Inertia and Newton's Third Law

Inertia

the tendency for motion to remain unchanged (unless acted upon by a resultant force - as shown in Newton’s First Law)

Inertial mass

a measure of how difficult it is to change the velocity of an object + it can be found using rearranged Newton’s Second Law of m = f / a → so inertial mass is just the ratio of force over acceleration

Newton’s Third Law

when two objects interact, the forces they exert on each other are equal and opposite

Newton’s Third Law when two objects collide

both objects feel equal and opposite force from the other (the normal contact force) → both objects feel same sized force in opposite directions and so accelerate away from each other → the object with the smaller mass will be accelerated more → these equally sized forces in opposite directions also explain the principle of conservation of momentum

Newton’s Third Law for an object in equilibrium (imagine a book set on a table)

the pairs of forces in this case: the book is pulled down by its weight due to gravity on Earth and the book also pulls up on the Earth + the normal contact force from table pushing up on book + the normal contact force from book pushing down on table