If a body is thrown vertically upward from a moving vehicle, then it will have two components of velocities. One is the horizontal component $(v\cos\theta)$ and other is the vertical component $(v\sin\theta)$. As the body goes upwards, the vertical component decreases and becomes zero at a point and then becomes negative (i.e. the body reverses its direction). But the horizontal component does not change. It is equal to the velocity of the moving vehicle. So, when the body completes its motion, both the body and the vehicle will have travelled same horizontal distance. Therefore, the body will fall on the vehicle.
Home Kinematics Reasonings If a body is thrown vertically upward from a vehicle moving with uniform velocity, where will the body fall?