IB Physics Dynamics Explained
Newton's Laws & Forces
Go beyond motion — understand the forces that drive it. Newton's Laws, free body diagrams, and IB exam strategies explained visually.
f = μN
Applied
Kinematics vs Dynamics
Kinematics
Describes how objects move — velocity, displacement, acceleration — without asking why.
WHAT & HOWDynamics
Explains why objects accelerate — the forces and masses behind every movement.
WHY & CAUSEIB Physics Dynamics: Newton's Three Laws
Law of Inertia
An object at rest stays at rest, and an object in motion stays in motion — unless acted on by a net external force.
F = ma
Net force equals mass times acceleration. Larger force → more acceleration. Larger mass → less acceleration.
Action–Reaction
For every action force, there is an equal and opposite reaction force — always acting on different objects.
IB Physics Dynamics: F = ma Explained
IB Physics Dynamics: Types of Forces
Gravitational Force
Every object with mass attracts every other. Near Earth's surface, this gives objects their weight.
W = mg, g = 9.8 m/s²Normal Force
A surface's support force — always perpendicular to the surface. Balances weight when on flat ground.
N = mg (on flat surface)Frictional Force
Opposes relative motion between surfaces. Static friction prevents motion; kinetic friction acts during sliding.
f = μNTension
Force transmitted through a string, rope, or cable when pulled. Acts along the string toward its attachment.
T = mg (hanging mass)Free Body Diagrams
Visualise Every Force
A Free Body Diagram strips away all surroundings and shows only the forces acting on a single object. Master this skill and IB exam questions become much easier.
Draw the object as a simple box or dot at the center.
Identify all forces: weight, normal, friction, tension, applied.
Draw arrows pointing in the exact direction of each force.
Label every arrow with its symbol and magnitude if known.
Solve an Exam Question
Calculate its acceleration.
Based on concepts from the official IB Physics syllabus: IB Physics Guide
Identify the formula from Newton's Second Law: F = ma
Rearrange for acceleration: a = F ÷ m
Substitute values: a = 20 ÷ 5
Common Mistakes
Forgetting Net Force
Acceleration depends on the net (resultant) force — not any single force acting alone.
Confusing Mass & Weight
Mass is the amount of matter (kg). Weight is the gravitational force acting on that mass (N).
Incomplete FBDs
Missing even one force from a free body diagram leads to the wrong net force and wrong answer.
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