All matter is made out of molecules and atoms. In solids, these molecules and atoms can be distributed in two different ways: the solids can be amorphous, which means their atoms are distributed randomly, or they can be crystalline, which means their atoms are in an orderly pattern. The crystalline state is more common

When an object is subject to external forces, it undergoes a change in size, shape, or both. A weight hanging on a spring stretches the spring. When the weight is removed, the spring returns to its original shape. We say that the spring is elastic. Elasticity is the property of changing shape when a deforming force acts on an object and returning to the original shape when the force is removed. Substances that do not resume their original shape after the deforming force is removed are inelastic.

When a weight is hung on a spring, the force of gravity acts on the spring, stretching it. The distance the spring stretches is directly proportional to the force on it. This is Hooke's law. F~x. If an elastic material is stretched beyond a certain point, it will become permanently distorted and not resume it's original shape. This point is called the elastic limit. Hooke's law only holds for substances that have not passed their elastic limit.

when something is stretched, it is said to be under tension. Tension pulls atoms apart. When something is pushed in, it is said to be under compression. Compression squashes atoms together. Consider a board hanging on a wall. If a weight is placed on one end, the top of the board is under tension, while the bottom of the board is under compression. If there is tension on top and compression on bottom, somewhere in between there must be a place where there is neither tension nor compression. This place is a line down the middle of the board, called the neutral layer.

Arches are structures where, when an load is placed on them, compression strengthens rather than weakens the arch. If you rotate an arch through a complete circle, you have a dome. A dome needs no columns to hold it up. Compression strengthens it.

As the size of a thing increases, it grows heavier much faster than it grows stronger. Scaling is the study of how the volume and size of an object affect the relationship to its weight, strength, and surface area. Strength comes from the area of the cross-section of an object. Weight depends on the volume of the object. Consider a cube one centimeter on each side. The volume of the cube is 1cm*1cm*1cm, or 1cm³. The cross-section is 1cm*1cm or 1cm². Now consider the same cube doubled, that is, being two centimeters on each side. the volume is 2cm*2cm*2cm, or 8cm³. The cross-section is 2cm*2cm or 4cm². You see that the cross-sectional area grows as the square of the linear dimension while the volume grows as the cube of the linear dimension.

Weight thus increases much faster than strength. A strong man whose every dimension was suddenly doubled would be able to lift more weight, but he would only be able to do half as many pushups!

A liquid contained in a vessel exerts a force against the walls of the vessel. To discuss this, we must talk about pressure. Pressure is force per unit area, or p=F/A. When you swim under water, you feel pressure. The deeper you swim, the more pressure you feel. So liquid pressure depends on depth. Liquid pressure also depends on the density of the liquid. The denser the liquid, the greater the pressure. Liquid pressure=density*depth. Pressure does not depend on the volume of the liquid.

Bouyancy is the apparent loss of weight of objects submerged in a liquid. Lifting a large rock off the bottom of a river is fairly easy as long as the rock is under water. But if you bring the rock out of the water, you have to exert more force to lift it up. This is because when the rock is submerged, the water exerts an upwards force on it that is exactly opposite in direction to gravity. This upward force is called the buoyant force. It occurs because of water pressure increasing with depth.

Consider the above image. Pressure acts on all points on the rock. The pressure on opposite sides of the rock cancel out. There is more pressure on the bottom of the rock than the top of the rock because pressure increases with depth, and the bottom of the rock is deeper than the top of the rock. So there is an upwards force on the rock. If the buoyant force on an object is less dense than the object's weight, the object will sink. If the forces are the same as the object's weight, the object will remain at any level in the liquid. If the force is greater than the object's weight, the object will float.

When an object is placed in a container filled with liquid, the container will overflow. Some liquid is displaced. Some thought will show that the volume of the liquid displaced is equal to the volume of the submerged object. Another relationship concerning displacement of liquid that is very important to buoyancy was discovered by Archimedes. Archimedes' principle states that an immersed body is buoyed up by a force equal to the weight of the fluid it displaces.

