here's the question (from mastering physics): "The weight of a person at the equator, as determined by a spring balance, is 750 N. By how much does this differ from the true force of gravitational attraction at the same point? Assume that the earth is spherically symmetric." Now, i would assume what needs to be done is first finding the mass of the person (750 N / 9.8 m/s^2). then plugging that into the equation: F = (G*m*M_e)/(R_e^2), using M_e = 5.92*10^24 kg and R_e = 6.38*10^6 m Then, subtracting that from 750 N. I did this, and came out with 3 different answers depending on my value for acceleration (9.8 or 9.81) and how many figures i used from that figured mass. They were 1.62, 2.09, and 1.32. Mastering Physics told me those were all wrong. ...what?