질량은물체의기본속성이자그관성(gydF4y2Ba뉴턴의제2법칙gydF4y2Ba)의측정치이며,개체에서물질의양의일반측정치입니다。개체의질량은물체의위치와관계가없습니다。즉,물체의질량은지구,달,먼우주공간에서동일합니다。질량의SI단위는公斤입니다。그림1에묘사된것처럼중력gydF4y2BaFgydF4y2BaGgydF4y2Ba= mgydF4y2BaggydF4y2Ba로인해개체에작용하는힘을무게라고합니다。중력가속도와무게를알면개체의질량을계산할수있습니다(gydF4y2Ba뉴턴의제2법칙gydF4y2Ba）.gydF4y2Ba
그림1:gydF4y2Ba질량m의사과의무게가gydF4y2BaFgydF4y2BaGgydF4y2Ba= mgydF4y2BaggydF4y2Ba로제시되어있음。gydF4y2Ba
저울은개체의무게를측정하는것이라기보다개체에작용하는수직항력과동일한크기이고규모별로공급되는힘을측정합니다。2에이런상황이그림묘사되어있습니다。사과의알짜힘이0일때(사과는가속화하지않음)수직항력gydF4y2BaFgydF4y2BaNgydF4y2Ba은사과의무게*gydF4y2BaFgydF4y2BaGgydF4y2Ba와동일한크기입니다。다시말해서:저울은사과의무게와동일한크기의힘으로위쪽으로밀고있습니다。gydF4y2Ba뉴턴의제3법칙gydF4y2Ba반에따르면,대방향에서저울에작용하는동등하게강한힘이있어야합니다。이런힘(저울의자유물체도상에서빨간색화살표로표시됨)은실제저울로측정되며알짜힘이0인경우에사과의무게와동등합니다。gydF4y2Ba
그림2:gydF4y2Ba왼쪽:저울에있는사과(회색직사각형으로표시됨)및2개의해당자유물체도(오른쪽)。gydF4y2BaFgydF4y2BaGgydF4y2Ba= mgydF4y2BaggydF4y2Ba는사과무게,gydF4y2BaFgydF4y2BaNgydF4y2Ba수직항력,mgydF4y2Ba年代gydF4y2BaggydF4y2Ba저울의무게이고,gydF4y2BaFgydF4y2BaFSgydF4y2Ba는바닥이저울에가하는수직항력입니다。gydF4y2Ba
가속하는체계의경우,가령위쪽으로가속하는승강기에서처럼,저울의판독값은사과무게와다릅니다。이경우,수직항력gydF4y2BaFgydF4y2BaNgydF4y2Ba의크기는사과의무게FgydF4y2BaGgydF4y2Ba= mg보다크므로저울의힘측정은사과의무게보다큽니다。“겉힘의척도측정을보기무게”라고하며척도가개체를상대로”밀지”않는자유낙하의경우에도0이될수있습니다。이런상황은궤도에있는우주비행사들에게적용되어비록그들의무게0이가아님에도불구하고무중력상태를경험하게합니다。gydF4y2Ba