a 70 kg person does a bungee jump from a bridge and falls straight down. the natural length of the bungee cord is 20 m and it has a spring constant k of 80 n/m. when the bungee cord has a total length of 25 m, what is the sign and magnitude of the y-component of acceleration of the jumper in units of m/s2 ? assume that the y direction is up and use g