Force and Motion

Force and Newton's laws

Use free-body diagrams, F = ma, and motion graphs to predict how objects move.

In HKDSE Physics, force and motion are linked by three core ideas: inertia, net force, and acceleration. This chapter gives you one practical flow: draw forces, find the net force, then predict acceleration with Newton's second law.

Newton's three laws

  1. First law: if net force is zero, velocity stays constant (including rest).
  2. Second law: acceleration is proportional to net force and inversely proportional to mass.
  3. Third law: forces come in equal and opposite interaction pairs.

Core equation

\[ \vec{F}_{\text{net}} = m\vec{a} \]

Problem-solving workflow

  1. Choose one object as your system.
  2. Sketch all external forces in a free-body diagram.
  3. Resolve forces by direction and compute net force.
  4. Use \(a = F_{\text{net}} / m\), then connect to motion equations.

Interactive force lab

Adjust mass, pulling force, and surface friction. The demo computes friction, net force, acceleration, and the motion state for a 3-second interval from rest.

Interactive motion graphs

Change initial velocity, acceleration, and elapsed time to inspect v-t and s-t behavior. This connects Newton's second law to kinematics directly.

Worked checkpoints

These quick prompts are designed for class discussion or self-check before moving to momentum and energy.

A 4 kg trolley is pulled right with 18 N while friction is 10 N left. What is its acceleration?

Net force \(= 18 - 10 = 8\text{ N}\) to the right, so \(a = F_{\text{net}}/m = 8/4 = 2\text{ m s}^{-2}\) to the right.

Why can a book stay at rest on a table even though gravity is acting?

Gravity pulls down, but the table exerts an equal normal force upward. Net force is zero, so acceleration is zero.

A rocket pushes exhaust gas backward. Which Newton's law explains forward thrust?

Newton's third law. The rocket pushes gas backward; the gas pushes the rocket forward with an equal and opposite force.

In the motion lab, what happens to the velocity-time line when acceleration is set to zero?

The line becomes horizontal, showing constant velocity. Position then changes linearly with time.

Last modified: