What is Newton's Second Law?
Newton's second law connects force, mass and acceleration into one of the most important equations in physics. It explains why a heavy object is harder to speed up than a light one, and why a bigger push produces a bigger acceleration.
Newton's second law states that the net force on an object equals its mass times its acceleration: F = m·a. Force and acceleration point in the same direction.
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Step-by-step worked examples
A 1000 kg car accelerates at 2 m/s². What net force is needed?
F = m · a F = 1000 × 2 F = 2000 N
A 50 N force pushes a 10 kg box on a frictionless floor. Find its acceleration.
a = F / m a = 50 / 10 a = 5 m/s²
A 0.5 kg ball is hit with a 10 N force. Find its acceleration.
a = F / m a = 10 / 0.5 a = 20 m/s²
Flashcards
Quick quiz
Q1.A net force of 20 N acts on a 4 kg object. What is its acceleration?
Q2.Which equation is Newton's second law?
Q3.What is the SI unit of force?
Q4.If the force on an object doubles while mass stays constant, acceleration...
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Common mistakes
Treating mass and weight as the same thing. — Correct: Mass (kg) is how much matter there is; weight (N) is the force of gravity, W = mg.
Plugging in just one applied force instead of the net force. — Correct: F in F=ma is the net (total) force after adding all forces as vectors.
Forgetting force units are kg·m/s², not just kg. — Correct: Always check units: mass in kg, acceleration in m/s², force in N.
Ignoring direction and only using magnitudes. — Correct: Force and acceleration are vectors — direction matters, especially with multiple forces.
FAQ
What is the formula for Newton's second law?
F = m·a: net force equals mass times acceleration, measured in newtons (N).
How do you calculate force using Newton's second law?
Multiply the object's mass (kg) by its acceleration (m/s²) to get force in newtons.
What are examples of Newton's second law?
A car accelerating, a ball being kicked, or a rocket launching — any time force changes an object's motion.
What are the units in Newton's second law?
Force in newtons (N), mass in kilograms (kg), and acceleration in metres per second squared (m/s²).