Newton's first and second laws

Newton's laws connect force to motion and are the backbone of mechanics.

• First law (inertia): an object stays at rest or in uniform motion unless a resultant force acts.
• Second law: Fnet = ma — acceleration is proportional to resultant force and inversely proportional to mass.
• Fnet is the VECTOR sum of all forces on the object.
• Acceleration is always in the direction of the resultant force.

• Inertia — resistance to change in motion.
• Resultant force — the net force after adding all forces.