{rofessor Dave here, let's talk about relative motion. By now we understand that physics is concerned with the motion of objects, and the quantitative analysis of that motion, but this can be tricky business if you really think about the fact that everything is in constant motion. When we say that a car is moving at 100 kilometers per hour on the freeway we are ignoring the fact that the surface of the earth is rotating around the center at about 1,700 km/hr at the equator. Earth is also moving around the Sun at around 30 km/s. The entire solar system moves around the galactic center at 20 kilometers per second and the Milky Way galaxy is moving through the universe at about 230 kilometers a second, so it's not completely wrong to say that the car is actually moving with this incredible speed as well, meaning that in a certain context, there's practically no difference between flying in a plane and sitting still in terms of absolute velocity. But that's why we tend to discuss relative motion. We don't notice the movement of the earth and the Sun and the galaxy in our everyday lives. To us, the earth seems to sit still, and a few dozen kilometers an hour may be insignificant in the grand scheme of the universe, but it's a big deal to the cop with a radar gun ready to pull you over. So we often discuss motion that is relative to the surface of the earth. When we do this we are assigning something called an inertial reference frame. This is the object or person or location that we pretend is completely still and we assess the motion of other objects relative to this inertial reference frame. For example if we treat the earth as an inertial reference frame and we ignore its rotation and movement through space, it becomes meaningful to say that this car is moving directly north at 100 km/hr relative to the ground. But if we are inside the car moving with constant velocity it seems as though we are not moving but rather that the surroundings are rushing past us at 100 km/hr. The car has become the inertial reference frame that is standing still and everything is in motion around us. It may seem strange to pretend that the car isn't moving, but remember it's just as incorrect to pretend that the earth isn't moving, so it really is all relative. Imagine you are sitting in a moving train. You toss a ball into the air and catch it. To you, the ball went straight up and down but to an observer on the ground the ball followed a parabolic path as the train went past. This is because when the ball leaves your hand it still has all of the forward velocity that the train does, but if you are inside the inertial reference frame of the train, the train is not moving at all, it is the surroundings that are moving, so the ball no longer has any horizontal velocity, only the vertical velocity you impart to it by moving your arm to throw the ball. Assigning frames of reference and understanding what they mean for the motion of objects is a big part of physics, and although some aspects of this kind of thinking seem obvious, they were not always well understood. Galileo was the first to rigorously describe relative motion after doing experiments in a moving ship. He found that if he dropped a ball while the ship was moving with constant velocity it fell straight down just like it did when the ship with stationary in the port. He then contemplated how a fish nearby would view the motion, with the ball moving forward as it falls, and how this differs from his own experience. He used these experiments to prove that velocity measurements depend on which reference frame you adopt, and he published his findings in a book called "Dialogue Concerning the Two Chief World Systems" in 1632. These ideas were extended to describe the motion of the earth around the Sun instead of the other way around by proving that the earth could be moving even if we don't feel it. This got Galileo and other scientists into hot water with the Catholic Church, but humanity eventually caught up to his revolutionary thinking. We refer to this portion of his work as Galilean relativity and these concepts will be built upon to enter very strange territory when we cover Einstein's special relativity in the modern physics course. But for right now, we won't go flying through space, we have much more work to do on earth, which despite its rotation and motion is actually quite a good approximation of an inertial reference frame, one that will be used frequently throughout our study of physics. With each topic we go through, whenever we examine an object's velocity, just remember it's all relative. Thanks for watching, guys. Subscribe to my channel for more tutorials, support me on patreon so I can keep making content, and as always feel free to email me: