{"id":3851,"date":"2018-04-18T03:11:20","date_gmt":"2018-04-18T03:11:20","guid":{"rendered":"http:\/\/fisicaevestibular.com.br\/novo\/?page_id=3851"},"modified":"2024-09-04T12:27:05","modified_gmt":"2024-09-04T12:27:05","slug":"resolucao-comentada-do-enem-2017","status":"publish","type":"page","link":"https:\/\/fisicaevestibular.com.br\/novo\/universidades-2018\/enem-2018\/resolucao-comentada-do-enem-2017\/","title":{"rendered":"Resolu\u00e7\u00e3o comentada do ENEM 2017"},"content":{"rendered":"

Resolu\u00e7\u00e3o comentada das quest\u00f5es de F\u00cdSICA (e de algumas de Matem\u00e1tica relacionadas com F\u00edsica) do<\/span><\/span><\/span><\/p>\n

ENEM-MEC-2017<\/span><\/span><\/span><\/p>\n

01-<\/b><\/span><\/span><\/span><\/p>\n

Como, <\/b><\/span><\/span><\/span>pelo enunciado, <\/b><\/span><\/span><\/span>numa <\/b><\/span><\/span><\/span>colis\u00e3o frontal a for\u00e7a que o cinto de seguran\u00e7a exerce sobre o t\u00f3rax e abd\u00f4men do motorista pode causar les\u00f5es graves nos \u00f3rg\u00e3os internos,<\/b><\/span><\/span><\/span> a inten\u00e7\u00e3o <\/b><\/span><\/span><\/span>\u00e9 <\/b><\/span><\/span><\/span>diminuir<\/b><\/span><\/span><\/span> essa for\u00e7a <\/b><\/span><\/span><\/span>de <\/b><\/span><\/span><\/span>maneira<\/b><\/span><\/span><\/span> que o cinto n\u00e3o <\/b><\/span><\/span><\/span>agrida o motorista, <\/b><\/span><\/span><\/span>apenas <\/b><\/span><\/span><\/span>o segure de forma uniforme com a menor for\u00e7a poss\u00edvel para n\u00e3o machuca-lo.<\/b><\/span><\/span><\/span><\/p>\n

Pela <\/b><\/span><\/span><\/span>segunda lei de Newton F<\/b><\/span><\/span><\/span>R<\/b><\/span><\/span><\/sub><\/span> = m.a <\/b><\/span><\/span><\/span>e, como a <\/b><\/span><\/span><\/span>massa \u00e9 constante <\/b><\/span><\/span><\/span>\u00e0 <\/b><\/span><\/span><\/span>menor for\u00e7a resultante<\/b><\/span><\/span><\/span> <\/b>sobre o boneco <\/b><\/span><\/span><\/span>corresponde \u00e0quela que produz a <\/b><\/span><\/span><\/span>menor desacelera\u00e7\u00e3o<\/b><\/span><\/span><\/span> sobre o mesmo.<\/b><\/span><\/span><\/span><\/p>\n

Assim, no <\/b><\/span><\/span><\/span>gr\u00e1fico, a curva que satisfaz \u00e9 a do cinto 2,<\/b><\/span><\/span><\/span> que produz <\/b><\/span><\/span><\/span>menor acelera\u00e7\u00e3o <\/b><\/span><\/span><\/span>e de maneira <\/b><\/span><\/span><\/span>mais cont\u00ednua <\/b><\/span><\/span><\/span>que as outras <\/b><\/span><\/span><\/span>correndo menor risco de lesionar o motorista. <\/b><\/span><\/span><\/span><\/p>\n

R- B<\/b><\/span><\/span><\/span><\/p>\n

02-<\/b><\/span><\/span><\/span><\/p>\n

\"\"<\/p>\n

\"\"<\/p>\n

03-<\/b><\/span><\/span><\/span><\/p>\n

Para <\/b><\/span><\/span><\/span>se obter boa ilumina\u00e7\u00e3o <\/b><\/span><\/span><\/span>como <\/b><\/span><\/span><\/span>deseja o arquiteto <\/b><\/span><\/span><\/span>voc\u00ea deve procurar <\/b><\/span><\/span><\/span>maior quantidade de intensidade emitida (\u00e1rea cinza) <\/b><\/span><\/span><\/span>que que <\/b><\/span><\/span><\/span>est\u00e3o dentro das linhas pontilhadas<\/b><\/span><\/span><\/span>, mais <\/b><\/span><\/span><\/span>sens\u00edveis, percept\u00edveis ao <\/b><\/span><\/span><\/span>olho humano, <\/b><\/span><\/span><\/span>que \u00e9 o caso da LED.<\/b><\/span><\/span><\/span><\/p>\n

Al\u00e9m disso, a <\/b><\/span><\/span><\/span>LED fornece muito pouca intensidade na faixa do infravermelho (IF), <\/b><\/span><\/span><\/span>que <\/b><\/span><\/span><\/span>emite calor, n\u00e3o aquecendo o ambiente, conforme desejo do arquiteto. Por esse motivo as l\u00e2mpadas de tungst\u00eanio e x\u00eanon devem ser evitadas (ampla regi\u00e3o de infravermelho).<\/b><\/span><\/span><\/span><\/p>\n

