Speed, SI unit for

Not all quantities can be measured with base units. For example, the SI unit for speed is meters per second (m/s). Notice that meters per second includes two SI base units—the meter and the second. A unit that is defined by a combination of base units is called a derived unit. Two other quantities that are measured in derived units are volume and density. 

The SI base units are used to obtain derived units. To do so, we use the defining equation for the quantity, substituting the appropriate base units. For example, speed is defined as the ratio of distance traveled to elapsed time. Thus, the SI unit for speed—m/s, read meters per second —is a derived unit, the SI unit for distance (length), m, divided by the SI unit for time, s. Two common derived units in chemistry are those for volume and density. 

Note that molar masses Ji must be converted to kilograms per mole for use in the equation. The final result is expressed in the SI unit of speed, meters per second. 

Calculate the average speed of a nitrogen molecule in air at room temperature (25 °C). Hint use SI units for R and for molar mass, and u will come out in m/s. 

As Table 1.1 shows, the SI system is based on a set of seven fundamental units, or base units, each of which is identified with a physical quantity. All other units, called derived units, are combinations of these seven base units. For example, the derived unit for speed, meters per second (m/s), is the base unit for length (m) divided by the base unit for time (s). (Derived units that occur as a ratio of... 

The SI unit for energy is the joule (pronounced jool ), J, in honor of James Joule (1818—1889), a British scientist who investigated work and heat 1 J = I kg-m s. Equation 5.1 shows that a mass of 2 kg moving at a speed of 1 m/s possesses a kinetic energy of 1 J ... 

The SI unit for force is the newton, (N). The newton is the force that will increase the speed of a one-kilogram mass by one meter per second each second that the force is applied. At Earth s surface, gravity has an acceleration of 9.8 m/s. Consider a haUet dancer with a mass of 51 kg, as shown in Figure 1.1 (on the previous page). A mass of 51 kg exerts a force of 500 N (51 kg X 9.8 m/s ) on Earth s surface. No matter how the dancer stands, she exerts the same force—500 N—against the floor. But the pressure exerted against the floor depends on the area of contact. 

This equation is valid, if all quantities are given in SI - units, for example in J T. In cgs-units it reads (c = speed of light)... 

From these seven base SI units, many others are derived. For example, speed can be denoted as distance per unit time (m.s-1) and acceleration as speed change per unit time (m.s-2). Some common derived units are given below. 

Pressure-sensitive adhesives are used in a great variety of applications, most commonly for adhesive tapes. In that case, they have to be tested by static shear test or dynamic shear test The difference between these two methods is that in static shear test a standard force is being applied to the test specimen and the adhesive failure is reported as the time it takes for failure to occur. The dynamic shear test involves a force being applied to the PSA tape at a specific rate of speed (typically 0.25 mm or 0.1 in. per minute). The value reported is as the peak force per unit area (Ib/in, also abbreviated as psi, and in SI units MPa) required to cause adhesive failure. The standards for adhesion shear tests are ASTM D3654, ISO EN 1943, and PSTC-107.i i ... 

Look back at the seven fundamental SI units given in Table 1.3 and you ll find that measures for such familiar quantities as area, volume, density, speed, and pressure are missing. All are examples of derived quantities rather than fundamental quantities because they can be expressed using one or more of the seven base units (Table 1.5). 

Table G Definitions of the Electric Field E, the (Di)electric Polarization P, the Electric Displacement D, the Magnetic Field H, the Magnetization M, the Magnetic induction or flux density B, statement of the Maxwell equations, and of the Lorentz Force Equation in Various Systems of Units rat. = rationalized (no 477-), unrat. = the explicit factor 477- is used in the definition of dielectric polarization and magnetization c = speed of light) (using SI values for e, me, h, c) [J.D. Jackson, Classical Electrodynamics, 3rd edition, Wiley, New York, 1999.]. For Hartree atomic u nits of mag netism, two conventions exist (1) the "Gauss" or wave convention, which requires that E and H have the same magnitude for electromagnetic waves in vacuo (2) the Lorentz convention, which derives the magnetic field from the Lorentz force equation the ratio between these two sets of units is the Sommerfeld fine-structure constant a = 1/137.0359895...

Many quantities you can measure need units other than the seven basic SI units. These units are derived by multiplying or dividing the base units. For example, speed is distance divided by time. The derived unit of speed is meters per second (m/s). A rectangle s area is found by multiplying its length (in meters) by its width (also in meters), so its unit is square meters (m ). 

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