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.