Young's modulus is a measure of how difficult it is to compress a material, such as steel. It measures pressure and is typically computed in terms of pascals (Pa). It is most commonly used by physicists to determine strain, a measurement of how a material, responds to a pressure, such as being squeezed or stretched.
Interatomic forces or the forces that hold a material together are strongly associated with an understanding of this measurement. The atoms in a solid exert both repulsive, or negative, and attractive, or positive, forces on each other. The forces the atoms create only achieve a balance when they are perfectly separated. For example, if the atoms are pushed too close together or if they are separated too far apart, the atoms will no longer be in perfect balance. Once they are no longer in balance, they will oppose the action being applied to them.
The stiffer interatomic forces are, the larger Young's modulus value will be, and the more the material will oppose the action. For example, if the interatomic forces are stiff, then this measurement will be large, and the material will not shrink as much when it is squeezed.
When studying the physics of steel, a scientist must first remember that steel is made up primarily of iron atoms. Consequently, the Young's modulus of steel is approximately the same as the Young's modulus of iron. Since the Young's modulus of iron is 195,000,000,000 Pa or 195 gigapascals (GPa), it considered to be very large and very difficult to compress.
Understanding that steel is extremely hard to compress is important in everyday life. For example, it can be used to construct buildings and not become compressed, ruining the integrity of the building structure. A cube of steel that is only 1 meter (3.28 ft) in width, height, and depth, would only compress about one micron while supporting the weight of one school bus, because of the amount of pressure it can withstand. In comparison, a cube that is comprised of the same dimensions and is made of lead has a lower value for Young's modulus. The lead cube would compress 14 times more than the steel cube.
Young's modulus can also be used in computing tension, where the atoms are pulled apart instead of squeezed together. In those cases, the strain is negative because the atoms are stretched instead of compressed. The main concept that should be understood is that in either case it is that the stiffness of the forces between the atoms that is being measured — whether they are being compressed or stretched. Consequently, the pressure computed for Young's modulus does not change for either kind of measurement.