Observe ainda que as <\/b><\/span><\/span><\/span>l\u00e2mpadas de Merc\u00fario e de Haleto met\u00e1lico possuem regi\u00f5es na parte vis\u00edvel,<\/b><\/span><\/span><\/span> mas <\/b><\/span><\/span><\/span>n\u00e3o t\u00e3o amplas quanto \u00e0s da LED.<\/b><\/span><\/span><\/span><\/p>\n

R- E<\/b><\/span><\/span><\/span><\/p>\n

04-<\/b><\/span><\/span><\/span><\/p>\n

Quando um <\/b><\/span><\/span><\/span>sinal diminui sua intensidade <\/b><\/span><\/span><\/span>para <\/b><\/span><\/span><\/span>valores inferiores a 10 dB, <\/b><\/span><\/span><\/span>este <\/b><\/span><\/span><\/span>precisa ser retransmitido.<\/b><\/span><\/span><\/span><\/p>\n

e <\/b><\/span><\/span><\/span>intensidades superiores a 100 dB n\u00e3o podem ser transmitidas adequadamente<\/b><\/span><\/span><\/span>. <\/b><\/span><\/span><\/span><\/p>\n

Assim, a faixa <\/b><\/span><\/span><\/span>de intensidade sonora em que a <\/b><\/span><\/span><\/span>linha pode operar, <\/b><\/span><\/span><\/span>sem retransmiss\u00e3o, <\/b><\/span><\/span><\/span>est\u00e1 <\/b><\/span><\/span><\/span>compreendida entre<\/b><\/span><\/span><\/span> 100 dB \u2013 10 dB <\/b><\/span><\/span><\/span>= 90 dB.<\/b><\/span><\/span><\/span><\/p>\n

\"\"<\/p>\n

05-<\/b><\/span><\/span><\/span><\/p>\n

Veja <\/b><\/span><\/span><\/span>nas figuras que as usinas e\u00f3licas<\/b><\/span><\/span><\/span> quando <\/b><\/span><\/span><\/span>funcionando produzem diretamente eletricidade <\/b><\/span><\/span><\/span>e, ao mesmo tempo <\/b><\/span><\/span><\/span>produzem hidrog\u00eanio que \u00e9 armazenado em setores espec\u00edficos da esta\u00e7\u00e3o.<\/b><\/span><\/span><\/span><\/p>\n

No curto per\u00edodo de <\/b><\/span><\/span><\/span>aus\u00eancia de ventos, <\/b><\/span><\/span><\/span>quando a <\/b><\/span><\/span><\/span>eletricidade n\u00e3o \u00e9 produzida diretamente <\/b><\/span><\/span><\/span>o <\/b><\/span><\/span><\/span>abastecimento de energia el\u00e9trica continua, <\/b><\/span><\/span><\/span>pois a esta\u00e7\u00e3o <\/b><\/span><\/span><\/span>possui biog\u00e1s armazenado <\/b><\/span><\/span><\/span>que, <\/b><\/span><\/span><\/span>misturado com o hidrog\u00eanio, em uma rea\u00e7\u00e3o qu\u00edmica de combust\u00e3o libera calor.<\/b><\/span><\/span><\/span><\/p>\n

Esse <\/b><\/span><\/span><\/span>calor liberado, <\/b><\/span><\/span><\/span>como numa <\/b><\/span><\/span><\/span>usina termel\u00e9trica<\/b><\/span><\/span><\/span>, \u00e9 utilizado para <\/b><\/span><\/span><\/span>transformar a \u00e1gua em vapor que gira <\/b><\/span><\/span><\/span>as <\/b><\/span><\/span><\/span>turbinas que transformar\u00e3o energia t\u00e9rmica em mec\u00e2nica <\/b><\/span><\/span><\/span>provocando o <\/b><\/span><\/span><\/span>movimento das turbinas.<\/b><\/span><\/span><\/span><\/p>\n

Essa energia <\/b><\/span><\/span><\/span>mec\u00e2nica aciona os geradores <\/b><\/span><\/span><\/span>que a transformam em <\/b><\/span><\/span><\/span>energia el\u00e9trica que \u00e9 enviada \u00e0s redes de distribui\u00e7\u00e3o. <\/b><\/span><\/span><\/span><\/p>\n

R- B<\/b><\/span><\/span><\/span><\/p>\n

06-<\/b><\/span><\/span><\/span><\/p>\n

Basta analisar o gr\u00e1fico e o enunciado:<\/b><\/span><\/span><\/span><\/p>\n

Pelo enunciado, <\/b><\/span><\/span><\/span>para <\/b><\/span><\/span><\/span>evitar que outros tecidos<\/b><\/span><\/span><\/span> sejam <\/b><\/span><\/span><\/span>danificados, <\/b><\/span><\/span><\/span>selecionam-se comprimentos de<\/b><\/span><\/span><\/span> <\/b>onda <\/b><\/span><\/span><\/span>que s\u00e3o <\/b><\/span><\/span><\/span>absorvidos pela melanina <\/b><\/span><\/span><\/span>(entre 400 nm e 1100 nm) <\/b><\/span><\/span><\/span>presente nos pelos, <\/b><\/span><\/span><\/span>mas que <\/b><\/span><\/span><\/span>n\u00e3o afetam a oxi-hemoglobina do sangue <\/b><\/span><\/span><\/span>(nulo em 700 nm)<\/b><\/span><\/span><\/span> e a <\/b><\/span><\/span><\/span>\u00e1gua <\/b><\/span><\/span><\/span>(nula inferior a 900 nm) <\/b><\/span><\/span><\/span>dos tecidos da regi\u00e3o <\/b><\/span><\/span><\/span>em que o <\/b><\/span><\/span><\/span>tratamento ser\u00e1 aplicado. <\/b><\/span><\/span><\/span>Conclus\u00e3o,<\/b><\/span><\/span><\/span> 700 nm.<\/b><\/span><\/span><\/span><\/p>\n

