Friction is among the most common forces, yet as of November 2026 a college physics text says its behavior is very complicated and still not completely understood, and HyperPhysics notes that almost every simple statement about friction can be countered by examples. Friction resists the sliding or rubbing of surfaces, and a rough surface makes more of it than a smooth one. Roughness is only part of the cause. Attraction between molecules in the two bodies accounts for much of friction, so even surfaces that are perfectly smooth still have it.
Textbooks usually describe friction with a formula that ties it in proportion to the force pressing two surfaces together, f = ΞΌN, with separate coefficients for static and kinetic friction, but HyperPhysics calls that model useful rather than precise. Everyday sliding shows the difference between the two: a crate that is heavy needs more force to start than to keep sliding, since friction on something at rest is bigger than on something moving. The rule that rougher surfaces stick more also fails at the extreme. Polish flat metal pieces ever smoother in a vacuum free of contaminants, and they can fasten to each other in a cold weld.
Newton's first law carries a deep idea. A thing at rest stays at rest until a push or a pull moves it, and a moving thing continues unchanged until some force acts to change that motion, so uniform motion in a straight line must be just as natural as being at rest. Nothing has to keep pushing a moving thing along. A book gliding over a table halts through friction, and with friction absent it would continue with unchanging speed and heading. Whether something is at rest depends on who is looking, because an object at rest for one observer moves in a straight line for another, and so no constant-velocity frame of reference is special. For that reason the law is often considered a statement about reference frames. It tells us where Newton's laws work, namely in frames that are not accelerating, which is also the condition for the second law, of which the first is a special case with zero net external force.
Near Earth, gravity is directed at the planet's middle, so a dropped ball falls, and the pull comes from all of Earth's mass, since gravity is stronger between things with more mass and weaker when they are farther apart. Pushes and friction need touching, while gravity and magnets operate over a gap, and Newton's theory said nothing about how a pull could cross that gap. Through much of the 1700s it stayed disputed, mostly because it gave no contact mechanism, and Leibniz and Huygens objected to an action at a distance. Weight depends on location, being a force, whereas mass has no such dependence: a thing's mass is unchanged between Earth, orbit and the Moon, though the Moon's gravity has roughly one-sixth the strength of Earth's. Weight is gravity's pull on an object, found near Earth by multiplying mass by g, which has the value 9.8 N/kg. Like every push or pull, gravity is described by both its size and its direction.
Newton noticed a link between a falling object and the Moon. The Moon's inward acceleration in its orbit is about 3,600 times smaller than g at Earth's surface, and the Moon lies roughly 60 Earth radii away, measured from Earth's center, so that, since 60 times 60 is 3,600, a single inverse-square force fits both. A source of strength spreads across a spherical surface, an area that grows with the square of the distance, and doubling the distance therefore leaves one quarter of the pull. The Principia went on to conclude that the force holding the planets in their orbits is the same kind of force as gravity on Earth, and with Kepler's laws it explained why planets move in ellipses rather than circles. That ended the old view, dating back at least to Aristotle, of a separate science for the heavens.
In 1916 Einstein's general relativity described gravity as mass warping space and time, and its predictions have been confirmed over the decades since, including the 2015 measurement of gravitational waves from two merging black holes. In its general form the second law says that momentum, meaning mass times velocity, changing over time gives the force, and F = ma covers only the case where mass stays unchanged. In that form the law is limited to net external force alone, and it does not apply directly when mass is changing, as for a burning rocket. Newton's laws still describe motion well, but only for objects that are neither near the speed of light nor as tiny as most molecules; beyond that range the work passes to relativity and quantum mechanics. The success of the Principia made it a main goal of physics to find the fundamental forces of nature and write them as laws, and philosophers have also asked what needing to replace Newton's gravity with Einstein's says about the limits of scientific knowledge.
The astronomer Fritz Zwicky noticed in 1933 that galaxies of the Coma Cluster were moving faster than the gravity of visible matter could hold, and his suggestion was that unseen matter supplied additional gravity that kept them bound. Decades later, in the 1970s, Vera Rubin saw that stars at the outer edges of spiral galaxies move as if held by more gravity than visible matter supplies, and she concluded that these galaxies must be held together by dark matter. NASA says her discovery was such strong evidence that scientists embraced the idea.
As of November 2026, NASA says scientists estimate that ordinary matter is only about 5% of the universe and dark matter about 27%, and the rest is thought to be dark energy. Dark matter interacts with ordinary matter through gravity, but it does not seem to interact with light. Many candidates have been proposed, such as WIMPs and axions, yet as of November 2026 NASA says searchers have not found evidence that would confirm any one of them. What dark matter is remains an open question.
Words to know
- observer: one who measures
- relativity: Einstein's gravity theory
- galaxies: star systems
- axions: proposed particles
