Fun With Physics

Fundamental Forces

Fundamental Forces

 

 

As you sit in front of your computer reading this article, you may be unaware of the many forces acting upon you. A force is defined as a push or pull that changes an object's state of motion or causes the object to deform. Newton defined a force as anything that caused an object to accelerate -- F = ma, where F is force, m is mass and a is acceleration.

The familiar force of gravity pulls you down into your seat, toward the Earth's center. You feel it as your weight. Why don't you fall through your seat? Well, another force, electromagnetism, holds the atoms of your seat together, preventing your atoms from intruding on those of your seat. Electromagnetic interactions in your computer monitor are also responsible for generating light that allows you to read the screen.

Gravity and electromagnetism are just two of the four fundamental forces of nature, specifically two that you can observe every day. What are the other two, and how do they affect you if you can't see them?

The remaining two forces work at the atomic level, which we never feel, despite being made of atoms. The strong force holds the nucleus together. Lastly, the weak force is responsible for radioactive decay, specifically, beta decay where a neutron within the nucleus changes into a proton and an electron, which is ejected from the nucleus.

Without these fundamental forces, you and all the other matter in the universe would fall apart and float away. Let's look at each fundamental force, what each does, how it was discovered and how it relates to the others.

Gravity

The first force that you ever became aware of was probably gravity. As a toddler, you had to learn to rise up against it and walk. When you stumbled, you immediately felt gravity bring you back down to the floor. Besides giving toddlers trouble, gravity holds the moon, planets,sun, stars and galaxies together in the universe in their respective orbits. It can work over immense distances and has an infinite range.

Isaac Newton envisioned gravity as a pull between any two objects that was directly related to their masses and inversely related to the square of the distance separating them. His law of gravitation enabled mankind to send astronauts to the moon and robotic probes to the outer reaches of our solar system. From 1687 until the early 20th century, Newton's idea of gravity as a "tug-of-war" between any two objects dominated physics.

But one phenomenon that Newton's theories couldn't explain was the peculiar orbit of Mercury. The orbit itself appeared to rotate (also known as precession). This observation frustrated astronomers since the mid-1800s. In 1915, Albert Einstein realized that Newton's laws of motion and gravity didn't apply to objects in high gravity or at high speeds, like the speed of light.

In his general theory of relativity, Albert Einstein envisioned gravity as a distortion of space caused by mass. Imagine that you place a bowling ball in the middle of a rubber sheet. The ball makes a depression in the sheet (a gravity well or gravity field). If you roll a marble toward the ball, it will fall into the depression (be attracted to the ball) and may even circle the ball (orbit) before it hits. Depending upon the speed of the marble, it may escape the depression and pass the ball, but the depression might alter the marble's path. Gravity fields around massive objects like the sun do the same. Einstein derived Newton's law of gravity from his own theory of relativity and showed that Newton's ideas were a special case of relativity, specifically one applying to weak gravity and low speeds.

When considering massive objects (Earth, stars, galaxies), gravity appears to be the most powerful force. However, when you apply gravity to the atomic level, it has little effect because the masses of subatomic particles are so small. On this level, it's actually downgraded to the weakest force.