A ball is thrown upward at an angle of 30° with the horizontal and lands on the top edge of a building that is 20 m away. The top edge is 5 m above the throwing point. The initial speed of the ball in metre/second is (take g = 10 m/s²):
Select all that apply

Question

A ball is thrown upward at an angle of 30° with the horizontal and lands on the top edge of a building that is 20 m away. The top edge is 5 m above the throwing point. The initial speed of the ball in metre/second is (take g = 10 m/s²):

Select all that apply

G1 Physics · Accepted Answer

Use y = x tan30° - gx²/(2u²cos²30°). With x = 20 m, y = 5 m and g = 10 m/s²: 5 = 20/√3 - 8000/(3u²). Solving gives u² = 1600(4 + √3)/(13√3). Hence u = 40√[(4 + √3)/(13√3)] m/s. Therefore option A.