Relative Motion and Inertial Reference Frames
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Relative Motion and Inertial Reference Frames

October 12, 2019


{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:

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  1. Your science is good but your history is simplistic. What got Galileo into trouble with the church was a great deal more nuanced than astronomical motions. The Roman church had previously supported Galileo and Copernicus, even Kepler. Cardinal Bellarmine cautioned Galileo that he should treat his heliocentric proposal as a hypothesis rather than as a proven fact. Most scientists (natural philosophers) at the time thought Galileo wrong. In truth, neither side comes out of this conflict with entirely clean hands. (The church had too great a power and Galileo was bombastic – always a dangerous combination.) However, the idea that there was at the time or before) a conflict between science and the church is rejected by most historians nowadays as a myth – a bit like the myth of the so-called 'dark ages'.

  2. I would recommend those who are only being introduced to this concept to take a look at the amazing properties of Einstein's Relativity. Great videos! Love your channel!

  3. When we stand still relative to the object, we are taking it's reference frame. Inertial reference frames are the frames that does not accelerate relative to other inertial frames.
    An easy way of testing inertial frames: take any object and release it in mid-air. If the object stands still, your frame is inertial.

  4. Let's talk about absolute position and reference frames. Positions that physicists calculate in each reference frame is a partial and ultimately incomplete calculation when it comes to position – in fact there is absolute position and all observers from each reference frames can agree on position if they have the proper equations gauges, sight, and instruments of all kinds. Absolute position is calculated by taking into consideration all the reference frames or the total reality of say a ball bouncing on a moving train. All viewers from every reference frame can agree with equations a gauges etc. As stated. For instance – a ball bouncing on a moving train – observer 1, a person standing at the side of the train tracks – observer 2, a person in outer space that is motionless. The person that is motionless in space sees all reference frames and can calculate absolute position quite easily – but infact they can all read or detect all reference frames to to the calculation, like the person on the train with a telescope that sees the person motionless in space, has a compas, has a speedometer, sees position outside looking through the window and all equations. The fact is they will all agree on position if the calculation of position of the bouncing ball is completed fully with all the reality behind the bouncing ball – all reference frames – which can all be detected and calculated- that is absolute position. Everything else I like to call a relative position – which ultimately are incomplete calculations of actual or absolute position.

  5. Excellent explanation! My book was not very helpful for this section and basically just gave me an equation without going into almost any detail.

  6. Velocity of Earth surface is 1700 Km/hr. (Inertial reference frame is the center of Earth)
    Velocity of Earth around Sun is 30 Km/s. (Inertial reference frame is Sun)
    Velocity of Solar system is 20 Km/s. (Inertial reference frame is center of Milky Way)
    Velocity of Milky Way through the Universe is 230 Km/s. ( WHERE IS INERTIAL REFERENCE FRAME? )

  7. By the logic discussed in this video then isn’t it perfectly legal to say that the earth is the center of our solar system and the sun does revolve around us if we take our planet as the inertial reference point?

  8. Best videos.
    Really appreciable.Your videos are not getting views but as you will give time the things will start getting better and better.
    Keep up the good work. Your work is competing khan academy and Physics Videos By Eugene Khutorya.
    May you live long and happy life.

  9. Prof.Dave yo rock
    Remember, you could be the best teacher the humanity has ever seen …
    Cause ya shrunk a hour class into 6 mins and still it is well understandable

  10. I have one doubt professor. From Kepler's II law the Earth moves faster nearer to the sun, why Earth's velocity is varying in its elliptical path?

  11. Hi professor ,I have one doubt the Earth moves faster nearer to the sun and moves slower farther to the sun the velocity is not constant it is varying between some values so from your point of view Earth is an inertial frame of reference but the condition for inertial frame of reference it obeys Newton's I law (constant velocity) . please explain?.I am confused.

  12. I don't get why you don't get recognition already…..you are THE GREAT EXPLAINER after Fynmen….I am not exaggerating. You have potential…..just keep making these outstanding lectures without being discouraged.

  13. WOW!! This guy is very good.. I been struggling with this for days, reading a lot and getting more confused. He made it simple to understand. Why cant more teachers do this???

  14. We should avoid the term "Inertial" in this context; as an inertial frame is on on which ,a body with zero net force acting upon it is not accelerating. The rotating earth is strictly not inertial

  15. How is it possible that he got those calculation you would literally have to be traveling threw outterspace to know that! …. And how is all this possible because the earth is Flat with a Dome!

  16. Thank you for speaking slowly and clearly unlike others in the profession of YouTube science presenters who seem to speak at the speed of light.

  17. Can this explain why it feels longer to go from Los Angeles to Arizona heading east then it would if we came from Arizona to Los Angeles. Some reason it feels faster

  18. Yet, a question is tickling my brain for days. Let a frame freely falling from a height above the earth. We know that since the frame and earth are accelerating w.r.t. each other, then one could be considered as being inertial and the other must not . Finally does it mean that being inertial is also relative ?

  19. Thank you very much, my "PHD" professor briefly covered inertial frames but this video was able to make up for her incompetence!

  20. The intro creeped me out, but still subscribing for teaching topics very important for the humanity.

  21. hmmmm hmmmm…
    The faster you move in space, the slower you move in time and vice versa
    But on what reference is your speed in space based?

    //for example,
    if you move at nearly the speed of light on a highway, the time will be very slow for you (time around you will be really fast) so if you decide to stop moving and you compare your watch with someone else's watch, your watch will be late for the rest of the world

    but then we could say that we werent moving, and in fact for us it was the rest of the world that was going really fast so when they (the world) stop moving (we stopped but everything is relative so we'll say they went fast and they stopped) you could compare your watches again but then this time it's the world that would be late not you because they went really fast with another reference

    the problem here is that for two different observers, there is two different result how is that possible

    this means that there must be idk a global reference or something so everybody can agree idk

  22. so if a fly is hovering inside a car and the car guy gasses the pedal, would the fly splat on the back window or stay in place as the car speeds

  23. Great explanation of relative motion. But CONSTANT velocity is poorly emphasized for inertial frame of references in order to differentiate from non-inertial which are accelerating.
    Isn't it: Frame of reference = coordinate system + object (upon coordinate system is attached/fixed)?

  24. Sir if 2 people are observing each other but one is at constant speed close to speed of light which person is gonna age?????

    I would really appreciate a detailed answer🙂

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