Key Concepts: Force & Interaction

What is Force?

• A force is a push or pull that can change an object's velocity (speed and/or direction).
• Force is a vector quantity — it has magnitude and direction. SI unit: Newton (N).
• 1 N = 1 kg · m/s² — the force needed to accelerate 1 kg by 1 m/s².

Types of Forces

• Contact forces: Require physical touch — friction, tension, normal force, applied force, air resistance.
• Non-contact (field) forces: Act at a distance — gravity, electromagnetic force, nuclear forces.

Free-Body Diagrams

• A free-body diagram shows all forces acting on a single object as arrows from its center of mass.
• Arrow length represents magnitude; direction represents the direction of the force.
• The net force (ΣF) is the vector sum of all forces. If ΣF = 0, the object is in equilibrium (no acceleration).