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June 2, 2022 at 3:41 pm #8524
恭喜以上的同學答對,是因為空氣振動的頻率。頻率越大,我們聽到的聲音就越高,反之亦然。
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May 26, 2022 at 5:39 pm #8519
答案: 這是與浮力作用有關。當一個物體的重力小於浮力時,它就能漂浮在水面,相反就會沉下去。潛水艇設計是有兩層外殼,內外殼之間有水艙。潛水艇透過加水和排水控制自身的重量, 若要潛水艇浮上水面,就排水;若要潛水艇下沉,就加水。這樣潛水艇便可以浮沉自如了。
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May 23, 2022 at 10:52 am #8516
The previous answer is that temperature causes star flashes.
There is another reason, which is related to the atmosphere of the earth. Since light has the property of bending through different substances, this is the refraction of light. When the air pressure in the atmosphere changes, resulting in a change in the density of the atmosphere, light refraction occurs. The stronger the wind, the more often the refraction of light occurs. Star twinkling is a phenomenon caused by differences in atmospheric density when light reflected or projected by stars passes through the atmosphere. Therefore, stars do not actually twinkle, and stars other than stars do not have the ability to emit light.
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May 12, 2022 at 4:01 pm #8510
Answer:
Cacti grow in hot deserts. In order to adapt to the arid environment, their leaves gradually degenerate into needle-like shapes to prevent the water from being easily lost in the body. Moreover, water sources are very scarce in deserts, because the stems of cacti can store a lot of water and become deserts. The thorns of the cactus also protect themselves and prevent them from being eaten by beasts. -
May 5, 2022 at 11:25 am #8507
答案公佈:
貓要捉老鼠因為貓是夜行動物,為了在夜間能看清事物,它們的身體裡需要一種叫作牛黃酸的物質。正好老鼠體內的牛磺酸含量十分豐富。其實幾乎所有的動物身體中都含有牛磺酸,但老鼠體內的含量相對較多,並且易於抓捕,所以,老鼠成了貓最愛的美食。如果貓長期不吃老鼠,夜視能力就會下降,甚至會逐漸喪失夜間活動能力。此外,牛磺酸還有利於貓咪繁育後代,貓媽媽補充牛磺酸後,生下的小貓會更健康。
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April 28, 2022 at 5:10 pm #8492
答案公佈:
不倒翁不會倒的原因是因為它上輕下重,底部有較重的鐵塊,重心很低;另一方面, 它的底面大而圓滑,當不倒翁向一邊傾斜時,由於支點(不倒翁和桌面的接觸點)發生變動,重心和支點就不在同一條鉛垂線上,這時候,不倒翁在重力的作用下會繞支點擺動,直到恢復正常的位置。不倒翁傾斜的程度越大,重心離開支點的水平距離就越大,重力產生的擺動效果也越大,使它恢復到原位的趨勢也就越顯著,所以不倒翁是永遠推不倒的。
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April 25, 2022 at 3:32 pm #8487
答案公佈: 雪是從大氣中的水蒸氣直接凝結而成。 雲中的低溫使得水蒸氣結成冰晶,當氣溫夠低時,冰晶落到地面仍是雪花時,就是下雪了。 雪形成的條件是,大氣中需含冷的冰晶核,充分的水氣,以及氣溫在0℃(冰點)以下。
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April 14, 2022 at 5:09 pm #8480
公佈答案!!!
酒精的學名叫做乙醇(C2H5OH),它具有很強的滲透力,能夠鑽入細菌內部,使菌體蛋白質凝固(化學上叫做變性),造成細菌因失去活性而死亡。
而單獨的純酒精或純水都不能使蛋白質變性,只有當水和酒精同時存在時,酒精溶液才顯示出使蛋白質變性的能力,使蛋白質失去活性,才能將細菌消滅。因此,只有一定濃度的酒精溶液,才能達到良好的消毒殺菌目的。
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