The fundamental units in mechanics are the units of length, mass, and time. The other units may be derived from these three. The connection between force and the fundamental units is provided by Newton's second law of motion. The units of other mechanical variables are connected by definition with the fundamental units. For example, if a particle traverses a unit length in unit time, its velocity is unity. Likewise, unit acceleration is the acceleration of a body that gains unit velocity in unit time.
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The use of dumbbells gives you a much more comprehensive strengthening effect because the workout engages your stabilizer muscles, in addition to the muscle you may be pin-pointing. Without all of the belts and artificial stabilizers of a machine, you also engage your core muscles, which are your body's natural stabilizers.