According to PBS, Galileo's inclined plane was an experiment performed to test the effects of gravity on falling objects. Galileo's inclined plane experiment eventually proved that in the absence of outside forces, gravity causes all objects to fall at an increasing rate.
Galileo's inclined plane is performed by rolling a ball down a grooved incline and measuring the time it takes for the ball to fall to different points. Galileo found that when he performed this experiment, the ball accelerated as it traveled down the ramp. Before Galileo's experiment, scientists believed that a falling object's acceleration did not increase with time.
From the inclined plane experiment, Galileo developed an equation to predict the velocity of a falling object. This equation stated that the speed of a falling object is directly related to the time the object fell. This concept helped form the basis for Newton's first law of motion, which states that an object at rest continues to remain at rest until an outside force acts upon it.
Galileo performed his inclined plane experiment while under house arrest for suggesting that the Earth revolves around the sun. While imprisoned, Galileo recorded his experiments to share his discoveries with the world.