A projectile motion is characterized by an arc-shaped direction of motion. It has two component vectors; horizontal and vertical, and both of them are independent of each other with respect to time. In order for the objects to reach the ground at the same time, Raoul should wait for the rock thrown over an arc to reach the level designated by the horizontal line. This is because at this point, only gravity is the force exerted on the object.
The time of flight for a projectile motion is
t = 2vsinθ/g
Hence, we should know the value of θ. Let's assume θ = 30°. Then,
t = 2(15)(sin 30°)/9.81
t = 1.53 seconds
Raoul should wait after 1.53 seconds.
