1 00:00:00,000 --> 00:00:27,360 Hello, Mrs. Savelle. 2 00:00:27,360 --> 00:00:32,560 Hi, you must be one of the treehouse detectives. I understand you need to learn about gears. 3 00:00:32,560 --> 00:00:34,800 We sure do. What exactly is a gear? 4 00:00:35,360 --> 00:00:41,920 A gear is one of the oldest simple machines invented. In fact, it's a wheel and axle, 5 00:00:41,920 --> 00:00:43,040 but with teeth. 6 00:00:43,040 --> 00:00:44,640 Teeth? What kind of teeth? 7 00:00:45,360 --> 00:00:47,440 I'll use these Lego gears to show you. 8 00:00:48,480 --> 00:00:48,960 That's cool. 9 00:00:49,920 --> 00:00:54,560 Gears work when the teeth of one gear mesh with the teeth of another. 10 00:00:58,480 --> 00:01:01,920 The first gear is called the driver or the primary gear. 11 00:01:02,800 --> 00:01:06,960 Notice when I turn it, the next gear or the follower gear 12 00:01:07,520 --> 00:01:12,640 rotates in the opposite direction, but at the same rotational speed. 13 00:01:12,640 --> 00:01:15,920 What happens when you put two gears together that aren't the same size? 14 00:01:15,920 --> 00:01:22,240 Let's see. Notice that the follower gear is two times larger and has twice as many teeth 15 00:01:22,240 --> 00:01:24,400 as the driving gear. Give it a try. 16 00:01:28,560 --> 00:01:31,040 The follower only rotates half as much as the driver. 17 00:01:31,600 --> 00:01:35,200 That's right. It has half the rotational speed of the driver. 18 00:01:35,840 --> 00:01:39,840 Changing rotational speed is one of the reasons we use gears. 19 00:01:39,840 --> 00:01:45,760 Another reason is to multiply the force on the axle of the follower gear. 20 00:01:46,000 --> 00:01:49,920 We definitely need more force, but how do you use gears to multiply force? 21 00:01:49,920 --> 00:01:54,000 It's simple. If you double the size of the follower gear, 22 00:01:54,000 --> 00:01:57,440 the force on the axle is twice what it used to be. 23 00:01:57,440 --> 00:01:58,480 Is it always that easy? 24 00:01:59,120 --> 00:02:04,640 You can actually calculate the increase in force by simply dividing the number of teeth 25 00:02:04,640 --> 00:02:08,240 on the follower with the number of teeth on the driver. 26 00:02:08,240 --> 00:02:11,680 So in this case, the follower has 40 teeth and the driver has 24. 27 00:02:12,240 --> 00:02:19,040 40 divided by 24 is about 1.7. That means that the force is multiplied about two times. 28 00:02:19,600 --> 00:02:22,160 That's right. You're really good with numbers. 29 00:02:22,160 --> 00:02:23,280 What if there are three gears? 30 00:02:23,840 --> 00:02:24,720 Let's try it. 31 00:02:26,560 --> 00:02:34,160 Here we have a driver with 8 teeth, a follower with 24 teeth, and another follower with 40 teeth. 32 00:02:34,720 --> 00:02:38,400 It looks like the driver gear goes around five times for every one time that the largest 33 00:02:38,400 --> 00:02:39,760 follower gear goes around. 34 00:02:39,760 --> 00:02:41,200 Good observation. 35 00:02:41,840 --> 00:02:48,640 To find the force multiplier, you divide the 40 teeth of the follower by the 8 teeth of the driver. 36 00:02:48,640 --> 00:02:51,600 That would be 5. But what about the middle gear? 37 00:02:51,600 --> 00:02:56,720 The middle gear doesn't make a difference, except for the fact that it changes the direction of rotation. 38 00:02:56,720 --> 00:03:00,880 Let's verify our results to see if the force is really multiplied by 5. 39 00:03:00,880 --> 00:03:09,200 I'll place five bricks on this lever arm and place one brick the same distance out on the lever arm on the driver gear. 40 00:03:09,200 --> 00:03:11,280 It balances, so we were correct. 41 00:03:12,000 --> 00:03:15,760 We've multiplied the force by 5, but the follower doesn't move very much at all. 42 00:03:15,760 --> 00:03:17,200 It's just like a pulley system. 43 00:03:17,200 --> 00:03:21,200 Whenever you multiply the force, you have to pull in a lot of rope to move the load a short distance. 44 00:03:21,200 --> 00:03:25,280 Right. Whenever you use simple machines, there's always a trade-off. 45 00:03:25,280 --> 00:03:27,040 Dr. D said the same thing. 46 00:03:27,040 --> 00:03:32,160 He also said that you could combine simple machines in order to get a greater multiplication of force. 47 00:03:32,160 --> 00:03:35,360 Check out this bicycle, for example. 48 00:03:35,360 --> 00:03:37,360 The crank works as a lever. 49 00:03:37,360 --> 00:03:39,360 We have a gear system. 50 00:03:39,360 --> 00:03:42,960 In this gear system, the teeth don't mesh together directly. 51 00:03:42,960 --> 00:03:44,960 They are connected by a chain. 52 00:03:44,960 --> 00:03:47,760 A bicycle uses a lot of simple machines. 53 00:03:47,760 --> 00:03:51,440 When you have more than one simple machine working together, 54 00:03:51,440 --> 00:03:54,960 like on the bicycle, it's called a compound machine. 55 00:03:54,960 --> 00:03:56,960 Here's another example. 56 00:03:56,960 --> 00:04:06,960 This crane is a combination of a crank or lever attached to a gear connected to pulleys. 57 00:04:06,960 --> 00:04:08,960 This is really cool. 58 00:04:08,960 --> 00:04:10,960 I'll bet that this helps us solve our problem. 59 00:04:10,960 --> 00:04:12,960 Thanks a lot, Ms. Sauvel. 60 00:04:12,960 --> 00:04:14,960 You're welcome.