Question

A ball is thrown at an angle θ with the horizontal. Its initial kinetic energy is 100 J and it becomes 30 J at the highest point. The angle of projection is

• A. 45⁰
• B. 30⁰
• C. cos^–1 (3/10)
• D. $\cos^{- 1}(\sqrt{3/10})$

Answer 1

Answer: (D)

$\cos^{- 1}(\sqrt{3/10})$

Answer 2

KE at highest point $K' = K\cos^{2}\theta$

$30 = 100\cos^{2}\theta$ ⇒ $\cos^{2}\theta = \frac{3}{10}$ ⇒ $\theta = \cos^{- 1}\left( \sqrt{\frac{3}{10}} \right)$