R- B<\/b><\/span><\/span><\/span><\/p>\n

07-<\/b><\/span><\/span><\/span><\/p>\n

J\u00e1 que <\/b><\/span><\/span><\/span>podemos desprezar o atrito <\/b><\/span><\/span><\/span>com o ar podemos <\/b><\/span><\/span><\/span>considerar o sistema como conservativo <\/b><\/span><\/span><\/span>e aplicar o <\/b><\/span><\/span><\/span>princ\u00edpio da conserva\u00e7\u00e3o da energia mec\u00e2nica <\/b><\/span><\/span><\/span>aplicando-o o <\/b><\/span><\/span><\/span>durante qualquer ponto trajeto <\/b><\/span><\/span><\/span>e considerando a <\/b><\/span><\/span><\/span>energia mec\u00e2nica como constante durante todo o trajeto.<\/b><\/span><\/span><\/span><\/p>\n

O referencial adotado \u00e9 a posi\u00e7\u00e3o de equil\u00edbrio da lona onde h = 0.<\/b><\/span><\/span><\/span><\/p>\n

\"\"<\/p>\n

\"\"<\/p>\n

Essa <\/b><\/span><\/span><\/span>fun\u00e7\u00e3o da energia cin\u00e9tica em fun\u00e7\u00e3o de h \u00e9 do segundo grau e o gr\u00e1fico \u00e9 uma par\u00e1bola com concavidade <\/b><\/span><\/span><\/span>para baixo, pois o <\/b><\/span><\/span><\/span>termo que antecede h<\/b><\/span><\/span><\/span>2<\/b><\/span><\/span><\/sup><\/span> \u00e9 negativo.<\/b><\/span><\/span><\/span><\/p>\n

Observe ainda que, <\/b><\/span><\/span><\/span>quando h<\/b><\/span><\/span><\/span> <\/b>= h<\/b><\/span><\/span><\/span>m\u00edn<\/b><\/span><\/span><\/sub><\/span> a energia cin\u00e9tica \u00e9 nula E<\/b><\/span><\/span><\/span>c<\/b><\/span><\/span><\/sub><\/span> = 0 <\/b><\/span><\/span><\/span>(ele para, V = 0).<\/b><\/span><\/span><\/span><\/p>\n

R- C<\/b><\/span><\/span><\/span><\/p>\n

08-<\/b><\/span><\/span><\/span><\/p>\n

Vamos <\/b><\/span><\/span><\/span>analisar apenas a parte inferior do circuito<\/b><\/span><\/span><\/span> onde <\/b><\/span><\/span><\/span>circulam as correntes i, i<\/b><\/span><\/span><\/span>1<\/b><\/span><\/span><\/sub><\/span> e i<\/b><\/span><\/span><\/span>2<\/b><\/span><\/span><\/sub><\/span>, <\/b><\/span><\/span><\/span>j\u00e1 que com <\/b><\/span><\/span><\/span><\/p>\n

\"\"<\/p>\n

apenas esses dados \u00e9 poss\u00edvel calcular a tens\u00e3o U pedida.<\/b><\/span><\/span><\/span><\/p>\n

Observe que o <\/b><\/span><\/span><\/span>trecho que possui o resistor de R<\/b><\/span><\/span><\/span>2<\/b><\/span><\/span><\/sub><\/span> = 120 \u2126 \u00e9 percorrido pela corrente i<\/b><\/span><\/span><\/span>2 <\/b><\/span><\/span><\/sub><\/span>= 500 mA = 0,5 A (corrente nominal suportada pelo fus\u00edvel).<\/b><\/span><\/span><\/span><\/p>\n

\"\"<\/p>\n

09-<\/b><\/span><\/span><\/span><\/p>\n

Quando o <\/b><\/span><\/span><\/span>trajeto ADC \u00e9 igual ao trajeto AEC (<\/b><\/span><\/span><\/span>mesma diferen\u00e7a de caminho) <\/b><\/span><\/span><\/span>capta-se um som muito intenso na sa\u00edda <\/b><\/span><\/span><\/span>o que indica que a <\/b><\/span><\/span><\/span>interfer\u00eancia \u00e9 construtiva e a fonte emitiu ondas em dire\u00e7\u00f5es opostas mas em concord\u00e2ncia de fase.<\/b><\/span><\/span><\/span><\/p>\n

Quando se <\/b><\/span><\/span><\/span>aumentou gradativamente o trajeto ADC <\/b><\/span><\/span><\/span>at\u00e9 que fique como <\/b><\/span><\/span><\/span>mostrado na figura <\/b><\/span><\/span><\/span>a <\/b><\/span><\/span><\/span>intensidade do som na sa\u00edda fica praticamente nula e a\u00ed voc\u00ea tem a primeira interfer\u00eancia destrutiva. <\/b><\/span><\/span><\/span><\/p>\n

