A car that reaches 60 mph in five and a half seconds pulls about half a g. Not two. Not one. Half. That figure surprises almost everyone who has ever described a launch as brutal, and it is the least surprising thing in this article.
Three instruments sit in most homes: a stopwatch on a phone, a kitchen scale, and a thermometer. Each of them reports something true and lets you conclude something false. The stopwatch tells you a car is quick without telling you how quick, the scale tells you what a thing weighs without telling you whether it floats, and the thermometer gives you a number that means nothing at all until you know which of four scales it was written in. What follows is seven beliefs about those three readings, each taken apart with a number, and one that looks like trivia and turns out to be exactly right.
Myth one: a fast launch means big g-forces
Zero to sixty in 5.4 s works out at 4.97 m/s², which is 16.30 ft/s² or 0.51 g. Half of what gravity does to you standing still. To feel a full g pushing you back into the seat, the same run has to finish in 2.7 s, which is supercar territory and puts the figure at 9.93 m/s².
The useful way to hold this in your head is not in g at all. It is 21.9 mph gained every second, which is what one g buys you. A hot hatch gains 11.1 mph per second. A freight train pulling out of a yard gains under two. Your elevator, on the way up, manages about 2.7 mph per second, which is 0.12 g, and that small number is enough to make a bathroom scale in the car read 12 percent heavy for a second and a half.
Myth two: twice the speed, twice the stopping distance
This one is not a rounding error. It is wrong by a factor that grows the faster you go, and it is wrong in the direction that hurts.
Braking hard on dry asphalt takes about 9 m/s² off your speed every second, which is 0.92 g and roughly what good tires give before the wheels lock. At that rate, here is what the road actually eats. The figures are pure braking, with no reaction time in them at all.
| Speed | Time to stop | Braking distance | Against 30 mph |
|---|---|---|---|
| 20 mph | 0.99 s | 14.6 ft | under half |
| 30 mph | 1.49 s | 32.8 ft | the baseline |
| 45 mph | 2.24 s | 73.8 ft | 2.25 times |
| 60 mph | 2.98 s | 131.1 ft | four times, not two |
| 75 mph | 3.73 s | 204.9 ft | 6.25 times |
| 90 mph | 4.47 s | 295.0 ft | nine times |
| 120 mph | 5.96 s | 524.5 ft | sixteen times |
The time column doubles neatly. The distance column does not, because distance goes with the square of the speed. Double the speed and you quadruple the road. Quadruple the speed, from 30 to 120, and you need sixteen times as much of it: 524.5 ft against 32.8 ft, which is the difference between stopping inside a city block and not stopping inside two.
And none of that includes thinking. At 60 mph you cover 88 ft in a single second of reaction, which is two thirds of the braking distance again, added before the pedal moves.
Myth three: heavy things sink
A 200 lb oak log floats. A 10 g steel nail sinks. The scale has nothing to say about it, because floating is decided by a ratio, not a weight: mass divided by the room that mass takes up.
Run the log through and it comes to 4.45 cubic feet, which is 126 liters of oak at 720 kg/m³. That is below water's 1,000, so it floats, and the share that sits below the waterline is simply the ratio: 72 percent wet, 28 percent dry. The nail is iron at 7,874 kg/m³, 7.9 times heavier than water, and it goes straight down whatever it weighs.
| Material | kg/m³ | lb/ft³ | In fresh water |
|---|---|---|---|
| Styrofoam | 25 | 1.56 | floats, 2.5 percent under |
| Cork | 180 | 11.24 | floats, 18.0 percent under |
| Pine | 530 | 33.09 | floats, 53.0 percent under |
| Oak | 720 | 44.95 | floats, 72.0 percent under |
| Ice | 917 | 57.25 | floats, 91.7 percent under |
| Concrete | 2,400 | 149.83 | sinks, 2.4 times heavier |
| Steel | 7,850 | 490.06 | sinks, 7.9 times heavier |
| Lead | 11,340 | 707.93 | sinks, 11.3 times heavier |
Myth four: ice floats because it is much lighter than water
Ice is lighter than water by 8.3 percent. That is the whole margin, and it is why 91.7 percent of every floating cube sits below the surface with only a sliver showing. In seawater at 1,025 kg/m³ the submerged share drops to 89.5 percent, which is the difference between a cube in a glass and an iceberg with a tenth of itself in the air.
Olive oil is the quiet case in that table. At 920 kg/m³ it floats on water with 92 percent of itself under, which is why a spill spreads into a film instead of sinking, and why the oil sits on top of the vinegar however hard you shake the jar.
Myth five: a pound is a pound, so a pound takes up a pound's worth of space
A pound of styrofoam and a pound of lead weigh the same. Of course they do. What nobody pictures correctly is the size of them.
One pound of lead is 2.44 cubic inches, about 40 milliliters, a lump smaller than a golf ball. One pound of styrofoam is 1,107 cubic inches, which is 18.1 liters, a box you would carry with two hands. Same reading on the scale, 454 times the volume.
