Newton's Laws of Motion
Published in 1687, Newton's three laws of motion form the foundation of classical mechanics across GCSE and A-Level Physics. They relate the forces acting on a body to its state of motion: equilibrium when resultant force is zero (First Law), acceleration proportional to resultant force via F = ma (Second Law), and equal and opposite interaction pairs between two bodies (Third Law).
- Newton's First Law (Law of Inertia)
- An object stays at rest, or keeps moving at constant velocity in a straight line, unless acted on by a resultant (net) external force. In other words, motion does not change unless a force causes it to.
- Newton's Second Law (F = ma)
- The resultant force on an object equals its mass multiplied by its acceleration (F = ma), so acceleration is proportional to the net force and inversely proportional to mass. Force is measured in newtons (N), where 1 N accelerates 1 kg at 1 m/s².
- Newton's Third Law
- For every action there is an equal and opposite reaction: if object A exerts a force on object B, then B exerts a force on A that is equal in size and opposite in direction. These paired forces always act on two different objects, so they never cancel out.
Newton's Laws of Motion: Wave Equation (v = fλ) & Force Vector (F = ma) Simulator
Adjust physical variables on the sliders below to see real-time changes in wave geometry, resultant forces, and acceleration.
F_net = 80 N − 25 N = 55 NAcceleration: a = F_net ÷ m = 55 ÷ 10 = 5.50 m/s²Key Concepts & Exam Definitions (5)
- Newton's First Law (Law of Inertia)
- An object stays at rest, or keeps moving at constant velocity in a straight line, unless acted on by a resultant (net) external force. In other words, motion does not change unless a force causes it to.
- Newton's Second Law (F = ma)
- The resultant force on an object equals its mass multiplied by its acceleration (F = ma), so acceleration is proportional to the net force and inversely proportional to mass. Force is measured in newtons (N), where 1 N accelerates 1 kg at 1 m/s².
- Newton's Third Law
- For every action there is an equal and opposite reaction: if object A exerts a force on object B, then B exerts a force on A that is equal in size and opposite in direction. These paired forces always act on two different objects, so they never cancel out.
- Resultant (Net) Force
- The single overall force left after adding up all the individual forces acting on an object, taking direction into account. If the resultant force is zero the object is in equilibrium (constant velocity or at rest); if it is non-zero the object accelerates.
- Mass vs. Weight
- Mass is the amount of matter in an object (in kilograms) and stays the same everywhere, while weight is the gravitational force on that mass (in newtons), given by W = mg. Weight changes with gravitational field strength, so an object weighs less on the Moon than on Earth even though its mass is unchanged.
Quick-Fire Flashcards (5)
Click any card to check the answer, then mark it as Known to track your progress.
State Newton's First Law of Motion.
What is the equation for Newton's Second Law, and what does each term mean?
What is inertia?
What is the difference between mass and weight?
Give an everyday example of Newton's Third Law.
Exam-Style Questions & Mark Schemes (3)
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- Q1
A resultant force of 12 N acts on a 3 kg object. Calculate its acceleration.
Click “Check Mark Scheme” to compare your answer, 4 m/s². Using Newton's Second Law, a = F ÷ m = 12 N ÷ 3 kg = 4 m/s², directed the same way as the resultant force.
- Q2
A car brakes suddenly and an unbelted passenger is thrown forward. Which law explains this, and why does it happen?
Click “Check Mark Scheme” to compare your answer, Newton's First Law (inertia). The passenger was moving with the car and tends to keep moving forward at the same velocity; because no backward force acts on their body, they continue forward until something (the seat belt or dashboard) stops them.
- Q3
When a person steps off a small stationary boat onto a dock, the boat moves backward. Explain this using Newton's laws.
Click “Check Mark Scheme” to compare your answer, Newton's Third Law. The person pushes backward on the boat, and the boat pushes the person forward with an equal and opposite force. The person moves forward onto the dock while the boat is pushed backward. (The boat, having less resistance in the water, visibly recoils)
Common Exam Questions (3)
What are Newton's three laws of motion?
1st Law: an object remains at rest or travels at constant velocity in a straight line unless acted upon by a resultant external force. 2nd Law: the resultant force acting on an object equals mass multiplied by acceleration (F = ma). 3rd Law: when body A exerts a force on body B, body B exerts an equal and opposite force of the same type on body A.
What is the difference between mass and weight?
Mass is the amount of matter in an object, measured in kilograms (kg), and remains constant everywhere. Weight is the gravitational force acting on that mass, measured in newtons (N) using W = mg, so it varies with gravitational field strength g.
What is inertia?
Inertia is the tendency of an object to resist any change to its state of rest or uniform motion. Inertial mass measures how difficult it is to change the velocity of an object (m = F ÷ a).
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