Newton's laws describe motion well only for objects moving much slower than light and larger than most molecules; for very fast or very small things, physicists use relativity and quantum mechanics. The kilogram is the unit of mass and the newton is the unit of force, with weight counted as a force, and in the United States the pound is also used as a unit of weight. The kilogram used to be a single lump of metal cast in the 19th century, but the SI was redefined so that it now rests on a fixed value of Planck's constant, which does not drift. Mass itself stays put as an object travels, since a thing keeps one mass whether it sits on Earth, circles in orbit or rests on the Moon.
Net force is all the forces on a thing added together, so two forces of equal size that point in opposite directions add up to nothing, and forces like that are called balanced. Now imagine a sled on a level field, free of friction and of air resistance: only gravity and the snow pushing upward act on it, they cancel, and the sled would keep going at constant velocity. An unbalanced force is what changes how a thing moves. Where none acts, nothing changes. That steadiness is inertia, the reluctance of a thing to have its motion changed. A book sliding on a table stops because friction acts on it, and with friction removed it would carry on in the same way.
Friction is the force that resists surfaces sliding or rubbing across one another, and the usual model says that it is proportional to the normal force pressing the surfaces together. It is written f = ΞΌN, where the coefficient ΞΌ differs for static and kinetic friction. HyperPhysics calls this model useful but by no means a precise description. A related fact is that a heavy crate demands more force to set going than to keep going, because resting friction is larger than sliding friction. You may have heard that a smoother surface always has less friction. Rougher surfaces usually do have more, but not always. If two flat metal pieces are made smoother and smoother in a vacuum with no contaminants, they can stick together in a cold weld.
Every push or pull comes with a size and a direction, and forces fall into two groups. A push or friction acts by touching, while gravity and magnets reach over a gap, and magnets can draw together or push apart with no contact at all. Every magnet has two poles, north and south, and like poles repel while unlike poles attract. The north end of a needle in a compass swings toward Earth's geographic North Pole; the magnetic pole in that region, however, is really a south magnetic pole. Gravity crosses the gap too. Near Earth it is directed at the middle of the planet, so a dropped ball falls down, and in general it grows with mass and fades as the distance between things grows.
Pushes and pulls change motion by speeding things up, slowing them or turning them, and a stronger push produces a larger change. An object that is sitting still remains so until some push or pull sets it going. Going around a corner, a car's passengers feel pushed outward, but in truth the car accelerates inward while their bodies tend to go straight, and a centripetal force, such as friction on the tires, is what turns the car. Centripetal means center-seeking, because anything moving in a circle needs a pull aimed at the center. Let the string on a spinning ball snap, and the ball leaves along a straight path, since nothing remains to turn it.
Air and water slow down whatever moves through them, and that force is called drag, while with almost no air to drag on them a hammer and a feather dropped together on the Moon fell at the same rate. Anything dropped gains speed as it falls, and the pull of gravity on it is its weight, but air friction differs from ordinary surface friction because it depends on velocity. If the weight of an object also acts, the object approaches a terminal velocity where weight and air resistance balance. In one textbook example, a 150-kilogram skydiver weighs more than a 100-kilogram one, so it takes more air resistance to match its weight. The heavier skydiver therefore gains a higher speed before settling at terminal velocity.
In its fuller form, Newton's second law says that force is the rate at which momentum, mass times velocity, changes, and F = ma covers the case in which mass stays fixed. In that form a larger force gives a bigger change in motion and a larger mass gives a smaller one, so under the same push a heavy thing changes its motion less than a light one. The DART spacecraft hit the asteroid Dimorphos at roughly 14,000 miles per hour, or 22,530 km/h. NASA's goal was to change its orbit by at least 73 seconds, and the measured change was 32 minutes. Debris blasted off the surface recoiled away and added to the push, much as a jet of air leaving a balloon sends the balloon the other way.
Words to know
- kilogram: unit of mass
- coefficient: ratio number
- centripetal: center-seeking
- recoiled: sprang back
- relativity: Einstein's physics