This is the gap that makes shipping weird: a truck fills up on space long before it fills up on weight for one cargo, and the other way round for another.
Myth six: you can spot a fake gold bar by weighing it
One cubic inch of gold weighs 11.16 oz. One cubic inch of tungsten weighs 11.13 oz. The densities are 19,300 and 19,250 kg/m³, a gap of 0.26 percent, which on a 316 g piece comes to under a gram.
A kitchen scale cannot see that, and neither can most jewelers' scales once the piece is plated and the edges are right. Density narrows the field, and on this particular pair it narrows it to two. That is why a serious test uses something else: conductivity, ultrasound, or a hole.
Myth seven: water boils at 212 degrees
At sea level, yes. A mile up it does not. Boiling point falls by roughly 1 °F for every 500 ft of elevation, so in Denver water boils near 202 °F and pasta takes longer for the simple reason that the water is cooler, not because the stove is weaker.
The other place the thermometer misleads is the oven. European recipes are written in Celsius and US dials are marked in Fahrenheit, and the four common settings do not land on round numbers going across.
| Recipe says | Exactly | US dial |
|---|---|---|
| 160 °C | 320 °F | 320, no rounding needed |
| 180 °C | 356 °F | 350, six degrees cool |
| 200 °C | 392 °F | 400, eight degrees hot |
| 220 °C | 428 °F | 425, three degrees cool |
None of those gaps matters for a casserole. All of them matter for bread and for anything with sugar in it, where a difference of ten degrees changes the color of the crust.
The one that sounds like a coincidence and is not
Minus forty is the same number in Fahrenheit and Celsius. People trot this out as a bar fact, the way they trot out that a duck's quack does not echo, and unlike that one it is exactly true and provable in a line.
The two scales are straight lines against each other: one gains 1.8 degrees for every degree of the other and starts 32 higher. Two straight lines with different slopes cross exactly once, never twice and never zero times, and solving for where gives -40. It is not a quirk of the numbers. It is the only answer the arithmetic allows, and it is the single temperature on Earth for which a converter is useless.
Kelvin and Rankine never cross anything, because both start at absolute zero: -459.67 °F, -273.15 °C, 0 K. A difference of 1 K is a difference of 1 °C, and a difference of 1 °R is a difference of 1 °F, which is why heat capacity tables quote joules per kilogram per kelvin and nobody ever converts the kelvin.
What the three readings have in common
Every one of these myths comes from the same habit: taking a number the instrument gives and skipping the ratio that turns it into an answer. The stopwatch gives seconds, and it is the speed change divided by those seconds that tells you anything. The scale gives pounds, and it is pounds divided by volume that decides whether the thing floats. The thermometer gives a reading, and it is the pair of anchors behind it that tells you what the reading means.
Two of those ratios are one line of arithmetic. The third has four scales in it and that is why it goes wrong most often.
Tools discussed in this article
Acceleration Formula Calculator gives any one of acceleration, final speed, time or distance from the other three, in mph, km/h, m/s or ft/s, and reports the rate in m/s², ft/s² and g at once. The braking table above came straight out of it.
Density Calculator takes a weight and a volume, in grams, ounces, pounds, cubic inches, cubic feet or gallons, and hands back the density, the specific gravity, the closest of 27 materials and a float or sink verdict with the submerged share.
Temperature Converter turns one reading into Fahrenheit, Celsius, Kelvin and Rankine together, shows the formula with your own number in it, and marks the nearest fixed point from absolute zero to the surface of the sun.
Ready-made numbers from this article
Several of the figures above have their own page, filled in and ready to run:
- What is 350 degrees Fahrenheit in Celsius and 400 degrees Fahrenheit, the two most used oven settings
- What is 180 degrees Celsius in Fahrenheit, the setting most recipes outside the US are written for
- What is 37 degrees Celsius in Fahrenheit, and 100 degrees Fahrenheit for the fever question
- What is 72 degrees Fahrenheit in Celsius, the thermostat setting
- How much do 5 gallons of water weigh, 15 gallons and a 55 gallon drum
More physics tools
The rest of the physics shelf, in the order it was built:
- Momentum Calculator and Force Calculator, the two that turn a mass and a speed change into something you can feel
- Kinetic Energy Calculator and Potential Energy Calculator, where the square of the speed shows up again
- Work Calculator and Mechanical Power Calculator, for the job done and how fast it is done
- Torque Calculator and Kinetic Friction Calculator, the grip behind every braking figure on this page
- Pendulum Period Calculator, Hooke's Law Calculator and Simple Harmonic Motion Calculator for things that swing and bounce
- Buoyancy Calculator, which takes the float verdict further into how much a thing can carry
- Thermal Expansion Calculator, Specific Heat Calculator and Thermal Conductivity Calculator for what heat does to a material once the thermometer has told you how much of it there is