The facts below come from well-established physics, with clear notes wherever a specific number needs independent verification before use in anything formal.
Falling Objects Ignore Mass
Drop a hammer and a feather in a vacuum, and they hit the ground at the same moment. This holds for any two objects, regardless of their weight, because gravitational acceleration depends on the gravitational field, not on the mass of the falling object. Apollo 15 astronaut David Scott demonstrated this on the Moon with a real hammer and feather, though the exact date of that footage is worth checking against NASA's own archive rather than secondhand summaries.
Our free-fall calculator runs this scenario directly, showing how velocity and distance change over time under constant gravitational acceleration.
Light Sets an Absolute Limit
The speed of light in a vacuum is fixed at 299,792,458 meters per second, a defined constant used to set the meter itself. Nothing with mass can reach it, and the value stays the same for every observer no matter how fast they are moving. This property is the foundation Einstein used to build special relativity, and everything about time dilation and mass-energy equivalence follows from it.
Clocks Run Slower at Speed
Because the speed of light never changes for any observer, time has to bend instead. A clock moving at high velocity ticks more slowly relative to a stationary one, an effect called time dilation. GPS satellites measurably experience this: their onboard clocks run faster than clocks on Earth due to weaker gravity at altitude, partly offset by a slowing effect from orbital speed. The correction applied is reported to be on the order of several dozen microseconds per day, though this figure should be confirmed against an official source such as the U.S. Naval Observatory before being quoted in technical writing.
Black Holes Trap Their Own Light
A black hole forms when enough mass compresses into a small enough volume that its escape velocity exceeds the speed of light. Since nothing travels faster than light, nothing crosses back out past the event horizon once it falls in, including light itself. Black holes cannot be observed directly; their presence is inferred from the effect they have on nearby stars, gas, and light.
Particles Occupy More Than One State
At the subatomic level, particles do not behave like small solid objects. Instead, they exist in superposition, occupying multiple states or positions simultaneously until measured. The double-slit experiment shows this directly: firing electrons or photons one at a time through two slits still produces an interference pattern, the same pattern expected if each particle passed through both slits at once.
A Few Constants Worth Knowing
A handful of fixed values recur across most of physics: the gravitational constant (G, approximately 6.674 × 10⁻¹¹ N·m²/kg², though the CODATA figure is periodically revised), Planck's constant (h, approximately 6.626 × 10⁻³⁴ J·s, the scale where quantum effects dominate), and the elementary charge (e, approximately 1.602 × 10⁻¹⁹ C). Working through these by hand invites small arithmetic errors, especially when unit systems get mixed. Calcmate's physics calculators (calcmate.org/physics) cover kinematics and thermodynamics using standardized values, so results can be checked rather than re-derived from scratch.
Sound Lags Far Behind Light
Lightning is seen before thunder is heard because light reaches the eye almost instantly, while sound is a mechanical wave that needs a medium like air, moving at roughly 343 meters per second at room temperature. That figure shifts with temperature and humidity, so treat it as an approximation.
Neutron Stars Pack Extreme Density
When a massive star collapses, what remains can be a neutron star, an object dense enough that a small volume of its material would weigh an extraordinary amount. Exact mass-per-volume figures vary by source and by the specific star in question, so any precise number should be checked against a current astrophysics reference. What holds a neutron star together is neutron degeneracy pressure, a quantum effect with no equivalent in everyday materials.
The Universe Keeps Expanding, Faster
Edwin Hubble's observations in the 1920s showed that distant galaxies recede from us, and the farther away a galaxy sits, the faster it moves. Later research indicated this expansion is accelerating, attributed to dark energy, a term for an effect that remains poorly understood. Percentage estimates for how much of the universe's energy content dark energy represents change as new data arrives, so current figures are worth checking against a recent primary source.
Everyday Physics in Four Examples
Physics is not confined to labs. A short list of it at work day to day:
- Friction keeps shoes gripping pavement and tires gripping the road.
- Inertia explains the forward lurch when a car brakes suddenly.
- Air pressure differences across a wing generate the lift that keeps a plane airborne.
- Thermal expansion is why bridges include expansion joints to prevent buckling as temperatures rise.
Testing these numbers directly, rather than taking them on faith, is where our physics calculators prove useful: entering real values shows how the equations behave.