Introduction to physics/lecture

Subject classification: this is a physics resource.

Resource type: this resource contains a lecture or lecture notes.

Action required: please create Category:Introduction to physics/Lectures and add it to Category:Lectures.

See Introduction to physics (MIT)

The Underpinnings of Physics

In its purest sense, physics is the study of the way matter and energy interact in nature. Since early civilization, humans have sought to describe the workings of the world around them. Physics attempts to predict the outcome of an event by knowing certain conditions beforehand. For example, physics can predict how long it will take for a rock to fall down a well, or how fast a pendulum will swing.

Basic Physical Properties and Concepts

All things in the universe have basic properties that describe the ways they act and interact with other bodies. In order to make precise predictions, we will describe these properties using numerical quantities. For example, we may put a simple coordinate system over the Earth to describe the positions of satellites over time as numerical quantities. A Cartesian coordinate system as you should already be familiar with is not very convenient for this, as the orbits of the satellites are elliptical. A spherical coordinate system is a little more convenient. There are two basic types of quantities that we will at first be concerned with measuring, scalars and vectors. A scalar quantity is one that does not depend on rigid motions (translations/rotations) of the coordinate system we use to describe the physical event, while a vector quantity may be different in different coordinate systems.

Motion is when an object changes position relative to another. In Newtonian physics, all motion is relative to some reference point. For example, two balls sitting next to each other on a moving train are said to be at rest with respect to each other, but are in motion relative to a stationary observer on the ground. This ties in very closely with the next concept.
Velocity describes how fast something is going and further, describes the direction of the object's motion. The difference between velocity and speed is that velocity always answers "how fast and in what direction?" while speed only answers "how fast?". More formally, speed describes the magnitude (hence, you can never have negative speed, even if you are going "backwards") of the velocity of an object. An example of a speed is the reading on a car's speedometer. A velocity might include the reading on the dashboard compass in addition to the speedometer. Velocity is a vector. Speed is a scalar.

A frame of reference is a point of view of a physical situation. Relative motion and rest will change if you change your frame of reference. Even though your chair may seem like it is at rest to you, it is actually rotating around with the earth at several hundred kilometers per hour. If you take another step back in your frame of reference, you will see that even the earth is not stationary; it is rotating around the sun. And the sun is in turn rotating around the center of our galaxy. This may seem confusing, but you will usually be using a very small part of the Earth as your frame of reference; that is, things not moving relative to the ground are said to be "at rest".

An inertial frame of reference is a sort of mental box we draw around a situation where Newton's law of inertia holds true. The importance of using these frames is that all physical events can be described from an inertial frame of reference using Newton's laws. We will learn about these laws in the next lesson.

Acceleration describes the change in velocity, much like velocity describes the change in distance. If an object does not change its speed, and does not change its direction, then its acceleration is 0. Even if the object is decreasing in speed, or decelerating, its loss of speed is still referred to as its acceleration. Note that since acceleration describes the change in velocity, which has a direction associated with it, acceleration also has a direction. This is different from the way the word is used in everyday language, which describes only the magnitude of acceleration. Ie., you hear that a car can go "Zero to sixty in three seconds", not "zero to sixty northwest in three seconds", because in that case, the direction doesn't really matter. In an inertial frame of reference, acceleration is always associated with a force. Acceleration is a vector quantity.

Mass answers the question of "how hard is it to change the motion of this object?". Given the same force, an object with a small mass will accelerate much faster than an object with a large amount of mass. Think of pushing a car versus pushing a shopping cart. The shopping cart (hopefully!) responds to your efforts quite easily, so it must have less mass than the car, which will take a while to get up to the same speed as your shopping cart under a similar effort. Mass is a scalar quantity.

Energy is associated with physical changes. Formally, energy is a force applied over a distance. There are many different forms of energy and they can be converted to other forms, but they all have the ability to cause some type of change. In general, there are two kinds of energy: Potential energy and Kinetic energy. Potential energy is energy that is ready to be released, like a rock on a cliff. Once the rock is pushed off the cliff, it begins to fall and loses potential energy. This loss in potential energy is compensated with a gain in kinetic energy. Once the rock hits the ground, all of the kinetic energy is transferred to the ground, perhaps causing an indentation in the ground or a fracture in the rock. Energy in the universe is always conserved; that is, it never disappears. However, a system can "lose" energy to heat. Energy is a scalar quantity.

Example:

A rock sitting on the top of a hill has potential energy due to being in a gravitational field. When pushed, this energy is converted into other forms. This energy cannot be made into which of the following?

Mass

Yes, Potential Energy cannot be transfered into mass

Kinetic energy

No, Potential Energy can be lost in the form of Kinetic energy

Heat

No, Potential Energy can be lost in the form of Heat

Acoustic vibrations

No, Potential Energy can be lost in the form of Acoustic, or sound, energy.

Topics

Newtonian Physics

Basics of Electricity and Magnetism

Learning resources

Questions

	scalar.
	vector.

	scalar.
	vector.

	scalar.
	vector.