A 5kg block is placed on a spring balance inside an elevator that starts from rest and moves upward such that its acceleration varies with time according to a(t)=0.5tms−2, where t is the time in seconds measured from the instant the elevator starts.
(i) Obtain an expression for the reading R(t) of the spring balance as a function of time during this upward motion.
(ii) Calculate the reading at t=4s.
(iii) At what instant will the reading become 25% greater than the true weight of the block? Give your answer correct to one decimal place.