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Newton's second law of motion states that any object will accelerate when acted upon by a net or unbalanced force. In other words, if a force is acting upon an object that is greater than the forces acting in the opposite direction, the object will accelerate in the direction of the larger force. This law can be summed up with the equation F = ma, where F is the force, m is the mass of the object, and a is acceleration. Using this law, we can calculate the force of gravity of any object on the surface of the earth, using the known acceleration due to gravity. On earth, the force of gravity causes objects to accelerate at a rate of 9.8 m/s2. On the earth's surface, we can use the simplified equation Fgrav = mg to calculate the force of gravity. If you want a more exact approximation of force, you can still use the above equation, Fgrav = (GMearthm)/d2 to determine force of gravity. For this particular equation, you must use metric units. The mass of the object needs to be in kilograms (kg) and the acceleration needs to be in meters per second squared (m/s2). You must convert to these units before continuing with the calculation. For smaller objects, you can weigh them on a scale or balance to determine its weight in kilograms (kg). For larger objects, you will have to look-up the approximate mass in a table or online. In physics problems, the mass of the object will generally be provided to you. Once you have defined the variables of your equation, you can plug them in and solve. Be sure that all of your units are in metric and on the right scale. Mass should be in kilograms and distance in meters. Solve the equation using the proper order of operations. Let's use the same equation from above and see how close the approximation is. Determine the force of gravity on a 68 kg person on the surface of the earth. Make sure all your variables have the proper units: m = 68 kg, g = 9.8 m/s2. Write your equation. Fgrav = mg = 68*9.8 = 666 N. With F = mg the force of gravity is 666 N, while using the more exact equation yields a force of 665 N. As you can see, these values are almost identical.
Understand Newton's Second Law of Motion, F = ma. Know the acceleration due to gravity on earth. Use the proper metric units. Determine the mass of the object in question. Solve the equation.