Reference frames

0:00- [Instructor] You stand on the ground and watch a train move at a constant velocity and you think to yourself, "Hey, the ground is at rest, and the train is moving." But your friend is sitting inside that very train and she thinks, "Uh-uh, my train is at rest," because she doesn't see her train moving at all.

0:16So she says, :the train is at rest," and she sees the ground moving and therefore she says the ground is the one that's moving. So who is right? You or your friend? Well, at first we might think that, you know, the this is right, the ground is really at rest and the train is really moving.

0:35Is it that your friend is also moving along with the train, giving her the illusion that the train is at rest? But she could also say the same thing about you. She could say, "Hey, the train is really at rest and it's you who is moving along with the ground, giving you the illusion that the ground is at rest."

0:59So how do we settle this? Well, maybe we do some kind of an experiment, but guess what? It turns out, whatever experiment you do both turn out to be right. If you do an experiment on ground, it'll convince you that the ground is at rest and the train is moving.

1:11If you do similar experiments on this train, that's moving at a constant velocity. If it was moving at a constant velocity, it'll convince you that the train is at rest and it's indeed the ground that is mowing. So both are right. This means as mind blowing as it sounds, motion is relative concept.

1:30It's relative to the reference frame that we're dealing with. So from the ground's reference frame, the ground is at rest and the train is mowing. But from the train's reference frame, it's the train that is at rest and it's the ground that is moving. So look, whether something is moving or not and the speed at which it's moving and the direction in which it's moving, all of that depends.

1:52It's relative to a reference frame. It depends on which reference frame you're looking at things from. Now, when I first thought about this, I was like, no, there was a part of me inside that said, "no, that can't be true. The ground is really at rest. Come on!"

2:06In this example, the train is really moving. But then we realize, wait a second, what we call is ground is actually the surface of the earth. And we start wondering, is it really at rest? I mean, because we know if you look at this, look at the earth, say from the point of view of the moon or say from the point of view of the sun, we do see the earth moving.

2:29We see it spinning. We see it moving around. So from the sun's perspective, the earth is moving and the ground is moving. Isn't that amazing? So ground is at rest only from the perspective of the ground itself, the ground's reference frame. But if you look at things from the sun's reference frame, the ground starts moving now and, again, at this point we might feel like, oh wow, okay, the ground is really moving, is just an illusion after all that it's a rest.

3:01But that would be wrong way of thinking about it. The right way of thinking about it is the ground is really at rest as seen from the ground's reference frame or with respect to ground reference frame, and the ground is really moving with respect to sun or as seen from the sun's reference frame.

3:15Motion is relative. There's no such thing as absolute motion, okay? There's no such thing as what's real motion. It's all relative. It's just that in our everyday life we think about motion or rest in terms of the grounds reference frame, because it's very convenient.

3:30So when we say that the bus is moving at 30 miles an hour, for example, what we really mean is that the bus is moving at 30 miles an hour with respect to ground. When we say that something is at rest, what we really mean is that that object is at rest or that object is not moving with respect to ground.

3:48You always think about a reference frame. It's just that in our day-to-day life, that reference frame is implied. It's the ground reference frame. So you don't say it out loud, but you can only talk about motion with respect to reference frames. And in general, if you zoom out, if you don't mention reference frames, motion or rest makes no sense.

4:06For example, think about space probes. If I ask you what is the velocity of a space probe, well, that question is meaningless because from its own reference frame, it's not moving at all. It sees everything else moving and that's valid. So from that perspective, the space probe would say its velocity is zero.

4:23But from the reference frame of the earth, the space probe will have a different velocity. It's definitely moving at some speed. With respect to sun, it'll have another velocity altogether. So in general, we have to mention which reference frame you're dealing with when you're talking about motion or velocity or speed or anything like that.

4:39So this means, again, as mind boggling as it sounds, the idea of constant velocity. If something is going at a constant velocity in one reference frame, it could be at rest in another reference frame. That is amazing if you ask me. So long story short, the key takeaway of this video is that motion has no absolute meaning, okay?

5:01You can only talk about motion in terms of reference frame. It's always relative to some reference frame. In our daily life, that reference frame is implied to be ground reference frame. But in general, all reference frames are equally valid. There's nothing special about ground.

5:16Remember, it's just very convenient for our everyday use case. But if you consider the motion of the planet's, for example, well now the ground reference frame may not be very convenient. In fact, now sun's reference frame might be more convenient. So you see all reference frames are equally valid.

5:33It is just that we choose the ones that are convenient to us, depending upon the situation.