IBDP Physics Handout: Circular Motion & Centripetal Force
Key Concepts
Circular motion occurs when an object moves along a curved path. Even if speed is constant,
velocity changes because direction changes. Therefore, there must be acceleration and a
resultant force.
Centripetal Force
A centripetal force acts toward the centre of the circular path. Examples include gravity,
tension, friction, and normal force.
Important Equations
v = ωr
ω = 2π/T = 2πf
f = 1/T
a = v²/r = ω²r
F = mv²/r = mω²r
Free-Body Diagrams
Always identify the real force responsible for circular motion. Do not draw a separate
'centripetal force' arrow.
Vertical Circular Motion
At the top of the circle, weight contributes to centripetal force. At the bottom, tension is
greatest. Minimum speed condition for maintaining circular motion: vmin = √gr.
Worked Example
A 0.072 kg ball moves at 3.4 m/s in a circle of radius 0.65 m.
a = v²/r = 18 m/s²
F = ma = 1.3 N
Common Mistakes
• Confusing centripetal force with a separate force
• Forgetting that velocity changes direction
• Using degrees instead of radians
Practice Questions
1. Explain why a car may skid on a bend.
2. Why do cyclists lean into corners?
3. Calculate the centripetal force on a 2 kg object moving at 5 m/s in a radius of 4 m.
4. Explain why released objects move tangentially.
IBDP Examination Advice
Show all working clearly, include units, and always explain which force provides the
centripetal force in each situation.