Understanding Newton’s Laws of Motion
Make sense of forces, motion and everyday examples using Newton’s three laws.
First law: motion does not change without a net external force
An object at rest stays at rest, and an object moving in a straight line at constant speed keeps doing so unless a resultant external force acts. This is the law of inertia.
A book on a desk does not accelerate because the upward support force balances its weight. A sliding object slows down mainly because of opposing forces such as friction.
Second law: a net force produces acceleration
For a body of constant mass, Newton’s second law is expressed as F = ma, where F is resultant force in newtons, m is mass in kilograms and a is acceleration in metres per second squared.
If a resultant force of 12 N acts on a 3 kg object, its acceleration is 12 ÷ 3 = 4 m/s² in the direction of the force. Remember to combine all relevant forces before calculating.
Third law: forces occur in pairs
When object A exerts a force on object B, object B exerts an equal-magnitude, opposite-direction force on object A. These forces act on different objects, so they do not cancel each other on a single object.
When you push against the floor while walking, the floor pushes back on you. This interaction helps propel you forward.
Common misunderstandings
A moving object does not necessarily need a forward net force to keep moving at constant velocity. A force is needed to change velocity, meaning speed, direction or both.
Likewise, action and reaction are not a pair of forces acting on the same object. Draw a free-body diagram for each object to identify the forces correctly.
Make the laws useful
Connect ideas to bicycles, lifts, vehicles and sports. Draw a simple diagram, choose a positive direction and identify the resultant force before using an equation.
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