Na <\/b><\/span><\/span><\/span>situa\u00e7\u00e3o da figura fornecida<\/b><\/span><\/span><\/span> observe que o <\/b><\/span><\/span><\/span>trecho AD \u00e9 10 cm maior que o trecho AE <\/b><\/span><\/span><\/span>e que o <\/b><\/span><\/span><\/span>trecho DC \u00e9 10 cm maior que o trecho EC<\/b><\/span><\/span><\/span>, o que <\/b><\/span><\/span><\/span>implica que o caminho que a onda percorre no trajeto ACD \u00e9 20 cm maior que o caminho que a onda percorre no trajeto AEC.<\/b><\/span><\/span><\/span><\/p>\n

Isso <\/b><\/span><\/span><\/span>significa que a diferen\u00e7a de caminho entre uma interfer\u00eancia construtiva e a destrutiva sucessiva \u00e9 de d<\/b><\/span><\/span><\/span>2<\/b><\/span><\/span><\/sub><\/span> \u2013 d<\/b><\/span><\/span><\/span>1<\/b><\/span><\/span><\/sub><\/span> = 20 cm = 0,2 m.<\/b><\/span><\/span><\/span><\/p>\n

Como na <\/b><\/span><\/span><\/span>sa\u00edda o som \u00e9 praticamente nulo, a\u00ed <\/b><\/span><\/span><\/span>voc\u00ea <\/b><\/span><\/span><\/span>tem interfer\u00eancia destrutiva, cuja teoria est\u00e1 a seguir:<\/b><\/span><\/span><\/span><\/p>\n

\"\"<\/p>\n

O <\/b><\/span><\/span><\/span>primeiro m\u00ednimo corresponde a n = 0 \"\"<\/b><\/span><\/span><\/span> <\/b>d<\/b><\/span><\/span><\/span>2<\/b><\/span><\/span><\/sub><\/span> \u2013 d<\/b><\/span><\/span><\/span>1<\/b><\/span><\/span><\/sub><\/span> = (2n +1).\"\"<\/b><\/span><\/span><\/span> \"\"<\/b><\/span><\/span><\/span> <\/b>0,2 = (2.0 + 1).\"\" \"\" 0,2 = 1.\"\"<\/b><\/span><\/span><\/span> \"\"<\/b><\/span><\/span><\/span>
\n\"\"<\/b><\/span><\/span><\/span> = 0,4 m.<\/b><\/span><\/span><\/span><\/p>\n

Utilizando a <\/b><\/span><\/span><\/span>equa\u00e7\u00e3o fundamental da ondulat\u00f3ria <\/b><\/span><\/span><\/span>com a velocidade do som no interior do tubo <\/b><\/span><\/span><\/span>V = 320 m\/s <\/b><\/span><\/span><\/span>\u0001<\/b><\/span><\/span><\/span> = 0,4 m \"\"<\/b><\/span><\/span><\/span> <\/b>V = <\/b><\/span><\/span><\/span>\u0001<\/b><\/span><\/span><\/span>.f \"\"<\/b><\/span><\/span><\/span> 320 = 0,4.f \"\" f = \"\" \"\" <\/b><\/span><\/span><\/span>f = 800 Hz.<\/b><\/span><\/span><\/span><\/p>\n

R- C<\/b><\/span><\/span><\/span><\/p>\n

10-<\/b><\/span><\/span><\/span><\/p>\n

A\u00a0<\/b><\/span><\/span><\/span>in\u00e9rcia\u00a0<\/b><\/span><\/span><\/span>de um corpo\u00a0<\/b><\/span><\/span><\/span>depende de sua massa<\/b><\/span><\/span><\/span>, pois para\u00a0<\/b><\/span><\/span><\/span>acelerar ou frear um \u00f4nibus<\/b><\/span><\/span><\/span>\u00a0devemos aplicar uma\u00a0<\/b><\/span><\/span><\/span>for\u00e7a maior que aquela que usamos para acelerar ou frear um carro<\/b><\/span><\/span><\/span>.<\/b><\/span><\/span><\/span><\/p>\n

Assim,\u00a0<\/b><\/span><\/span><\/span>quanto maior a massa de um corpo, maior ser\u00e1 sua in\u00e9rcia.<\/b><\/span><\/span><\/span><\/p>\n

Um corpo em\u00a0<\/b><\/span><\/span><\/span>movimento circular<\/b><\/span><\/span><\/span>\u00a0<\/b><\/span><\/span><\/span>como, por exemplo, uma\u00a0<\/b><\/span><\/span><\/span>pedra presa a um fio<\/b><\/span><\/span><\/span>, tem, em\u00a0<\/b><\/span><\/span><\/span>cada ponto<\/b><\/span><\/span><\/span>, o\u00a0<\/b><\/span><\/span><\/span>vetor velocidade<\/b><\/span><\/span><\/span>\u00a0<\/b><\/span><\/span><\/span>sempre\u00a0tangente \u00e0 trajet\u00f3ria.<\/b><\/span><\/span><\/span><\/p>\n

Se voc\u00ea\u00a0cortar <\/b><\/span><\/span><\/span>o fio<\/b><\/span><\/span><\/span>, <\/b><\/span><\/span><\/span>a pedra, <\/b><\/span><\/span><\/span>por\u00a0in\u00e9rcia,<\/b><\/span><\/span><\/span> tender\u00e1 a manter <\/b><\/span><\/span><\/span>a\u00a0<\/b><\/span><\/span><\/span>dire\u00e7\u00e3o e o sentido do vetor velocidade e sair\u00e1 pela tangente.<\/b><\/span><\/span><\/span><\/p>\n

