Newton's laws of motion. Why moving things don't need a push to keep moving, what acceleration means exactly, and how force, mass and acceleration are linked by F = ma. Concept tutorials with a live force and motion board.
Inertia, acceleration as a defined quantity, and F = ma (AC9S10U05).
A force is a vector: it has a size and a direction. Forces along one line add if they point the same way and subtract if they point opposite ways. What is left is the net (resultant) force, and that is what changes motion.
Newton's first law: an object at rest stays at rest, and a moving object keeps moving at a constant velocity, unless a net force acts. Moving things slow down because of friction, not because their push runs out.
Acceleration is the rate of change of velocity, a = Δv ÷ Δt, in m/s². It has a direction, the same as the net force. Slowing down is acceleration too (opposite to the motion), and on a speed–time graph acceleration is the gradient.
Newton's second law: the net force on an object equals its mass times its acceleration, F = ma. Double the net force and the acceleration doubles; double the mass and the acceleration halves.
Forces come in pairs. When object A pushes or pulls object B, B pushes or pulls A back with a force of the same size in the opposite direction. The two forces act on different objects, so they never cancel each other out.
A free body diagram shows one object and every force acting on it, as arrows from the object: size by length, direction by the way it points. Add the forces along each direction to find the net force, then use Newton's laws to say what the object's motion will do.
A multi-step problem becomes manageable with a routine: list what you know, work out the net force, find the acceleration with F = ma, use a = Δv ÷ t to link it to speed and time, then check the answer makes sense. Keep units (N, kg, m/s², m/s, s) with every number.
Newton's laws explain why safety features and techniques work. Seat belts and head restraints deal with inertia (first law); airbags and crumple zones lengthen the stopping time to cut the force (F = ma); rockets push gases back and are pushed forward (third law); and a spearthrower (woomera) extends the thrower's arm so the force acts for longer and over a greater distance, so the spear leaves faster.
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