The speed of a boat in still water is v. The boat is to make a round trip in a river whose current travels at speed u. Part A Derive a formula for the time needed to make a round trip of total distance D if the boat makes the round trip by moving upstream and back downstream. Express your answer in terms of the variables D, v, and u.

r=rho l/a where a is the area, l is the length and rho is resistivity so the greater the area the lesser the resistance thats why the earth pin is thicker bcos current follow the shortest path, so less resistive .

i don't know that i agree with that but what's the question ?

the skeletons of most mammals reach a certain size and then stop growing. however, many animals, including some mammals, keep growing throughout their lives. kangaroos, for example, just keep growing and growing until they die. also, the longer an animal lives the greater the chance it might catch some deadly disease.