# Interlude 1 - The Main Question: How do things move?

The main topic that is typically chosen for a first physics class is motion:

How do things move?

There are lots of good reasons for choosing this question.  The reason that motion is covered first in traditional physics courses is that it sets the framework for what you need to pay attention to in order to describe, understand, and predict how things move. This builds a strong mathematical model of motion, introducing and mathematizing such basic concepts as position, velocity, acceleration, force, and energy. The fundamental conceptual idea is to describe how position changes. The mathematical structures to describe rates of change come from calculus -- derivatives and integrals.

Motion also deserves the top spot among topics among fundamentals of physics for biologists.  One obvious reason is that for biological organisms, critical goals are to be able to get food, escape predators, and reproduce. Although some organisms take a fixed position for most of their lives, most animals and protists move -- a lot -- and even those that don't depend heavily on the motion of external fluids.  To describe these phenomena quantitatively, we need to understand the physics of motion.  Measuring motion allows you to understand the costs and limiting factors for motion for many organisms.

A second reason that is not so obvious is that for any organism, even for organisms that stay fixed for most of their life (such as plants), there is a lot of motion going on at the cellular scale.  During the development of multi-cellular organisms, cells have to rearrange and organize into functioning larger structures. In trees, for example, cells arrange themselves so as to use physical principles to carry fluids and nutrients to great heights. This kind of motion is tightly controlled with all sorts of biological and chemical signals but it also is controlled by the basic physical laws of motion.

A third reason is that at the microscopic level, inside a cell, the basic chemicals of life are in constant motion. Some of the motions are actively guided by the cell; others rely on random motion from the interaction with thermal energy. These molecular interactions and motions are controlled by forces and the physical laws of motion.

To analyze motion we have to answer a number of questions.  These will be the broad topics that take us through the first term.

1. How can we describe motion? (Kinematics)
To do this, we have to specify where something is and when. This leads us to consider the fundamental issues of measurement, rate of change, and multiple representations -- all basic and useful things to understand and learn to use.
2. What is responsible for motion? (Dynamics)
In our study of dynamics we will be exploring a number of questions.
• How do objects move and change their motions? This leads us to understand the fundamental issues of inertia (mass) and forces (things that tend to change an object's motion).
• What forces are there and what are their properties? This will lead us to study ways of describing forces -- spring forces, gravity, electrical forces, and resistive forces as objects move in fluids (air, water, ...). These forces will play critical roles in understanding mechanisms in biological systems.
• What useful tools are there to describe and think about motion? While forces, velocities, and accelerations are basic in developing the concepts of the fundamental laws of motion, additional concepts such as energy, momentum, and angular momentum can simplify complex motion problems because they turn out to satisfy conservation laws. Energy is a particularly important concept in biology and will become of critical interest in the rest of the class.

In our first descriptions of motion, we will make a number of simplifications so as to be able to get our heads around the basic physics before we make things complicated.  So we will be modeling objects as "point masses" (that is, objects whose size and shape are irrelevant) or as "rigid bodies" (objects whose size and shape matter, but whose internal structure is irrelevant). When we are pretty comfortable with the basic description and laws of motion, we will turn to objects that can change shape.

1. How do real objects that can deform behave?  This includes solids that can bend and break, and fluids, like liquids and gases. Applying our physical principles to the parts of an object helps us develop insights into how fluids flow and bones break, and also how DNA twists or how much the scaffolding in a cell can bend.

As we go down into the structure of matter, we learn that a phenomenon that seems unrelated to motion -- heat and temperature -- is in fact about the motion of the molecules a substance is made up of.  And we learn that it arises from the random motion of the internal parts of matter. This result is of primary importance in understanding biological mechanisms at the cellular and sub-cellular levels. So we will spend a significant amount of time answering the fourth question.

1. How do you get a net movement from random motion? That you can do this is a bit surprising. The conversion of random thermal motion into net movement leads to a wide range of results producing phenomena such as diffusion, osmosis, heat flow, and Brownian motion. Even the apparent "direct flow" of water in a pipe or electric currents are strongly affected by random motion of their microscopic components (water molecules or electrons respectively).  At the molecular scale, the "engines of life"  - molecular motors that are present in many forms inside cells - all operate on this basis.  Therefore the interplay of random thermal motion and motion caused by "explicit" forces (such as gravity or electricity) is of great importance.

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