How can we explain Galileo's experiment?

Galileo deduced that objects move with constant speed when no force acts on them. This was the basis for Newton’s first law of motion.


By watching the motion of objects on an inclined plane Galileo understood that objects move with a constant speed when no force acts on them. He assumed a simple example that when a marble rolls down an inclined plane, its velocity increases.

Interia explanation by Galileo

Galileo hypothesized that a falling object gains an equal amount of velocity in equal intervals of time. This also says that the speed increases at a constant rate as it falls.

But, he faced a problem in testing this hypothesis. It was impossible for Galileo to observe the object’s free-falling motion and at the time, technology was unable to record such high speeds. As a result, Galileo tried to decelerate its motion by replacing the falling object with a ball or a marble rolling down an inclined plane. Since free-falling is fundamentally equivalent to a completely vertical ramp, he assumed that a ball rolling down a ramp would speed up in the same way as a falling ball would.


  • Galileo took the marble, a ramp and started the experiment by rolling it down the ramp.
  • Using a water clock, Galileo measured the time it took for the rolling ball to reach a known distance down the inclined plane.
  • After numerous trials, it was noted that the time taken for the ball to roll the entire length of the ramp was equal to double the amount of time it took for the same ball to only roll a quarter of the distance.
  • In short, if one doubled the amount of distance the ball travelled, it would travel four times as far. Through this experiment, Galileo concluded that, if an object is released from rest and gains speed at a steady rate (as it would in free-fall or when rolling down an inclined plane), then the total distance, s, travelled by the object is proportional to the time squared needed for that travel.
  • Mathematically s ∝ t2

This was the basis for Newton’s first law of motion.

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