Class 11 Physics Chapter 3 Motion in a Plane – Revision Notes

Motion in a plane calls for vector tools rather than single numbers, since both direction and magnitude matter. Chapter 3 of Class 11 Physics uses this to derive the projectile motion formulas for time of flight, height, and range, then applies the same vector thinking to uniform circular motion.

Last Updated: September 23, 2026

Key Terms

• Scalar: magnitude only. Vector: magnitude + direction.
• Resolution: A_x=Acosθ, A_y=Asinθ.

Projectile Motion Formulas

• T = 2u sinθ/g
• H = u²sin²θ/2g
• R = u²sin2θ/g (max at θ=45°)

Circular Motion

• Centripetal acceleration a_c=v²/r, directed toward centre.
• ω=v/r; T=2π/ω.

One-Line Summary

Motion in a plane uses vector algebra to describe two-dimensional motion, covering projectile motion (parabolic path) and uniform circular motion (constant speed, changing direction, centripetal acceleration).

Quick visual: a worked diagram from the full Solutions page, for reference.

Frequently Asked Questions

Why do we need vectors to describe motion in a plane, unlike motion in a straight line?
Motion in a plane involves both magnitude and direction changing in two dimensions, so displacement, velocity and acceleration must be represented as vectors with components along two perpendicular axes, not just single numbers.

What is projectile motion, and why is it treated as two independent motions?
Projectile motion is the motion of an object thrown into the air under gravity alone. It is analysed as independent horizontal (constant velocity) and vertical (uniformly accelerated) motions because gravity only affects the vertical component.

Written by Satish

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