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 Fundamental Forces Every day, we interact with the fundamental forces of science. These are known as gravity, electromagnetism, the strong force, and the weak force. The strong and weak forces work at only very small distances of less than a trillionth of a centimeter, and typically play their role inside the nucleus of an atom. The other two forces are much more common in our everyday lives. 
We experience electromagnetic forces when our hair stands up after combing it. The comb removes electrons from the hair strands, and all of the hair becomes positively charged. Strands repel each other, and the hair appears to stand up. Because all of the hair has the same charge, it repels. Since your hair is positively charged and the comb is negatively charged, you will find that your hair is attracted to the comb. Electromagnetism is a force, either attractional or repulsive, between all charged particles in the universe. Gravity is the force with which we interact most frequently. As you are walking down the sidewalk, gravity is the force that keeps you on the ground. When you're lying in your bed, gravity prevents you from floating up into the ceiling. In our lives, the strongest gravitational force is our pull towards Earth. Although many other planets are more massive, none has as much attractional force as Earth because they are too far away to make much of a difference. Gravity is an attractional force between all matter with mass in the universe. We can calculate the force of gravity and of electromagnetism by knowing the mass or charge of the two objects and their distance. First, the force of gravity on two objects is defined by the following equation: 
where G is the universal constant for gravity, m1 and m2 are the masses in kilograms for the objects, and d is the distance in meters between them. G is defined to be: 
In the value above, N represents Newtons, a unit of force where 1 N = 1 (kg * m) / (s^2). The value of G was calculated by Henry Cavendish in 1798. Even though the force of gravity between most objects we can experiment with is very small, Cavendish designed a method for detecting and calculating this value (Trefil and Hazen, 46). You can read more about his experiment from this page. Now, we have enough information to calculate the gravitational force between two protons spaced 1nm apart. The mass of a proton is about 1.6726e-27 kg, and 1nm = 1e-9 m. Applying the formula:  
With these formulas, you can calculate the gravitational force between any two objects. In the case of these two protons, the force is very small. In everyday life, the force of the Earth is really the only gravitational force we experience. Similarly, we can calculate the force of electromagnetism between the two protons. Because they both have the same charge, they will repel each other. Charges are often measured in Coulombs. The charge of one proton is 1.6e-19 C (Trefil and Hazen, A-17). The formula for calculating electromagnetic force is very similar to that for gravity:  
We can use this information to calculate the electromagnetic force pushing the two protons apart. 
By comparing the two force values that we calculated from the two protons, the electromagnetic repulsion is 1,235,000,000,000,000,000,000,000,000,000,000,000 (1.235e36) times greater than the gravitational attraction. That's a lot by any measure! Even though the force of gravity seems most prevalent in our lives, electromagnetic forces play an even stronger role at the subatomic level. Adjust the slider below to change the distance between the two particles. Observe how the forces compare at different distances. Click here to view the source code.
© 2008 Michael F. Robbins |