The normal force is directed perpendicular to the surface. The force between two solids in contact that prevents them from occupying the same space. An object's weight is directed down, toward the center of the gravitating body like the Earth or moon, for example. The force of gravity acting on an object due to its mass. How about we build up the concept of force with real world examples? Here we go… Nearly everyone on the planet knows what distance and time mean. No real attempt was made to define either of these quantities formally in this book (so far) and none was needed. The two quantities that play this role in kinematics are distance and time. Each new topic begins with elemental concepts, called axioms, that are so simple that they cannot be made any simpler or are so generally well understood that an explanation would not help people to understand them any better. What is "a push or a pull"? How would you measure such a thing? Most importantly, how does "a push or a pull" relate to the other quantities already defined in this book?
This is a reasonable informal definition to help you conceptualize a force, but it is a terrible operational definition. Many introductory textbooks often define a force as "a push or a pull". The mathematical description of motion that includes these quantities is called dynamics.
It is now time to expand our studies to include the quantities that affect motion - mass and force.
With the exception of falling bodies and projectiles (which involve some mysterious thing called gravity) the factors affecting this motion were never discussed. The first chapter of this book dealt with the topic of kinematics - the mathematical description of motion.
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