1 00:00:04,910 --> 00:00:10,910 Well, let's start explaining what dynamics means. 2 00:00:11,910 --> 00:00:16,910 You can have in the palette my map. 3 00:00:17,910 --> 00:00:21,910 I will try to explain it in a few videos. 4 00:00:22,910 --> 00:00:28,910 Dynamics deals with movement. Movement and what causes it. 5 00:00:28,910 --> 00:00:38,909 Normally we admit that movement is made by a force, but it is wrong, as explained in the lab last day. 6 00:00:38,909 --> 00:00:43,909 Things can move just by inertia. This is a very important concept. 7 00:00:43,909 --> 00:00:56,909 For example, this meteorite has come from very far away, perhaps beyond solar system limits, and it follows a straight line trajectory. 8 00:00:56,909 --> 00:01:05,909 This is because no force has been applied to it, so it moves at a constant velocity 9 00:01:05,909 --> 00:01:14,909 until it reaches the atmosphere, then by friction it will stop or will disintegrate. 10 00:01:14,909 --> 00:01:23,409 Thus, if no force is applied to an object, just by inertia it will keep moving in a straight 11 00:01:23,409 --> 00:01:29,810 line at constant velocity. This is the principle of inertia. 12 00:01:31,409 --> 00:01:38,769 This principle was first stated by Galileo Galilei more than 400 years ago. 13 00:01:41,400 --> 00:01:49,239 Inertia means that if no force is applied to an object, it will keep at rest 14 00:01:49,239 --> 00:01:55,480 or moving straight at a constant velocity until a fault is applied to it. 15 00:01:57,959 --> 00:02:05,480 Perhaps we do not appreciate what inertia means in our daily life because we are surrounded by 16 00:02:05,480 --> 00:02:12,680 a lot of friction forces. Were we on the moon, where there's no atmosphere, then we could feel 17 00:02:12,680 --> 00:02:22,219 this inertia. Perhaps you have practiced skiing or skating where friction is 18 00:02:22,219 --> 00:02:29,419 really reduced you can notice what inertia is. I mean when you initiate a 19 00:02:29,419 --> 00:02:39,150 movement it's very difficult for you to stop. Finally we can understand this 20 00:02:39,150 --> 00:02:45,930 This inertia principle that states that when something is at rest, it will keep at rest 21 00:02:45,930 --> 00:02:46,930 forever. 22 00:02:46,930 --> 00:02:54,830 Or, if it's moving at a constant velocity, it will keep moving at a straight constant 23 00:02:54,830 --> 00:02:59,370 velocity forever until the fall is applied. 24 00:02:59,370 --> 00:03:02,469 Why these two similar sentences? 25 00:03:02,469 --> 00:03:06,770 Well, this is because movement is always relative. 26 00:03:07,650 --> 00:03:14,770 Relative means that there's no way to find out whether we are at rest or moving 27 00:03:14,770 --> 00:03:21,330 in a straight forward direction at constant velocity. This is the relativity of movement. 28 00:03:23,169 --> 00:03:31,250 Yes, this is the principle that Einstein used to give us a new concept of space and time. 29 00:03:31,250 --> 00:03:38,250 One can appreciate this in a train station or subway station, if you prefer. 30 00:03:38,250 --> 00:03:43,250 You are in the wagon and there is another wagon in parallel. 31 00:03:43,250 --> 00:03:50,250 You cannot be sure whether you are moving or is the other train that is moving forward. 32 00:03:50,250 --> 00:03:59,250 In fact, what the relativity of movement says is that you could say that you are always at rest 33 00:03:59,250 --> 00:04:09,490 rest and the whole world, the station of course, is moving. Provide you keep moving in a straight line 34 00:04:10,689 --> 00:04:16,689 and at constant velocity, nobody will tell you that you are not right. 35 00:04:18,449 --> 00:04:27,009 Then, to describe the movement of someone inside our wagon, for example a boy running in our wagon, 36 00:04:27,009 --> 00:04:37,629 we can choose either the station as a reference framework or we can say that our reference system 37 00:04:37,629 --> 00:04:46,930 is the weapon itself and so the station will keep moving. It doesn't matter. If we want to describe 38 00:04:46,930 --> 00:04:55,930 the movement of a car on a road we don't take into account that the earth itself is moving around the 39 00:04:55,930 --> 00:05:05,800 sun. Well, this is all for the moment. Next videos we are studying non-constant or non-uniform 40 00:05:05,800 --> 00:05:12,040 movement. Then we shall deal with forces. Remember, if there is no force, there is inertia.