A large number of ants behaving as a liquid, and as a solid.
The job of a physicist is to find simple rules that predict the things that happen in the world around us. From the size of avalanches and forest fires, to the shape of red blood cells, to the flow of traffic in cities, all of these things can be explained by simple rules and principles that govern the dynamics of nature. Once we can explain the world around us, we can attempt to change it by turning these principles around and using them to design things like electric cars, or rockets, or hydroelectric dams. In this gentle introduction to mechanics, you'll be exposed to some interesting phenomena in the world around us, and asked to explain how or why they happen. Although these are elementary examples, they encapsulate the essential experience of doing real physics research. Here are some tips to get started.
- Try to make your question as crisp as you can in your head, and strip it down as much as you can without throwing away essential details.
- If your question overwhelms you, try to simplify it.
- Use your intuition and understanding of other processes to reason about the one at hand.
- Use extreme cases to help rule out incorrect solutions.
When you spin a cup of tea, the leaf particulate at the bottom tends to come together at the center of the cup. Why is that?
The spinning of the liquid in the cup sets up a circular motion. In order to maintain circular motion, there must be a force pushing the liquid toward the center of the cup (think of how when you go around a turn in the car, either the seat belt of the wall of the car strongly pushes you toward the center of the turn). This is accomplished by a gradient of pressure that increases from the center of the cup toward the outer rim. Particles at the bottom of the cup are slowed by the friction of dragging along the surface, and thus the force they feel from the pressure gradient exceeds what's needed to keep them on a circular path. This causes the pressure gradient to push them toward the center of the cup until they reach the middle (where they feel no pressure), where they stop.
One of the best features of physics is the universality of the principles. Mechanisms identified in the dynamics of quantum fields can very well be applied to problems in materials science. Nonetheless, we can broadly classify the kinds of things we'll study in mechanics.
- Particles: Point particles are an essential fiction in mechanics. With good justification, they allow us to represent objects by a point at its center of mass. Though an obvious simplification, particles greatly simplify our mathematical reasoning and end up being a pretty faithful representation. Great progress has been made by characterizing the behavior of particles which can then be applied to extended bodies. Explore the range of particle motions in Newton's laws, rocket physics, and the Lagrangian formulation of mechanics.
- Fields: While particles are familiar to everyday experience, fields are what govern the interaction of particles and provide for the interesting dynamics in the world around us. A simple example is the gravitational field which arises from the intrinsic mass of a given object. When you jump into the air, what brings you back to the ground is the gravitational field of Earth. Remarkable, the same field also governs the motion of the planets. Start to find out more about fields and how they shape the motion of matter in Gravitation, magnetic field lines , deriving Kepler's laws, and charge, and electric fields .
- Conservation laws: Physics would be very hard if all there were was particles and fields. Mathematical analysis often becomes intractable in physics, and it can even be the case that choosing the wrong way to look at a problem will make the difference between it being easy, or impossible. Happily, there are deep principles buttressing the structure of physics which state that certain things must always be conserved. It is often the case that we can shortcut very tedious calculation by appealing to these conservation principles. Get started with conservation of energy, and conservation of momentum.