Units
Paying attention to units is the most important rule for understanding, solving and checking that physics problem has been solved correctly. Always check your units.
Units are definite magnitude of a physical quantity.
Distance can be measured in inches (in), feet (ft), miles (mi), meters (m) etc.
Time can be measured in units of seconds (sec), hours (hr), days (day), years(yr) etc.
Mass can be measured in kilograms (kg), pounds (lb) etc.
You cannot mix units from different system to solve problems. Convert all the units in a given problem into a single set of units before solving a problem.
Mechanics:
Fundamental to Mechanics are: Distance, Time, and mass.
All of mechanics involves taking the fundamental concepts of distance, time and weight and manipulating using ratios.
Motion:
The chapter is about motion, and more specifically about describing how things move. The first thing you need to know about motion is that as a concept, it only makes sense to talk about things moving with respect to something else. Typically in mechanics motion is relative to the earth, but is can be relative to anything. A car can move relative to the ground, or to another car.
Speed is the ratio of the changes in two fundamental physics concepts: distance and time. If we use the metric system, the distance can be measured in meters (m) and time in seconds (sec), and the ratio of the two is expressed in meters per second. It could also be expressed in feet per second or miles per hour. In mechanics and math generally, whenever you hear or read the work “per”, it always means to divide by, so speed is distance divided by time.
Velocity is a special kind of speed. It is speed in a straight line, or speed in a particular direction. To understand what velocity means you need to know the speed of an object and its direction.
Acceleration is the ratio of change in velocity to change in time. Taking velocity, which is distance per time in a particular direction, and dividing by the change in time, we see that the units of acceleration are distance per time per time or distance per time squared
Newton’s 1st Law of Motion
Newton’s first laws of motion, also known as Newton’s law of inertia, states that objects at rest will stay at rest unless acted on by an outside force, and objects in motion will continue moving at a constant velocity, that is, at a constants speed and in a straight line, until acted on by an outside force. The tendency to remain still or remain at a constant velocity is described as inertia. Inertia is a measure of mass. Massive objects are harder to move than less massive objects. They are also harder to stop.
Common forces that act on objects and change their direction, velocity, or speed are gravity and friction.
Gravity is a force that acts on all objects containing mass. If you threw a ball straight in from of you and there was no gravity (or other forces) the ball would continue in a straight line forever.
Friction is a force that arises when two massive objects rub against each other. Without friction a hockey puck moving on an ice rink would never slowdown or stop.
It is important to clearly understand the difference between weight and mass. Mass is fundamental and describes the amount of matter an object is composed of. Weight of an object is the gravitational force acting on the object. Mass does not change, but the weight of an object changes depending on gravity. There is less gravity on the moon than there is on earth. That is why objects way less on the moon than they do on earth despite the fact their mass is always the same.
Newton’s 2nd Law of Motion
Forces produce accelerations. Mass resists acceleration. Newton observed these facts and proposed the following relationship know as his second law of motion:
In the international system of units, force is measured in units of Newtons. One Newton is equal to one kg times one meter per second squared, because mass is measured in kilograms and acceleration is measured in meters per second squared