Take any ol mass. Something uniformly shaped with a hole in it works best, like a disk or something. Tie a piece of thin cord to it through the hole (if it has a hole, heh) and tie the other end to a crossbar or something from which it can hang freely. The idea is to make a pendulum.

OK, now carefully measure the length of the cord from the knot where it ties to the crossbar to the middle of the mass which is attached to it. Either measure metrically, or convert to meters. (2.5 cm = 1 in. ~ 100 cm = 1 meter).

Now, you are going to find the acceleration of gravity on earth. (Ie: the rate objects fall to the earth due to gravity, the reason they all would fall at the same rate if there were no atmosphere). You need to clock how long it takes the pendulum to make one full oscillation (ie: swing forward and then back to where you let it go). To do this accurately, time how long it takes for 25 full oscillations, and then divide that time by 25. That gives you average time for one oscillation.

To be more accurate, change the length of your cord, and do it again. The more you do, the better average value you'll get.

OK, you're swinging the pendulum, and you know the length of its cord. Once you get your time for one oscillation, plug into this formula:

g = ((4)(pi-squared)(L))/T-squared

pi = 3.14; L = cord length; T = time for one oscillation (square this)

OK, now you have found the value of g, or acceleration due to gravity of the earth.

Take that value for g, and plug it into this formula:

M = ((g)(R-squared))/G

M = mass of the earth; g = the value you got by swinging the pendulum; R = radius of the earth (approx 6700km -- convert to meters); and G is the gravitational constant: 6.67 x 10 (superscript neg 11) N m-sq/kg-sq

It's not as hard as it looks, if anyone tries it, be sure to use scientific notation in both the numerator and the denominator in the final equation, or it won't turn out right. Don't forget to square where things need to be squared. Oh yes, N (a unit of measure called a newton) = (kg)(m)/s-squared; you have to know this to get your final unit right on the mass of the earth.