According to Sir Isaac Newton, the force of an entity equals its mass, multiplied by acceleration. This basic principle is what is used to calculate load force, which is the force that opposes that entity. Any time one does work, such as lifting a coffee mug off a table or pushing a ball up a hill, energy is transferred from the entity to the object, causing a desired effect. The object's mass is the resistance acted uponits load force.
Things You'll Need
 Balance

Pick something upreally, anything. It has a mass that remains constant no matter where you go (even in the vacuum of space). The truth is that everything has a mass, and an object at rest has an acceleration of zero meters/second.

Calculate the mass of the object to be moved with a balance. A balance is the only way to get an accurate measure of an object's mass; the standard unit for mass is the gram.

Apply Sir Isaac Newton's formula: force = mass x acceleration. Since we now know the acceleration (0) and the mass (1), the force of an object at rest has a force of 0 newtons. However, it still has potential energy.

Multiply the mass of the object by the gravitational acceleration of the earth (9.8 m/sec2), and the height in meters. This equation is the object at rest's potential energy. Potential energy is measured in joules; this is the load force.

Imagine a box on the floor, of which the weight is unknown. Measure the mass of the box on a balance, and say it weighs 5 kilograms. Because the box is stationary, it has no acceleration, and thus no load force. Once the box is lifted off the ground at any distance, it now has potential energy in addition to its mass. If the box is lifted to a height of 1 meter, we apply the formula: 5 (mass) x 9.8 (gravitational acceleration of the earth) x 1 (height) = 49 joules of energy. This means that 49 joules of energy are required to lift the box to a height of 1 meter, and the force at which the box is pushing down on you is equal and opposing (49 joules).
Tips & Warnings
 An object in motion (a ball rolling down a hill) has kinetic energy, and thus an acceleration (meters/second); therefore, the potential energy need not be calculated, and the load force can be measured in the first three steps.
References
 Photo Credit apple on scale (focus on apple) image by Pontus Edenberg from Fotolia.com