ANSWERS: 1
  • No, no, no. 15 N force? Are you kidding? That's not enough to draw a bow! Medieval longbows had a draw weight of at least 350 N, often above 500 N. 0.25m? That's ridiculous. Draw weight is measured at 70 cm or more. Also, 150 grams is far too heavy for an arrow from that period. The heftiest war arrows were just barely over 100 grams, but target arrows were typically only 10-15 grams. A legendary archer such as Robin Hood would have never made such a wimpy attempt to shoot an arrow. Let's say, instead, that Robin used a proper pull of 400 N to a distance of 0.70 m with an arrow with a mass of 0.020 kg. I would calculate the energy used to draw the bowstring as the dot product of the force and the distance. E = 400 N (dot) 0.70 m = 280 J. The, I would use the definition of kinetic energy E = 1/2 m v^2, and solve for v. 280 J = 1/2 0.020 kg v^2, so v = sqrt(2 x 280 J / 0.020 kg) = 167 m/s. Oh yeah, that's much more realistic, of course, there are actually some losses due to friction between the arrow and the string, between the arrow and the air, especially between the string and the air and internally inside of the bow as it flexes, so the actual answer would be something slightly less than that. But, Robin Hood was a badass, so I like this answer much better! Now repeat exactly what I did for calculations, but with the silly made-up numbers your teacher gave you, and you'll see that your teacher's arrow wouldn't catch grandma if she had a 2 m head start. In fact, even if grandma didn't move, your teacher's arrow would have a tough time hitting her above the knee from 2 m away LOL.
    • Linda Joy
      LOL!! I wish I'd known you when I was in school! I'd have submitted this to my teacher! With the correct answer at the bottom, of course! Thanks, I needed that!

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