Newton's Laws of Motion

2026 Syllabus Objectives

By the end of this topic, you should be able to:

  1. Apply Newton's laws of motion to the linear motion of a particle of constant mass moving under the action of constant forces, which may include friction, tension in an inextensible string and thrust in a connecting rod
  2. Use the relationship between mass and weight W = mg
  3. Solve simple problems which may be modelled as the motion of a particle moving vertically or on an inclined plane with constant acceleration
  4. Solve simple problems which may be modelled as the motion of connected particles

Main Content

https://cambridgeassistant.com

What Are Forces?

A force is a push or a pull acting on an object. Forces can make objects speed up, slow down, change direction, or stay still. We measure force in newtons (N).

Forces are vectors, which means they have both:

  • Magnitude (size/strength) — how strong the push or pull is
  • Direction — which way the force acts

Common forces you'll work with include:

  • Weight — the pull of gravity on an object (always acts downward)
  • Tension — the pulling force in a rope, string, or cable
  • Thrust — the pushing force in a rod or bar
  • Friction — the force that opposes motion when surfaces rub together
  • Normal reaction — the force perpendicular to a surface (pushes away from the surface)

Newton's Three Laws of Motion

Newton's First Law

Statement: An object will remain at rest or continue moving at constant velocity unless acted upon by a resultant (unbalanced) force.

In simple terms: Things don't start moving, stop moving, or change direction by themselves — a force must make them do it.

What this means:

  • If an object is stationary and no resultant force acts on it, it stays stationary
  • If an object is moving and no resultant force acts on it, it keeps moving at the same speed in the same direction
  • If forces are balanced (they cancel each other out), the object behaves as if there's no force at all

Example: A book resting on a table stays at rest because the weight (downward force) is balanced by the normal reaction from the table (upward force). The resultant force is zero.

Sign in to view full notes