03Class 11 Physics

Motion in a Plane formulas

Vectors, projectile motion and circular motion formulas for Motion in a Plane. Split every 2D problem into x and y parts.

Vectors

Resultant of two vectors
R=A2+B2+2ABcos⁡θ\displaystyle R = \sqrt{A^2 + B^2 + 2AB\cos\theta}
Direction of the resultant
tan⁡α=Bsin⁡θA+Bcos⁡θ\displaystyle \tan\alpha = \frac{B\sin\theta}{A + B\cos\theta}

α is the angle from A.

Components
Ax=Acos⁡θ,Ay=Asin⁡θ\displaystyle A_x = A\cos\theta, \quad A_y = A\sin\theta

Projectile motion

Time of flight
T=2usin⁡θg\displaystyle T = \frac{2u\sin\theta}{g}
Maximum height
H=u2sin⁡2θ2g\displaystyle H = \frac{u^2\sin^2\theta}{2g}
Horizontal range
R=u2sin⁡2θg\displaystyle R = \frac{u^2\sin 2\theta}{g}

Largest at θ = 45°. Angles θ and 90° − θ give the same range.

Path equation
y=xtan⁡θ−gx22u2cos⁡2θ\displaystyle y = x\tan\theta - \frac{g x^2}{2u^2\cos^2\theta}

Circular motion

Angular speed
ω=ΔθΔt=2πT=2πν\displaystyle \omega = \frac{\Delta\theta}{\Delta t} = \frac{2\pi}{T} = 2\pi\nu
Linear and angular speed
v=rω\displaystyle v = r\omega
Centripetal acceleration
ac=v2r=rω2\displaystyle a_c = \frac{v^2}{r} = r\omega^2

Where students slip up

  • Using the range and height formulas when launch and landing heights differ. They only hold when both are at the same level.
  • Measuring θ from the vertical. The formulas here use the angle from the horizontal.
  • Forgetting that the vertical velocity is zero at the top, while the horizontal velocity stays the same throughout.

Try it on your weakest chapter.

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