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These include the equivalence of mass and energy (E = mc 2), length contraction (moving objects shorten), [Note 8] and time dilation (moving clocks run more slowly). The factor γ by which lengths contract and times dilate is known as the Lorentz factor and is given by γ = (1 − v 2 / c 2 ) −1/2 , where v is the speed of the object.
The name comes from the Russian word перо (pero) meaning feather, reflecting the form of pernach that resembled an arrow with fletching. The most popular variety of pernach had six flanges and was called shestopyor (from Russian shest ' and pero, that is six-feathered). Pernach was the first form of the flanged mace to find wide usage.