Physics: The Story of How Things Work

Physics begins with a promise: that ordinary things can be put in order and tested again. Dew on grass, echoes in hallways, thermometers and motors, ships that float and balloons that rise, magnets that pull nails and lightning that crackles across the sky. This is not just curious stuff on the side of life—it is the very stuff science is made of. Ordinary knowledge is a mix of experience and rumor; scientific knowledge arranges it so that effects are tied to causes and ideas are linked to measurements. In this work, we use some simple words: a hypothesis is a suggested explanation, a theory is a well-tested framework that ties many findings together, and a law is a rule that always holds, often so clear that it can be calculated. To speak together, we use common measures, especially the metric system with its steady units.
Matter fills space. Energy is the ability to make something happen. The molecular idea ties all this together: all substances are made of incredibly small molecules that never stop moving. How tightly they lie and how strongly they attract each other decide whether something is solid, liquid, or gas. Heat them up, and the molecules move faster. In solids, they hold their neighbors and resist changes in shape. In liquids, they slide past each other. In gases, they roam widely and fill spaces. Some changes—melting ice, boiling water, dissolving sugar—preserve identity and are called physical. Others, like burning magnesium or scorching sugar, make something new and are chemical. Physics mostly cares about physical changes and the measurable relationships they obey.

Proof of molecular motion is everywhere. A smell spreads across a room even if no one blows. If you let out vapors of ammonia and hydrochloric acid a short distance apart, they rush toward each other and form a visible cloud about midway. Gases also run at different speeds through porous cups: hydrogen outruns air, swells a container, and can make a small jet; when the supply is cut, hydrogen slips out faster than air can get in, creating suction. The pressure a gas exerts is not because it is tightly packed—steam shrinks to a fraction of its volume when it condenses—but because molecules keep hitting the walls without stopping. Heat makes the hits harder; pressure rises or the gas expands.
Look closely at very small particles suspended in a liquid, and they dance jerkily. That is Brownian motion: uneven little hits from molecules hitting them from all sides. Liquids also spread dissolved substances, they evaporate into drier air and cool surfaces because the fastest molecules escape first. Water can go through thin membranes and be pulled toward stronger solutions. That is called osmosis. Liquids and gases flow, but not equally easily. Some are thicker than others, and even gases have a little 'thickness' that slows the flow, just as syrup flows more slowly than water.

Molecules are drawn to each other, both inside a substance and between different substances. When similar molecules hold together, it is called cohesion. When different surfaces attract each other, it is called adhesion. A thin skin on top of a liquid—surface tension—can do strange things: soap bubbles become tight and shiny, a thin steel needle can rest on the water's surface, wet brush hairs bunch into a point, and a tiny insect can actually be carried along on a stretched liquid skin. When two liquids with different surface tension meet, one can spread suddenly or pull together on the other.
In narrow spaces, adhesion sometimes wins over cohesion. Then water creeps up inside thin tubes, called capillary action. When the wall does not get wet, like mercury in glass, the liquid is pulled down. Wicks, towels, blotters, and sponges work that way. Waxed cloth lets water vapor through but not liquid water. Soil and forest floors save and share moisture through such tiny forces, and dry-land farming lives on them.
Dissolving and crystallization show that molecules 'know' each other. Over time and right conditions, substances build crystals with impressive order. Porous solids take up gas, and liquids dissolve gases more or less depending on temperature and pressure. That makes soda fizzy and determines how much oxygen fish get in warm summer water.

The holding together of solids shows in strength and springy properties. If you pull a thread within reasonable limits, it becomes the same length again. If you press down a spring and let go, it bounces back. Within a certain limit, elasticity is simple to describe: double the force gives double the stretch. If you go beyond the limit, the shape does not come back completely. Elasticity exists in push and pull, in bending and twisting, in shape and in volume. Outside forces disturb the balance between attraction and heat motion. If you remove the forces in time, the original shape rises again.
Other properties vary greatly because the internal structure varies: gold can be beaten as thin as leaf and still hold together; spring steel endures sudden blows; glass resists changing shape for a long time and then suddenly gives way with a snap. Such differences—toughness, brittleness, hardness—are not just words. They decide what we can build and how we must treat materials.

