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Publicado em 19/9/2012 por Ciência 2.0

Na água do mar conseguimos flutuar e ficar à tona. O que faz com que seja possível para nós fazê-lo? Eis a explicação científica...

Foi o matemático grego Arquimedes, de Siracusa, o primeiro a debater-se com esta questão no século III a.C., tendo ficado famoso por descobrir como determinar o volume de um corpo irregular, sem o danificar. Conta-se que ao entrar para dentro de um tanque para tomar banho, e ao ficar completamente submergido, reparou que o nível da água subiu. Pensou… pensou e… EUREKA! O volume de água do tanque deslocado para cima teria que equivaler ao seu próprio volume! Ao dividir a massa do seu corpo (o peso em quilos) pelo volume da água deslocada, obteria a densidade (massa volúmica) do seu corpo.

Tendo como referência que a densidade relativa (ou gravidade específica) da água é 1 e a correspondente da água do mar é 1,025, faz com que no mar o nosso corpo sofra uma impulsão maior do que, por exemplo, na banheira ou piscina (água doce). Quando estamos submersos na água, essa força de impulsão é proporcional ao nosso volume imerso e à densidade relativa do nosso corpo.

Podemos alterar ligeiramente a densidade relativa do nosso corpo; quando estamos relaxados ela diminui, aumentando quando estamos tensos. Pessoas diferentes possuem densidade relativa diferente. Por exemplo, uma pessoa com uma maior densidade de massa gorda pode flutuar mais facilmente, dado que a densidade relativa da massa gorda é menor do que a da massa muscular. Adicionalmente, para facilitar que o corpo flutue na água comummente inspiramos ar, que tem uma densidade relativa mil vezes inferior à da água e permite reduzir a densidade relativa do corpo. A densidade relativa da massa muscular, ossos e massa gorda, por exemplo, é 1,0, 1,5 e 0,8 respetivamente, resultando numa gravidade específica média do corpo humano de aproximadamente 0,97. Como vimos, este valor é inferior à densidade relativa da água, daí que conseguimos flutuar, de uma forma relaxada, com facilidade.

Bons mergulhos!

Consultor científico: Carlos Tavaresdocente de Física da Escola de Ciências da Universidade do Minho

Foto: Flickr/miyoneza

 

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