Brought new philosophy to physics: we should carry out experiments again and again to understand the true laws of nature. Studied dynamics (the relationship between forces and motions) Studied balls rolling on slopes and flat planes, recognizing the importance of friction. Tried to smooth the flat planes on which the balls were rolling to eliminate that effect. Recognized air resistance and its effect on falling objects. Showed that heavier objects do not fall to the ground faster than light objects. Galilean relativity principle: the behaviour of objects depends on your point of view. Example: man on ship dropping heavy weight from top of mast. From person on lands pov, falls at a curve. Lesson: the behaviour of stones tossed in the air can"t tell us if the earth is moving or not (which was an ancient argument) The concept that an object in motion will tend to stay in that state of motion unless something acts to oppose it.