A person sitting in an open car moving at constant velocity throws a ball vertically up into air. Where will the ball fall? (A) Outside the air (B) In the car ahead of the person (C) In the car to the side of the person (D) Exactly into the hand from which it was thrown up
Hint To answer this question, we need to consider the forces acting on the ball during its time of flight. Then, considering the motion of the ball with respect to the person, we can get the answer from the options.
Complete step by step answer Before the instant of time, when the person throws the ball up into the air, the ball and the person together move with the velocity equal to that of the car. Now, when the ball is thrown upwards, then it experiences a downward force, which is the force of gravity equal to its weight. As there is no information regarding the force due to the air is mentioned, we assume that there is vacuum. So apart from the gravitational force acting in the vertically downward direction no other force acts on the ball. This means that there is no force in the horizontal direction on the ball. So, the initial velocity of the ball, which is equal to the velocity of the moving car, remains unchanged. Thus during the whole journey of the ball, from moving upwards to returning downwards, the ball moves with a velocity equal to that of the car in the horizontal direction. Now, the car and hence the person is not accelerated during this time interval. So both the ball and the person move with the same velocity in the horizontal direction. Therefore, with respect to the man, the ball is at rest in the horizontal direction and thus it simply goes upward and returns downward. This means that the ball will fall exactly in the hand of the person from which it was thrown up. Hence, the correct answer is option D.
Note This question could also be answered with the help of the pseudo force. As the car is moving with the constant velocity, so its acceleration is zero. Hence, with respect to the person, no pseudo force acts on the ball which can move it in the horizontal direction.