Box pushed on rough surface: applied force forward, friction backward → if equal → constant velocity.
FINDING RESULTANT FROM FBD:
Add all force vectors (tip-to-tail or by resolving into components).
If forces are not at right angles, use scale drawing or trigonometry.
Resolving Forces from Free Body Diagrams
When forces act at angles, resolve them into HORIZONTAL and VERTICAL COMPONENTS.
FOR A FORCE F AT ANGLE θ TO HORIZONTAL:
Horizontal component: Fx = F cos θ
Vertical component: Fy = F sin θ
This allows calculation of resultant in each direction separately.
EQUILIBRIUM using components:
Sum of horizontal forces = 0
Sum of vertical forces = 0
EXAMPLE — inclined plane:
Object on slope at angle θ:
Component of weight along slope: W sin θ (down the slope)
Component of weight perpendicular to slope: W cos θ (into slope)
Normal contact force = W cos θ
Friction force (if stationary) = W sin θ
⚠️ Common Mistake
All forces in a free body diagram act ON the object — not forces the object exerts on others. Weight always acts downward from the centre of mass. Normal contact force is PERPENDICULAR to the surface — not vertical (unless surface is horizontal).
📐 Key Equations
Horizontal component: Fx = F cos θ
Vertical component: Fy = F sin θ
📌 Key Note
FBD: all forces on one object as arrows (length = magnitude). Balanced: resultant = 0, object stationary or constant velocity. Unbalanced: net force → acceleration. Resolve angled forces: Fx = F cosθ, Fy = F sinθ. Equilibrium: sum horizontal = 0 AND sum vertical = 0.
🎯 Matching Activity — Free Body Diagrams
Match each situation to the correct FBD description. — drag the symbols on the right to match the component names on the left.
Book at rest on table
Drop here
Skydiver at terminal velocity
Drop here
Car accelerating
Drop here
Object on slope at rest
Drop here
Normal force perpendicular to slope = W cosθ; friction along slope = W sinθ
Weight down = normal contact up — balanced forces, resultant = 0
Weight down = drag up — balanced, constant velocity
Thrust > drag — net forward force, accelerates in direction of motion
⭐ Higher Tier Only
HT only — draw and interpret free body diagrams. Resolve forces into perpendicular components. Determine resultant force from FBD using scale drawing or components. Identify whether an object is in equilibrium from a FBD.
🔬 Triple Science Only
Free body diagrams as a separate topic (HT only) — part of the physics-only extended forces content.
🎯 Test Yourself
Question 1 of 2
1. A 10 N force acts at 30° to the horizontal. What is the vertical component?
2. In a free body diagram of a skydiver falling at terminal velocity, what must be true?
⭐ How Well Do You Understand This Topic?
Be honest with yourself — this helps you know what to revise!
Don't get itGetting thereNailed it!
🤖 Ask Mr Badmus AI
Stuck? Just ask! 💬
I'll use FIFA for calculations and flag Higher/Triple content clearly.