Density, not weight, is the deciding factor in whether an object will sink or float. Density determines buoyant force. If you have a dense object in water, it will not have a large enough volume to displace enough water to buoy itself up. Here are three rules of floatation:
1.)If an object is denser than the liquid it is in, it will sink.
2.)If an object is less dense than the liquid it is in, it will float.
3.)If an object is the same density as the liquid it is in, it will remain at any level.
An iron block will sink, because it is denser than water. But an iron boat will float. This is because the shape of the boat makes it have a much greater volume, therefore less density, therefore a larger buoyant force.

One of the most important facts of fluid pressure is that a change in pressure at one point in a fluid will be transmitted undiminished to all other points in the fluid. This is Pascal's principle. Consider a u-shaped tube of water with a piston on either end. Apply a force to one piston, and you change the pressure in the water. The other piston will rise. Now in crease the area of the second piston, making it ten times as large as the first piston. Say you place a 10 kg weight on the first piston. The second piston will support a 100 kg weight! (This doesn't violate conservation of energy. The increase of force is compensated by a decrease in distance. If the first piston moves down 10 cm., the second piston will only move up one cm.)

The surface of a liquid tends to contract. We call this tendency surface tension. Surface tension is caused by molecular attraction. In a liquid, molecules are pulled in all directions by attractive forces. On the surface of a liquid, molecules are only attracted down and to the side. This causes the surface of a liquid to contract to the smallest possible surface area.

If you put a narrow tube in water, water will rise up the tube. This is called capillary action. This happens because the water molecules are attracted more to the glass molecules than to themselves. Attraction between unlike substances is called adhesion. Attraction between like substances is called cohesion. The water will rise until the weight of the water equals the adhesive force, so water will rise higher in narrower tubes.

We live at the bottom of an ocean of air. This air exerts pressure. If we remove all the air from a cylinder surrounded by air, a piston in the cylinder will be forced up. so strong a force will act on the piston that, for a cylinder with a ten centimeter diameter, a person could be lifted by the piston! The force is caused by atmospheric pressure.

Pressure in a bicycle tires is caused by the molecules of air in the tire hitting the walls of the tire. Suppose you doubled the amount of air in the bike tire. The density of the air would be doubled. The number of collisions with the wall, or the pressure, would also be doubled. So pressure is proportional to the density of the gas. We can also make pressure double by taking a fixed amount of air and compressing it to half its original volume (twice its original density.) Notice that here the pressure of the gas times the volume of the gas before and after the compression are the same. (PV=2P*V/2) This occurrence is called Boyle's law, which states that PV_original=PV_{compressed or expanded}.

Archimedes principal holds true for gases as well as liquids. An object surrounded by air is buoyed up by a force equal to the weight of the air displaced. Balloons float because they displace a greater weight of air then they displace. Again, for an object to float in air, it must be less dense then air. A balloon is filled with gas to increase its volume while adding little to its weight, thus decreasing its density.

An eighteenth century scientist named Bernoulli studied the relationship between gas speed and pressure. Bernoulli knew that when a gas (or any fluid) passes through a narrow space its speed increases. He deduced that a fluid gains this speed at the expense of pressure. Bernoulli's principle states that when the speed of a fluid increases, pressure in the fluid decreases. This is a consequence of conservation of energy. In most situations, only three types of energy need to be considered: Kinetic, Potential, and Pressure. If a fluid flows at a constant elevation, potential energy remains constant. That leaves kinetic energy and pressure. If kinetic energy increases, pressure must decrease.

Bernoulli's principle is the principle behind an airplane wing. An airplane's wing is curved on top and flatter on bottom. Air passing over the curved wing tries to keep up with air passing under the wing (because of something called streamlining, which we won't go into here), and to do this it must travel faster than the air under the wing because it has a longer distance to cover. So there is less pressure on the top of the wing and more pressure on the bottom, and the airplane rises.

Plasma is the fourth phase of matter. Plasma is an electrified gas. It occurs when all the molecules in a gas are ionized, or stripped of their electrons. Plasma conducts electricity and absorbs some kinds of radiation. Plasma can be moved and molded by electric or magnetic fields.