Ent\u00e3o, <\/b><\/span><\/span><\/span>quando a centr\u00edfuga gira em alta velocidade <\/b><\/span><\/span><\/span>ela faz com que a <\/b><\/span><\/span><\/span>subst\u00e2ncia mais densa <\/b><\/span><\/span><\/span>(maior massa, mesmo volume), de <\/b><\/span><\/span><\/span>maior in\u00e9rcia tenha maior tend\u00eancia de \u201csair pela tangente\u201d indo mais para o fundo do recipiente.<\/b><\/span><\/span><\/span><\/p>\n

Portanto, nesses aparelhos, a <\/b><\/span><\/span><\/span>separa\u00e7\u00e3o das subst\u00e2ncias ocorre em fun\u00e7\u00e3o das diferentes densidades.<\/b><\/span><\/span><\/span><\/p>\n

R- A <\/b><\/span><\/span><\/span><\/p>\n

11-<\/b><\/span><\/span><\/span><\/p>\n

A <\/b><\/span><\/span><\/span>resist\u00eancia el\u00e9trica do chuveiro, <\/b><\/span><\/span><\/span>quando aquecida, <\/b><\/span><\/span><\/span>transforma integralmente energia el\u00e9trica (W<\/b><\/span><\/span><\/span>el\u00e9trica<\/b><\/span><\/span><\/sub><\/span>) <\/b><\/span><\/span><\/span>em <\/b><\/span><\/span><\/span>energia t\u00e9rmica (calor Q) \"\"W<\/b><\/span><\/span><\/span>el\u00e9trica<\/b><\/span><\/span><\/sub><\/span> = Q.<\/b><\/span><\/span><\/span><\/p>\n

\"\"<\/p>\n

\"\"<\/p>\n

12-<\/b><\/span><\/span><\/span><\/p>\n

O <\/b><\/span>olho humano possui\u00a0dois tipos de c\u00e9lulas respons\u00e1veis<\/b><\/span><\/span><\/span> por nos <\/b><\/span><\/span><\/span>fazerem enxergar: os cones e os bastonetes.<\/b><\/span><\/span><\/span><\/p>\n

Os <\/b><\/span><\/span><\/span>bastonetes s\u00e3o c\u00e9lulas que necessitam de pouca luz para serem sensibilizadas.<\/b><\/span><\/span><\/span>\u00a0\u00a0Entretanto <\/b><\/span><\/span><\/span>n\u00e3o conseguem formar imagens coloridas ou n\u00edtidas. <\/b><\/span><\/span><\/span>\u00c9 <\/b><\/span><\/span><\/span>por isso <\/b><\/span><\/span><\/span>que a <\/b><\/span><\/span><\/span>noite ou em locais escuro \u00e9 muito dif\u00edcil se distinguir cor.<\/b><\/span><\/span><\/span><\/p>\n

\"\"<\/p>\n

J\u00e1 os <\/b><\/span><\/span><\/span>cones <\/b><\/span><\/span><\/span>s\u00e3o sensibilizados com uma quantidade grande de luz e geram as imagens <\/b><\/span><\/span><\/span>n\u00edtidas <\/b><\/span><\/span><\/span>e coloridas.<\/b><\/span><\/span><\/span> Existem <\/b><\/span><\/span><\/span>3 tipos de cones, os azuis, os vermelhos e os verdes<\/b><\/span><\/span><\/span>.\u00a0<\/b><\/span><\/span><\/span><\/p>\n

As\u00a0<\/b><\/span><\/span><\/span>cores vermelha, azul e verde s\u00e3o as 3 cores que nossos olhos captam. <\/b><\/span><\/span><\/span><\/p>\n

Todas as <\/b><\/span><\/span><\/span>outras cores que vemos s\u00e3o formadas a partir dessas 3 cores.<\/b><\/span><\/span><\/span> Por isso essas <\/b><\/span><\/span><\/span>3 cores s\u00e3o <\/b><\/span><\/span><\/span>consideradas as <\/b><\/span><\/span><\/span>cores\u00a0\u00a0prim\u00e1rias da vis\u00e3o e tamb\u00e9m da s\u00edntese aditiva de cor.<\/b><\/span><\/span><\/span><\/p>\n

Assim, <\/b><\/span><\/span><\/span>degenera\u00e7\u00e3o desse tipo celular (cones) <\/b><\/span><\/span><\/span>ir\u00e1 <\/b><\/span><\/span><\/span>comprometer a capacidade de vis\u00e3o em cores<\/b><\/span><\/span><\/span>.<\/b><\/span><\/span><\/span><\/p>\n

