Hello folks, here is a problem: The effciency of a wheel and axle system is 80% and the ratio of radius of wheel of axle is 4:1. In other to lift a mass of 20 kg, the effort required is A.60N B.62.5N C.32.5N D.250N E.50N I started by bringing out the data in the question: Effiency, E.f=80%. Radius of bigger wheel, WR=4. Radius of axle of smaller circumfrence wr=1 since their are in the ratio 4:1. Mass lifted=20kg. Using Velocity ratio, V.R= distance moved by effort/distance moved by load =circumfrence of bigger wheel of R/circumfrenc of axle of smaller radius r =2piR/2pir=R/r. But in the absence of friction, V.R, R/r =mechanical advantage, M.A. putting my data in equation: R/r=load, L/effort, E=M.A. They only give the mass of the object lifted as 20kg. There is no instruction that acceleration due gravity, g which is 10ms-2 should be used. But knowing that load and effort has it units in Newton, N and that force, F=mass, m times g I decided to use it. Thus: R/r= L/E 4/1=20*10/E 4E=200N E=200/4=50N. You can see clearly that the effort required is 50N which is option E. But the book gives the answer as B. i.e 62.5N. .Can this be true? If it is true then how?