Force – is any action that causes a push or pull.
Units are newtons.
Forces are vector quantities - they have a magnitude and a direction.
A special force called friction acts between different materials when they touch as they move against each other.
Although a surface may seem smooth, as you move closer to the molecular level, bumps and grooves on the surface cause materials to lock into each other, causing friction.
Atomic Force Microscope:
Galileo reasoned (correctly) that only in the presence of friction is a force necessary to keep an object moving:
Newton's first law (Law of Inertia) states:
"Every object continues in a state of rest, or of motion in a straight line at constant speed, until it is compelled to change that state by other forces acting upon it."
Chemists define mass simply as the measure of the amount of material in a substance.
Physicists define mass as the measure of the inertia of an object. That is, the more force that is required to change the motion of an object (and hence its inertia), the greater the mass the object has.
Mass vs. Weight
Weight is the force of gravity acting on an object - will be discussed in one of Newton's later laws.
Mass and weight are directly proportional to one another regardless of location.
The net force is the combination (vector sum) of all forces acting on an object.
When an object is at rest on another object, there is an equilibrium in place. This is because the net force on the object is 0 N - it is at rest.
The force of support that acts upward on the object is called the normal force.