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⚖️ Newton's Second Law

F_net = ma — net force, mass, and acceleration. Resolve components, separate mass from weight, and check units before you trust the number.

lessons

~16 min

to learn

🔬 Science

subject

Adults

level

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What you’ll learn

Net force, not biggest forceState Newton's second law as F_net = ma and identify net force as the vector sum of external forces.Acceleration follows the net external force. Equal opposite forces cancel; leftover force times mass sets how quickly velocity changes.
Axes and componentsResolve an angled force into perpendicular components and write separate second-law equations along chosen axes.Components are descriptions of one force, not new forces. Sum x with x and y with y, then read acceleration from each equation.
Mass is not weightDistinguish mass (inertia, kilograms) from weight (gravitational force, newtons) and predict how mass changes acceleration for fixed F_net.For fixed net force, larger mass means smaller acceleration. Weight is a force; mass is what resists acceleration in F = ma.
Check before you trustAudit a dynamics result with units, free-body diagram, and acceleration direction before accepting the number.A newton is one kilogram meter per second squared. Diagram first, sum forces, then interpret negative signs as direction — not as failure.

Questions this course answers

What is F_net in Newton's second law?

OpenStax and standard mechanics treat net force as the vector sum of the external forces included for that system.

If the net external force on an object is zero, what must be true?

F_net = ma implies zero net force gives zero acceleration; constant velocity (including rest) is allowed.

Why resolve an angled force into components?

Components let you sum along perpendicular axes; they describe one force, they do not multiply it.

For a fixed net force, what happens if mass doubles?

Because a = F_net / m, doubling mass halves acceleration when F_net is unchanged.

What is one newton in base SI units?

NIST lists the newton as the SI derived unit of force equal to kg·m·s⁻².

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