R- A<\/b><\/span><\/span><\/span><\/p>\n

13-<\/b><\/span><\/span><\/span><\/p>\n

Veja na <\/b><\/span><\/span><\/span>figura e no esquema abaix<\/b><\/span><\/span><\/span>o onde <\/b><\/span><\/span><\/span>E = 10 000 V <\/b><\/span><\/span><\/span>\u00e9 a <\/b><\/span><\/span><\/span>for\u00e7a eletromotriz <\/b><\/span><\/span><\/span>do gerador, <\/b><\/span><\/span><\/span>r <\/b><\/span><\/span><\/span>sua <\/b><\/span><\/span><\/span>resist\u00eancia interna, R<\/b><\/span><\/span><\/span>h <\/b><\/span><\/span><\/sub><\/span>= 1 000<\/b><\/span><\/span><\/span> <\/b>\u2126 <\/b><\/span><\/span><\/span>\u00e9 a <\/b><\/span><\/span><\/span>resist\u00eancia corporal da pessoa e i = 0,01 A \u00e9 a m\u00e1xima corrente n\u00e3o letal atrav\u00e9s do corpo da pessoa.<\/b><\/span><\/span><\/span><\/p>\n

\"\"<\/p>\n

\"\"<\/p>\n

14-<\/b><\/span><\/span><\/span><\/p>\n

\"\"<\/p>\n

\"\"<\/p>\n

\"\"<\/p>\n

\"\"<\/p>\n

15-<\/b><\/span><\/span><\/span><\/p>\n

Se, ao <\/b><\/span><\/span><\/span>fazer<\/b><\/span><\/span><\/span> o <\/b><\/span><\/span><\/span>gerador funcionar,<\/b><\/span><\/span><\/span> o <\/b><\/span><\/span><\/span>estudante obteve uma tens\u00e3o m\u00e1xima V <\/b><\/span><\/span><\/span>e uma <\/b><\/span><\/span><\/span>corrente de curto-circuito i, ent\u00e3o a tens\u00e3o m\u00e1xima U \u00e9 a for\u00e7a eletromotriz induzida \"\"<\/b><\/span><\/span><\/span> <\/b>do gerador, ou seja, V = \"\".<\/b><\/span><\/span><\/span><\/p>\n

Para <\/b><\/span><\/span><\/span>n espiras a for\u00e7a eletromotriz induzida \"\"<\/b><\/span><\/span><\/span> <\/b>\u00e9 fornecida por \"\"<\/b><\/span><\/span><\/span> <\/b>=<\/b><\/span><\/span><\/span> <\/b>n<\/b><\/span><\/span><\/span>.\"\" <\/b><\/span><\/span><\/span>= n. \"\"<\/b><\/span><\/span><\/span>.<\/b><\/span><\/span><\/span><\/p>\n

Para <\/b><\/span><\/span><\/span>variar o fluxo magn\u00e9tico no interior da associa\u00e7\u00e3o de espiras, podemos mudar <\/b><\/span><\/span><\/span>o <\/b><\/span><\/span><\/span>valor do campo magn\u00e9tico (B), da \u00e1rea (S), do \u00e2ngulo de giro (\"\") ou o n\u00famero de espiras (n).<\/b><\/span><\/span><\/span><\/p>\n

Mas o <\/b><\/span><\/span><\/span>enunciado pede tamb\u00e9m que, al\u00e9m de dobrar \"\"<\/b><\/span><\/span><\/span> <\/b>deve-<\/b><\/span><\/span><\/span>se <\/b><\/span><\/span><\/span>manter constante<\/b><\/span><\/span><\/span> a corrente de <\/b><\/span><\/span><\/span>curto circuito <\/b><\/span><\/span><\/span>i<\/b><\/span><\/span><\/span>cc<\/b><\/span><\/span><\/sub><\/span> <\/b>que \u00e9 fornecida por <\/b><\/span><\/span><\/span>i<\/b><\/span><\/span><\/span>cc<\/b><\/span><\/span><\/sub><\/span> = \"\" , <\/b><\/span><\/span><\/span>onde <\/b><\/span><\/span><\/span>r \u00e9 a resist\u00eancia interna devida \u00e0s n espiras<\/b><\/span><\/span><\/span>, o que <\/b><\/span><\/span><\/span>s\u00f3 pode acontecer se voc\u00ea dobrar r.<\/b><\/span><\/span><\/span><\/p>\n

Mas, pela <\/b><\/span><\/span><\/span>segunda lei de Ohm a resist\u00eancia r <\/b><\/span><\/span><\/span>\u00e9 fornecida por:<\/b><\/span><\/span><\/span><\/p>\n

\"\"<\/p>\n

Como o <\/b><\/span><\/span><\/span>exerc\u00edcio<\/b><\/span><\/span><\/span> pede <\/b><\/span><\/span><\/span>o dobro da d.d.p. induzida\"\", sem mudar a corrente de curto-circuito i<\/b><\/span><\/span><\/span>cc<\/b><\/span><\/span><\/sub><\/span>, <\/b><\/span><\/span><\/span>al\u00e9m de <\/b><\/span><\/span><\/span>dobrarmos o fluxo \"\", <\/b><\/span><\/span><\/span>devemos <\/b><\/span><\/span><\/span>dobrar tamb\u00e9m a resist\u00eancia r do material. <\/b><\/span><\/span><\/span>Isso pode acontecer <\/b><\/span><\/span><\/span>atrav\u00e9s da mudan\u00e7a do material \"\", do comprimento L ou da \u00e1rea S de se\u00e7\u00e3o do fio.<\/b><\/span><\/span><\/span><\/p>\n

