No, they are not equivalent. The numerators are equals, but the denominators are not, they are not equivalent. If the first one would be 6/8, they would be equivalent, because we can simplify the fractions by 2 and we got 3/4 again. Good luck !
Logically if you double weight on something it means that you are adding the exact same weight as the first object to the same thing to make it twice the weight as the original, so yes... the weight will double if the length is doubled
