First Law of Motion Second Law of Motion
| F=ma
Mass---------> Inertia |
Time -Space }-----> Velocity--> Acceleration---> Force---> Energy
| |
conservation of momentum <------momentum conservation of Energy
Don't be intimidated by the derivations of formulae in the class or in the textbook. The usual starting point is a definition. A definition represents a short hand of some intutive relationship between physical quantities.
Simple algebra manipulations are used to express the unknown quantity (what we want to find) and what is known (what is given by observation or by the problem in the textbook). Formulae represent relationship between physical quantities, space, time, velocity, acceleration and similar. Physics uses these relationships to predict the behavior of the world around us (at least in simplified situations).
To understand the formulae use pencil and paper when you read your textbook. Don't get frustrated if at first you don't understand. Follow the examples and try to derive the results by yourself. Sometime it is needed some "digestion" time before you get a concept. Learning difficult ideas, like the ones we study in Physics and Mathematics, is more like a spiral motion then a motion in a straight line. Good journey!