When something tends to rub or does rub against something else
What direction does friction always go in?
The opposite way of motion
What is inertia and is it a force?
An object's resistance to changing its state of motion. Inertia is not a force.
When two quantities both change by the same factor
When two quantities don't change by the same factor
Newton's Second Law
The acceleration produced by a net force on an object is erectly proportional to the net force, is in the same direction as the net force, and is inversely proportional to the to the mass of the object
Newton's Third Law
Whenever one object exerts a force on a second object, the second object exerts an equal and opposite force on the first