravity fits with quantum theory, what dark matter is, and why there is more matter than antimatter. Follow it by asking which of them could be settled by an experiment now running or being built, since that separates the problems with a path to an answer from the ones still waiting for an idea. It works well as the last question of a class or a talk.
Getting a real answer to a physics question
Practical guidance for the conversation itself
Before you ask
Say what you already think
Open with your own guess, even a wrong one: 'I assumed the heavier ball lands first.' It tells the person where to start, and a wrong guess corrected in front of you stays corrected longer than a fact handed over.
Ask for the picture first, the equation second
A teacher short of time will reach for the formula. Ask what is physically going on first, what pushes on what and where the energy goes, and then ask for the equation as a summary of that. If the order is reversed, the symbols have nothing to attach to.
Match the group to the moment
In class, the Motion and Energy and Light and Electricity questions fit the units most courses teach. With a physicist at a dinner table or after a public talk, Space and Time and Quantum get the liveliest answers. How We Know works anywhere, and is the group to use when an answer sounded too tidy.
Bring something you saw
A question tied to an observation gets a better answer than one read cold from a list. Mention the spoon that looked bent in the glass, the phone that warmed on its charger or the documentary that lost you halfway, and then ask the nearest question on the page.
Follow-ups that work on any answer
What would happen if...
Change one thing and ask again: what if there were no air, twice the mass, half the distance? Physics is a set of rules for answering exactly this, so a person who understands the idea can run the change in their head, and one who is reciting it cannot.
How big is that?
Ask for a rough number. How much slower does the fast clock run, how far does the electron drift in a second, how much heavier is the hot kettle? Scale is what separates an effect you could notice from one that needs an atomic clock, and popular accounts usually leave it out.
Where does that picture stop working?
Every analogy in physics is borrowed: water in pipes for current, a rubber sheet for gravity, a rising loaf for the expanding universe. Each one fails somewhere. Asking where is not rude; people who teach the subject tend to enjoy it, and the failure point often teaches more than the analogy did.
Say it back
Finish by putting the answer in your own words and asking if you have it right. It is the fastest way to find the step you skipped, and it gives the other person a second try at the part that did not land.
Four answers you can check at home
Drop a sheet of paper twice
Drop a flat sheet of paper beside a book and the book lands first. Crumple the sheet into a tight ball and they land together. The paper weighs what it did before, so the difference was the air, which is the answer to the falling-balls question shown in ten seconds.
Time a pendulum
Hang a weight on a string about a meter long, let it swing gently and time ten swings: close to twenty seconds. Double the weight and the time does not change; halve the string and it drops. A pendulum ignores mass for the same reason falling does, and for a long time timing one was the most accurate way to measure the strength of gravity.
Slide an arrow behind a glass of water
Draw a sideways arrow on a card and move it slowly away behind a full glass of water. At some distance the arrow flips to point the other way, because the curved glass of water is bending the light like a lens. Ask why it flips left to right and not top to bottom.
Touch the table and its leg
Rest one hand on a wooden tabletop and the other on a metal leg or a steel pan that has sat in the same room all day. They are at the same temperature and the metal still feels colder. Then ask the heat and temperature questions with your hands as the evidence.
When the answer does not help
A name is not an explanation
'That is refraction' or 'that is just inertia' labels the thing you asked about. Thank them for the word and ask what is happening, in terms of what moves and what pushes it. Many of the notes on this page point out the label answer for that reason.
When the right answer feels wrong
Physics is the subject where daily experience teaches the wrong rule: things seem to stop unless they are pushed, and heavy things seem to fall faster. If a correct answer still feels wrong, say so, and ask what in ordinary life hides the true behavior. It is nearly always friction or air.
'Nobody knows' is a real answer
Several questions here, on what came before the Big Bang, what dark matter is and where quantum fuzziness ends, have no settled answer. A person who says so plainly is being accurate. Be more careful with a confident, complete answer to one of those than with an admission.
These are not homework questions
Nothing on the list has a number to hand in. If you are stuck on an assigned problem, bring the work you have done and ask where it went wrong. If you want to understand why the method works, the questions here are the ones to ask afterward.
Quantum and energy as magic words
Outside physics, 'quantum' and 'energy' get attached to claims about healing, consciousness and luck. If someone leans on the physics to support one of those, ask which experiment shows it and what result would have counted against it. The How We Know group was written for that conversation.