Survey
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
Levers in the Human Body lever (LEV uhr) a simple machine that consists of a bar that pivots at a fixed point called a fulcrum mechanical advantage (muh KAN i kuhl ad VANT ij) a number that tells you how many times a machine multiplies force The action of a muscle pulling on a bone often works like a type of simple machine called a lever. A lever is a rigid bar that pivots at a fixed point known as a fulcrum. Any force applied to the lever is called the effort force. A force that resists the motion of the lever, such as the downward force exerted by a weight on the bar, is called the load. Figure 3 shows the action of three types of levers in the human body. In your body, the rigid bar is a bone. The effort force is supplied by muscles. And the fulcrum at which the bone pivots, is a joint. Levers increase the amount of work that can be done by the effort force applied to a load. This increase in work is called mechanical advantage. Figure 3 Levers in the Human Body Load Effort Fulcrum First-class lever Third-class lever Second-class lever 478 Chapter 15 Body Organization and Structure Three Classes of Levers As Figure 3 shows, there are three classes of levers. First-class and second-class levers increase the amount of force applied to a load. Third-class levers increase the speed of the motion. The class of a lever is determined by the location of the fulcrum, load, and effort force. In a first-class lever, the fulcrum is between the effort force and the load. First-class levers work like a car jack or seesaw. To lift a load, a downward effort force must be applied to the other end of the lever. There are few first-class levers in the body. The joint in your neck acts as the fulcrum in a first-class lever that lifts your head. The effort force is supplied by the muscles at the back of your neck. In a second-class lever, the load is between the fulcrum and the effort force. Second-class levers allow you to use less effort force than the force exerted by the load. Second-class levers work like a shovel. There are also few second-class levers in the body. The action of rising onto your toes represents the action of a second-class lever. The effort force in a third-class lever is between the fulcrum and the load. Third-class levers increase the distance through which the load is moved. Third-class levers work like a dolly that is used to carry heavy boxes. Most movable joints in the human body work like third-class levers. You use a third-class lever to lift your textbook. Your elbow is the fulcrum, and the biceps muscle in your upper arm provides the effort force. Give an example of a lever in the body. Mechanical Advantage Force is measured using the SI unit called a newton (N). Use the equation below to answer the questions that follow. mechanical advantage ! force applied to load " effort force If a second-class lever in your body has a mechanical advantage of 8. What is the effort force applied to a load of 40 N? Where is this lever in your body? 7.6.i Quick Lab How Do Levers Work? 7.6.i You can use simple materials to model how levers in the human body work. 1. Draw a table like the one below. Class of lever First Second Third Sketch of model Human body sketch E RIT W T NO OK DO N BO I 2. Use a meterstick, small wooden block, and 100 g weight, to build a lever from each class. Refer to Figure 3 as needed. 3. Sketch each of your models in your table. 4. Which part of the human body moves like each class of lever? Sketch this body part in the table that you created. 5. How does the location of the fulcrum change the action of the lever? 6. What advantage does each class of lever provide? 25 min Section 3 The Muscular System 479