Inertia and Newton's Third Law

Inertia is the tendency for motion to remain unchanged

• Until acted upon by a resultant force, objects at rest stay at rest and objects moving at a steady speed will stay moving at that speed. This tendency to continue in the same state of motion is called inertia
• An object’s inertial mass measures how difficult it is to change the velocity of an object
• Inertial mass can be found using Newton’s Second Law of F=ma. Rearranging this gives m=F/a, so inertial mass is just the ratio of force over acceleration

Newton’s third law: equal and opposite forces act on interacting objects

• When two objects interact, the forces they exert on each other are equal and opposite
• If you push something, say a shopping trolley, the trolley will push back against you, just as hard
• And as soon as you stop pushing, so does the trolley
• The important thing to remember is that the two forces are acting on different objects
• It can be easy to get confused with Newton’s third law when an object is in equilibrium. A book resting on the ground is in equilibrium. The weight of the book is equal to the normal contact force. But this is NOT Newton’s third law because the two forces are different types, and both acting on the book