Where liquids and gases rest or flow, they press against everything around. Pressure increases with depth and depends on density. If you hold a card against the end of a tube and lower it into water, the water is kept out by air pressure until the forces inside and outside the card balance. Measure it, and do it again, a rule appears: at the same level in a liquid, pressure equals the weight of the column of liquid above that level. The total force on a surface depends on the area, the average depth, and how heavy the liquid is per unit volume.
Pressure in a liquid is the same in all directions at the same point. If you close the liquid in and add extra pressure at one spot, this pressure goes unchanged through the whole liquid. This insight was clarified by a thinker who saw what we now see everywhere: a small cylinder pushing on a liquid can lift heavy loads in a wide cylinder. Hydraulic presses and jacks do just that. We trade movement length for force, and the area ratio tells how much gain. It is like using a long arm to lift something heavy.
Water systems enjoy this—and suffer from it. An artesian well can flow by itself when water in a sloping layer lies higher than the hole we drill far away. Large steel pipes in a city get their flow needs smoothed out by high water towers. Small air cushions at taps dampen the shocks when we shut off quickly.

Liquids also carry things. An old law says that a body immersed in a liquid experiences an upward push equal to the weight of the liquid it displaces. A floating object pushes down exactly as much as the water pushes up, and how deep it sinks tells how heavy a load it carries. If you weigh a stone in air and then in water, the difference in weight equals the weight of the displaced water, and so you get the volume. That is how we find density and specific gravity. Floaters sink deeper in light liquids and shallower in heavy ones. They are called hydrometers and sit as tiny spies on density in everything from milk to acid.
Gases share a lot with liquids. They flow, they carry, but they also let themselves be squeezed easily. Air has weight. A column of mercury in a glass tube stands about three-quarters of a meter high, held up by air pressure. If we remove the air around, the column falls. If we carry the instrument up a mountain, it falls with height. That is how it was shown once and for all that air presses.
Another rule says that for a fixed amount of gas at the same temperature, volume will decrease when pressure increases, and vice versa. That gives us a simple tool to understand everything from real clouds to play with syringes.

From these properties grow clever tools. An air pump pulls air out of a container through valves. A water jet in a tube can create suction and empty air from a device; it is called an aspirator. Pumps that push gas into bottles or tires use the same game. Lift pumps get well water by letting air pressure push water into a cylinder when the piston makes the air inside a little thinner. But because air pressure can only lift water about ten meters, deeper wells need the cylinder down at the water level.
Force pumps push water out through valves. Air chambers at the outlet smooth the flow so it doesn't just come in jerks. A siphon, once filled, carries liquid over a rim as long as the outlet is lower than the surface we start from. A little glass diver in a water-filled bottle shows how pressure goes everywhere equally, and that gas can be squeezed more than liquid. Squeeze the bottle, the diver sinks. Let go, it rises.
Water hammer, which we usually avoid in pipes, can still be used in a smart machine: a hydraulic ram lifts part of a falling water stream upward with no other power than the fall itself. Balloons rise because they displace more air than they weigh. Parachutes slow falls by giving a large surface to air. Compressed air stops trains, measures gas in houses, and drives pumps that throw liquid upward by spinning blades.

A force is a push or a pull. We can measure it by how much it stretches something elastic, or by how much it changes motion. A spring scale is reliable because, within the elastic limit, equal extra push gives equal extra stretch. A force has size and direction and acts at a point. We can draw it as an arrow and add arrows together. Equal forces in the same line become bigger together. Two forces at an angle can be replaced by one—the resultant—which points along the diagonal between them. The force that would hold everything still points exactly opposite to the resultant and is the same size.
We also have to distinguish mass and weight. Mass tells how much stuff something has and how hard it is to change its speed. Weight is the Earth's pull on the thing, and changes a little with where we are. Motion comes in several kinds: sliding along, spinning, swinging. Speed is how fast and which way, acceleration is how quickly speed changes. A falling object accelerates. A ball thrown up is slowed by the same downward acceleration.
The amount of motion, momentum, is mass times speed. Changes in it measure what a force has done.