PHYS101: Introduction to Mechanics

Our first step in this course is to gain a basic understanding of the language and analytical techniques that are specific to physics. This unit will include a brief outline of physics and the scientific method, measurement units and scientific notation, and the concepts of significant figures, order-of-magnitude estimates, and scaling.

Our formal study of physics begins with kinematics, which is defined as the study of motion without considering its causes. The word "kinematics” comes from a Greek term meaning "motion.” In this unit, we will study motion without worrying about what forces cause or change it. Such considerations come in later units. In this unit, we will examine the simplest type of motion - namely, motion along a straight line, or one-dimensional motion.

Most motions in nature follow curved paths rather than straight lines. Motion along a curved path on a flat surface or a plane is two-dimensional and thus described by two-dimensional kinematics. Two-dimensional kinematics is a simple extension of the one-dimensional kinematics covered in the previous unit. This simple extension will allow us to apply physics to many more situations, and it will also yield unexpected insights about nature.

The study of motion is kinematics, which describes the way objects move, their velocity, and their acceleration. Dynamicsconsider the forces that affect the motion of moving objects. Newton's laws of motion are the foundation of dynamics. These laws provide examples of the breadth and simplicity of principles under which nature functions. They are also universal laws in that they apply to similar situations on Earth as well as in space.

In this unit, we will study the simplest form of curved motion: uniform circular motion, or motion in a circular path at constant speed. In some ways, this unit is a continuation of the previous unit on dynamics, but we will introduce new concepts such as angular velocity and acceleration, centripetal force, and the force of gravity.

Energy is the capacity of a physical system to perform work. It plays an essential role both in everyday events and in scientific phenomena. You can probably name many forms of energy from that provided by our foods to the energy we use to run our cars to the sunlight that warms us on the beach. Not only does energy have many interesting forms, but it is involved in almost all phenomena and is one of the most important concepts of physics.

Energy can change forms, but it cannot appear from nothing or disappear without a trace. Thus, energy is one of a handful of physical quantities that we say is conserved.

We use the term momentum in various ways in everyday language. We speak of sports teams gaining and maintaining the momentum to win. Generally, momentum implies a tendency to continue on course - to move in the same direction - and is associated with mass and velocity. Momentum has its most important application in analyzing collision problems, and, like energy, is important because it is conserved. Only a few physical quantities are conserved in nature, and studying them yields fundamental insight into how nature works, as we shall see in our study of momentum.

What might desks, bridges, buildings, trees, and mountains have in common - at least in the eyes of a physicist? The answer is that they are ordinarily motionless relative to the Earth. Thus, their acceleration in the Earth frame of reference is zero. Newton's second law states that net F = ma,so the net external force is zero on all stationary objects and for all objects moving at constant velocity. There are forces acting, but they are balanced. That is, the forces are in equilibrium.

Why do tornadoes spin so rapidly? The answer is that the air masses that produce tornadoes are themselves rotating, and when the radii of the air masses decrease, their rate of rotation increases. An ice skater increases her spin in an exactly analogous way. The skater starts her rotation with outstretched limbs and increases her spin by pulling them in toward her body. The same physics describes the spin of a skater and the wrenching force of a tornado. Clearly, force, energy, and power are associated with rotational motion. These and other aspects of rotational motion are covered in this unit. We will see that all important aspects of rotational motion either have already been defined for linear motion or have exact analogs in linear motion.

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