Mechanical Comprehension

Force is a push or pull that can change an object's motion. Newton's 2nd law: F = m × a (force = mass × acceleration). Unit: newton (N) = kg·m/s².

F = m × a = 10 kg × 3 m/s² = 30 N.

Mass is the amount of matter (kg), constant everywhere. Weight is the gravitational force on that mass: W = m × g, where g ≈ 9.8 m/s² on Earth. Weight changes with gravity; mass does not.

W = m × g = 5 kg × 9.8 m/s² = 49 N.

An object at rest stays at rest, and an object in motion stays in motion at constant velocity, unless acted on by a net external force. This tendency is called inertia.

For every action there is an equal and opposite reaction. Forces always come in pairs acting on different objects (e.g., a rocket pushes gas down, gas pushes rocket up).