A <\/b><\/span><\/span><\/span>\u00fanica op\u00e7\u00e3o que satisfaz <\/b><\/span><\/span><\/span>as <\/b><\/span><\/span><\/span>duas condi\u00e7\u00f5es ao mesmo tempo \u00e9 dobrar o n\u00famero n de espiras, <\/b><\/span><\/span><\/span>o que <\/b><\/span><\/span><\/span>dobra a d.d.p. e a resist\u00eancia do material ao mesmo tempo, <\/b><\/span><\/span><\/span>e que mant\u00e9m <\/b><\/span><\/span><\/span>a corrente de curto inalterada.<\/b><\/span><\/span><\/span><\/p>\n

R- A<\/b><\/span><\/span><\/span><\/p>\n

16-<\/b><\/span><\/span><\/span><\/p>\n

\"\"<\/p>\n

17-<\/b><\/span><\/span><\/span><\/p>\n

\"\"<\/p>\n

\"\"<\/p>\n

Se <\/b><\/span><\/span><\/span>at\u00e9 o encontro A se moveu durante 40 s <\/b><\/span><\/span><\/span>e <\/b><\/span><\/span><\/span>B durante 50s, <\/b><\/span><\/span><\/span>ent\u00e3o <\/b><\/span><\/span><\/span>B partiu 10 s depois de A.<\/b><\/span><\/span><\/span><\/p>\n

R- B<\/b><\/span><\/span><\/span><\/p>\n

Ou, <\/b><\/span><\/span><\/span>simples racioc\u00ednio: <\/b><\/span><\/span><\/span>Se, quando eles se <\/b><\/span><\/span><\/span>encontram A se moveu durante 40 s, <\/b><\/span><\/span><\/span>ent\u00e3o com a <\/b><\/span><\/span><\/span>mesma velocidade, B se moveu durante <\/b><\/span><\/span><\/span>(90 \u2013 40 = <\/b><\/span><\/span><\/span>50 s<\/b><\/span><\/span><\/span>) o que significa que <\/b><\/span><\/span><\/span>B partiu 10 segundos depois de A<\/b><\/span><\/span><\/span><\/p>\n

18-<\/b><\/span><\/span><\/span><\/p>\n

Basta <\/b><\/span><\/span><\/span>prestar aten\u00e7\u00e3o no enunciado <\/b><\/span><\/span><\/span>que afirma que o <\/b><\/span><\/span><\/span>n\u00famero de voltas efetuadas pela roda traseira a cada pedalada \u00e9 calculado dividindo-se a quantidade de dentes da coroa pela quantidade de dentes da catraca <\/b><\/span><\/span><\/span>e, fazendo isso voc\u00ea <\/b><\/span><\/span><\/span>obt\u00e9m a tabela abaixo:<\/b><\/span><\/span><\/span><\/p>\n

\"\"<\/p>\n

Voc\u00ea <\/b><\/span><\/span><\/span>deve escolher a menor raz\u00e3o<\/b><\/span><\/span><\/span> que \u00e9 aquela em que <\/b><\/span><\/span><\/span>a roda traseira percorre a menor dist\u00e2ncia para fazer o percurso o mais devagar poss\u00edvel, fornecida pela alternativa IV.<\/b><\/span><\/span><\/span><\/p>\n

R – D<\/b><\/span><\/span><\/span><\/p>\n

19-<\/b><\/span><\/span><\/span><\/p>\n

\"\"<\/p>\n

20-<\/b><\/span><\/span><\/span><\/p>\n

\"\"<\/p>\n

21-<\/b><\/span><\/span><\/span><\/p>\n

Trata-se de um <\/b><\/span><\/span><\/span>movimento circular uniforme (MCU) cuja equa\u00e7\u00e3o da velocidade est\u00e1 definida abaixo:<\/b><\/span><\/span><\/span><\/p>\n

\"\"Como o <\/b><\/span><\/span><\/span>piv\u00f4 deve efetuar uma volta completa a cada 25 h <\/b><\/span><\/span><\/span>o <\/b><\/span><\/span><\/span>per\u00edodo do piv\u00f4 e de cada uma das torres que o acompanha em seu movimento \u00e9 de T<\/b><\/span><\/span><\/span>1<\/b><\/span><\/span><\/sub><\/span> = T<\/b><\/span><\/span><\/span>2<\/b><\/span><\/span><\/sub><\/span> = T<\/b><\/span><\/span><\/span>3<\/b><\/span><\/span><\/sub><\/span> = 25h.<\/b><\/span><\/span><\/span><\/p>\n

Se as dist\u00e2ncias entre torres consecutivas bem como da base \u00e0 torre T<\/b><\/span><\/span><\/span>1<\/b><\/span><\/span><\/sub><\/span>\u00a0s\u00e3o iguais a 50 m, voc\u00ea tem R<\/b><\/span><\/span><\/span>1<\/b><\/span><\/span><\/sub><\/span> = 50m, R<\/b><\/span><\/span><\/span>2<\/b><\/span><\/span><\/sub><\/span> = 100 m e R<\/b><\/span><\/span><\/span>3<\/b><\/span><\/span><\/sub><\/span> = 150m.<\/b><\/span><\/span><\/span><\/p>\n

Torre 1 \"\" V<\/b><\/span><\/span><\/span>1<\/b><\/span><\/span><\/sub><\/span> = \"\"<\/b><\/span><\/span><\/span> <\/b>=<\/b><\/span><\/span><\/span> \"\" <\/b><\/span><\/span><\/span>=<\/b><\/span><\/span><\/span> \"\"<\/b><\/span><\/span><\/span> \"\" <\/b><\/span><\/span><\/span>V<\/b><\/span><\/span><\/span>1<\/b><\/span><\/span><\/sub><\/span> = 12 m\/h.<\/b><\/span><\/span><\/span><\/p>\n

