Centrifugal vs. Centripetal

I went to my daughter’s open house at school last Friday and afterwards we played a bit of tether ball before heading home. And while I’m not that great at tether ball, I can explain the forces involved in the game.

   Centrifugal force is what is often used to describe what happens to the ball as it rotates around the pole—it’s being pushed as far away from the pole as possible. But in actuality, centrifugal force is a fictitious force. The only force being applied to the ball, pulling it toward the center of rotation, is a centripetal or center-seeking force. There is nothing actually pulling the ball away from the string, what you have is just inertia as described by Newton in his First Law of Motion: An object at rest remains at rest unless acted upon by a force and an object in motion remains in motion—at a constant velocity—unless acted upon by a force. 
   Newton based his first law on the work of Galileo, who described what he called the Law of Inertia: “A body at rest remains at rest and a body in motion continues to move at constant velocity along a straight line unless acted upon by an external force.” Until Galileo, it was thought that one must exert a force in order to keep an object in motion. Galileo recognized that the reason moving bodies eventually come to rest is because of resistance forces such as friction. Without friction, bodies would continue to move at constant velocity. But I digress…
   So if you were to cut the rope as the ball is rotating what would happen? Some might think that the ball would fly away from the pole, but that’s not correct. The ball would actually move perpendicular to the pole, due to inertia. The centripetal force of the rope works against inertia by keeping the ball from travelling in a straight path. It is this constant struggle against inertia that makes it seem that the ball is trying to move away from the pole. What we call a centrifugal force is actually just the effect of inertia working against the centripetal force. Your welcome.

The Oh-My-God Particle

One of the two Fly's Eye Cosmic Ray Detectors.

The Oh-My-God particle was an ultra-high-energy cosmic ray—most likely a proton—detected on October, 1991 in the skies over western Utah. Its observation by the University of Utah's Fly's Eye Cosmic Ray Detector was a shock to astrophysicists, who estimated its energy to be approximately 50 J. In other words, a subatomic particle with kinetic energy equal to that of a baseball traveling at about 90 kilometers per hour.
   The particle was traveling at almost the speed of light. Assuming it was a proton, its speed was only about 1.5 quadrillionth of a meter per second less than the speed of light. In other words, if it were in a race with a beam of light, the Oh-My-God particle would fall behind only one centimeter every 220,000 years.
   The energy of this particle is some 40 million times that of the highest energy protons that have been produced by the Large Hadron Collider. However, only a small part of this energy would be available for an interaction with another proton or neutron. Most of the energy would remain as kinetic energy. The effective energy available for such an interaction is still 50 times greater than the collision energy of the Large Hadron Collider.
   Applying special relativity to such a fast particle yields some incredible results. Time passes more slowly as velocity increases, and for anyone hypothetically travelling on the back of this particle time would nearly stop. For example, a trip to the Andromeda Galaxy, which is more than two million light years away, would have a perceived travel time of only three and a half minutes. Special relativity also tells us that there is a length contraction in the direction of motion. If the Earth were somehow able to match the speed of the Oh-My-God particle, it would pancake down to a thickness of less than four hundredths of a millimeter!
   The University of Utah experiment relied on two telescopes searching the sky for the characteristic flashes of ultraviolet light that are produced when a cosmic ray collides with a molecule in Earth’s atmosphere and creates a shower of secondary particles. The two telescopes were covered in photomultiplier tubes and looked like the compound eyes of a fly. By capturing almost all the light in the shower, they were able to make a good measurement of the particle’s energy.
   These ultra-high-energy cosmic rays are very rare. Since the first observation, only about fifteen similar events have been recorded to confirm the phenomenon. What cosmic process transforms an ordinary particle into an Oh-My-God particle? A supernova or supermassive black hole might explain it, but when astronomers followed the impact track back to its source they found nothing unusual in that direction.

When the Lights Go Out

One of my readers posed the question: When I turn off the light in my bedroom at night, where does all the light go? Before we answer this, we need to make a few assumptions. Let’s assume that the bedroom is a perfect container: It has no windows for light to escape from and no cracks around the door so light can’t escape there either. Also, let’s say the walls are perfectly solid, and consist of a regular structure of atoms. Imagine a grid of hard spheres laying next to  each other. This is the surface of the walls. 
   First, we need to realize that light is a form of energy. While the light switch is on, it closes an electric circuit and electrons flow through the light bulb. The light bulb converts the energy from the electric current to light energy in the form of photons. Photons are tiny packet of electromagnetic energy and momentum. When you turn the light off, the circuit is broken, the energy stops flowing, and the light goes away. But where does it go?
   The photons travel across the room at the speed of light. When a photon hits the wall, its energy and momentum is either absorbed by the atoms in the wall, or are reflected to another wall where it again may get absorbed or reflected. One of the fundamental concepts of physics tells us that both energy and momentum are conserved, which means that an atom will get a small kick from absorbing a photon. It will move, and kick against its neighbor, etc. If enough photons get absorbed, this will result in the wall warming up slightly. So the light gets converted into thermal energy in the wall. This is what is meant by having a temperature. 
   If the wall were at absolute zero, these atoms do not move and are simply at rest, each one just touching the next. By saying that the wall has a temperature, we are really saying that it contains thermal energy. This thermal energy is the random vibration of the atoms around their equilibrium point. Such a vibration can travel through the grid of atoms in the form of a wave. One atom pushes the next, which pushes the next, etc. 
   When you turn off the light switch, the process just stops—the bulb stops generating photons, and the last set of photons hit walls until they’re all absorbed, all within a fraction of a second.