1. The problem statement, all variables and given/known data The average orbital radii about the Sun of the Earth and Mars are 1.5*11^11 m and 2.3*11^11 m respectively. How many (Earth) years does it take Mars to complete its orbit? Answer: 1.9 years. 2. Relevant equations F = G * ((m1*m2) / r^2) F = m*(w^2)*r T^2 = (4Pi^2 / G*m)*r^3; m = m sun g = (4Pi^2*rm^3)/(T^2*re^2); rm = radius Moon, re = radius Earth. 3. The attempt at a solution I get the answer using Kepler's third law. http://www.studyphysics.ca/newnotes/20/unit02_circulargravitation/chp08_space/lesson34.htm K = T^2 / r^3 T^2 = K * r^3 T^2 = 3.95*10^-29 * 2.3*10^11 = ans T = ans^1/2 T = 693.25 days -> 1.899 years = 1.9 years. But I can't get the required answer using book formulas (A-Level Physics). E.g.: using the last equation I got (4*(pi^2)*((2.3*10^11 m)^3))/((9.8 ms^-1*((1.5*10^11 m)^2))^1/2)=1.02291 × 10^24 and if it is seconds -> 32.436 quadrillion years... Not even close to the 1.9 years answer. Any suggestions? Thank you in advance.