Nails hold in wood,and screws hold in either wood or metal by reason of friction,without which they would be useless.

 
  • 釘子能釘住木頭,螺絲能擰住木頭或金屬,都是由于有摩擦力,如果沒(méi)有摩擦力,它們就毫無(wú)用處。
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