LAW OF INERTIA
If we throw a ball up in a very fast-moving vehicle (like an aircraft or a train), why does it come straight back to our hands and not fall behind us?
SAJITH K., KOCHI, KERALA
According to Newton’s first law, every material object continues in its state of rest, or of uniform motion in a straight line, unless it is compelled to change that state by an unbalanced (net) force impressed upon it.
When a person throws a ball up in a moving vehicle, say, a train, the ball does come straight back to thrower as though the train were at rest. This is an iron-clad evidence for the law of inertia as the horizontal motion of ball before, during and after the catch is the same. To paraphrase, owing to the inertia of motion, the horizontal component vector of the velocity of the ball is equal to the velocity of the train at the moment the ball leaves the thrower’s hand. The former remains constant during the ball’s flight. So if the train moves with a constant velocity, the ball will exactly return to the thrower.
Here, it should be noted that the gravitational force only pulls the ball down, but not back or forth. This would also mean that the ball does not experience any acceleration that could change its forward velocity. Hence, the vertical force of gravity affects only the vertical motion of the ball.
Therefore, in accordance with the law of inertia, it is logical to expect the vertically thrown ball return to thrower’s hands in a train moving with a constant velocity. However, if the train accelerates during the flight, the ball will fall behind the thrower. On the other hand, if the train decelerates during the flight, the ball will fall ahead of the thrower.
Dr. P. RAMESH BABU
Professor of Physics, VIT University, Vellore, Tamil Nadu
THIS WEEK’S QUESTION
What technique is used to measure the distance of a celestial object from Earth and also of its distance from other celestial objects?
K.Srinivasan, Chennai
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