Rule 17 of 36 · Chapter III — Force & Motion
Inertia resists your intentions
Why this rule exists
Mass measures how strongly a body resists being accelerated: the same push produces less change in a heavier object. Newton's second law, F = m*a, makes this exact — acceleration is force divided by mass. Inertial mass is a property of the body alone, independent of the force applied, which is why a single number predicts its response to every push.
In practice
To find how something moves, add the forces to get the net force, then divide by mass for the acceleration. Keep the mass in the denominator in mind: doubling mass halves the acceleration for the same force. For rotation, use the moment of inertia, which weights each bit of mass by the square of its distance from the axis.
When it doesn't apply
Near light speed, inertia grows without bound, so F = m*a breaks; use the momentum form F = dp/dt with relativistic momentum. Mass is also not always constant — a rocket sheds it as it burns — so differentiate momentum, not velocity alone.