Torre 2 \"\" V<\/b><\/span><\/span><\/span>2<\/b><\/span><\/span><\/sub><\/span> = \"\"<\/b><\/span><\/span><\/span> <\/b>=<\/b><\/span><\/span><\/span> \"\" <\/b><\/span><\/span><\/span>=<\/b><\/span><\/span><\/span> \"\"<\/b><\/span><\/span><\/span> \"\" <\/b><\/span><\/span><\/span>V<\/b><\/span><\/span><\/span>1<\/b><\/span><\/span><\/sub><\/span> = 24 m\/h.<\/b><\/span><\/span><\/span><\/p>\n

Torre 3 \"\" V<\/b><\/span><\/span><\/span>1<\/b><\/span><\/span><\/sub><\/span> = \"\"<\/b><\/span><\/span><\/span> <\/b>=<\/b><\/span><\/span><\/span> \"\" <\/b><\/span><\/span><\/span>=<\/b><\/span><\/span><\/span> \"\"<\/b><\/span><\/span><\/span> \"\" <\/b><\/span><\/span><\/span>V<\/b><\/span><\/span><\/span>1<\/b><\/span><\/span><\/sub><\/span> = 36 m\/h.<\/b><\/span><\/span><\/span><\/p>\n

R- A<\/b><\/span><\/span><\/span><\/p>\n

22-<\/b><\/span><\/span><\/span><\/p>\n

Observe no <\/b><\/span><\/span><\/span>gr\u00e1fico <\/b><\/span><\/span><\/span>que o <\/b><\/span><\/span><\/span>reservat\u00f3rio A (bolinha) <\/b><\/span><\/span><\/span>entre <\/b><\/span><\/span><\/span>8 h e 9 h possui volume de 30 000 L <\/b><\/span><\/span><\/span>e que o<\/b><\/span><\/span><\/span><\/p>\n

\"\"<\/p>\n

reservat\u00f3rio B do lado direito (setinha) <\/b><\/span><\/span><\/span>tamb\u00e9m entre <\/b><\/span><\/span><\/span>8 h e 9 h possui volume de 30 000 L.<\/b><\/span><\/span><\/span><\/p>\n

Ent\u00e3o, <\/b><\/span><\/span><\/span>eles cont\u00e9m a mesma quantidade de \u00e1gua durante<\/b><\/span><\/span><\/span> (9 \u2013 8) = <\/b><\/span><\/span><\/span>1 h.<\/b><\/span><\/span><\/span><\/p>\n

R- A<\/b><\/span><\/span><\/span><\/p>\n

 <\/p>\n

Voltar para os exerc\u00edcios<\/span><\/a><\/h3>\n","protected":false},"excerpt":{"rendered":"

Resolu\u00e7\u00e3o comentada das quest\u00f5es de F\u00cdSICA (e de algumas de Matem\u00e1tica relacionadas com F\u00edsica) do ENEM-MEC-2017 01- Como, pelo enunciado, numa colis\u00e3o frontal a for\u00e7a que o cinto de seguran\u00e7a exerce sobre o t\u00f3rax e abd\u00f4men do motorista pode causar les\u00f5es graves nos \u00f3rg\u00e3os internos, a inten\u00e7\u00e3o \u00e9 diminuir essa for\u00e7a de maneira que o cinto n\u00e3o agrida o motorista,<\/p>\n","protected":false},"author":1,"featured_media":0,"parent":3849,"menu_order":0,"comment_status":"closed","ping_status":"closed","template":"","meta":{"om_disable_all_campaigns":false,"_monsterinsights_skip_tracking":false,"_monsterinsights_sitenote_active":false,"_monsterinsights_sitenote_note":"","_monsterinsights_sitenote_category":0,"footnotes":""},"class_list":["post-3851","page","type-page","status-publish","hentry"],"aioseo_notices":[],"_links":{"self":[{"href":"https:\/\/fisicaevestibular.com.br\/novo\/wp-json\/wp\/v2\/pages\/3851","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/fisicaevestibular.com.br\/novo\/wp-json\/wp\/v2\/pages"}],"about":[{"href":"https:\/\/fisicaevestibular.com.br\/novo\/wp-json\/wp\/v2\/types\/page"}],"author":[{"embeddable":true,"href":"https:\/\/fisicaevestibular.com.br\/novo\/wp-json\/wp\/v2\/users\/1"}],"replies":[{"embeddable":true,"href":"https:\/\/fisicaevestibular.com.br\/novo\/wp-json\/wp\/v2\/comments?post=3851"}],"version-history":[{"count":3,"href":"https:\/\/fisicaevestibular.com.br\/novo\/wp-json\/wp\/v2\/pages\/3851\/revisions"}],"predecessor-version":[{"id":3856,"href":"https:\/\/fisicaevestibular.com.br\/novo\/wp-json\/wp\/v2\/pages\/3851\/revisions\/3856"}],"up":[{"embeddable":true,"href":"https:\/\/fisicaevestibular.com.br\/novo\/wp-json\/wp\/v2\/pages\/3849"}],"wp:attachment":[{"href":"https:\/\/fisicaevestibular.com.br\/novo\/wp-json\/wp\/v2\/media?parent=3851"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}