CHAPTER 13 2 Simple Machines SECTION Work and Energy KEY IDEAS As you read this section, keep these questions in mind: What are simple machines? What simple machines are in the lever family? What simple machines are in the inclined plane family? What are compound machines? What Are Simple Machines? We are surrounded by many different electronics and machines. In physics, a machine is a mechanical device that changes the motion of an object. Remember that machines make work easier by changing the way a is applied. Many machines, such as cars and bicycles, are complicated. However, even the most complicated machine is made from a combination of just six simple machines. Simple machines are the most basic machines. Scientists divide the six simple machines into two families: the lever family and the inclined plane family. The lever family includes the simple lever, the pulley, and the wheel and axle. The inclined plane family includes the simple inclined plane, the wedge, and the screw. The lever family READING TOOLBOX Compare As you read this section, make a chart showing the similarities and differences between the six simple machines. Describe how each machine affects input and output s and distances. Include the mechanical advantage each machine provides. The inclined plane family Simple lever Simple inclined plane Pulley Wedge Wheel and axle Screw 1. Infer What do you think is the reason that the wedge and the simple inclined plane are in the same family of simple machines? How Do Levers Work? If you have ever used a claw hammer to remove a nail from a piece of wood, you have used a simple lever. All levers have a rigid arm that pivots, or turns, around a point. This point is called the fulcrum. Interactive Reader 275 Work and Energy
2. Identify What part of the hammer acts as the fulcrum when the hammer is used to remove a nail? CLASSES OF LEVERS Scientists divide levers into three main classes based on where the fulcrum, input, and output are. In a first-class lever, the fulcrum is between the input and output s. The mechanical advantage of a firstclass lever depends on the position of the fulcrum. First-class lever In a first-class lever, the fulcrum is between the input and the output. 3. Compare How do the directions of the input and output s in a secondclass lever compare? 4. Apply Concepts How does the input distance of a third-class lever compare to the output distance? In a second-class lever, the output is located between the fulcrum and the input. Wheelbarrows are examples of second-class levers. The mechanical advantage of a second-class lever is always greater than one. Therefore, second-class levers multiply. Second-class lever In a second-class lever, the output is between the fulcrum and the input. In a third-class lever, the input is located between the fulcrum and the output. The output of a third-class lever is less than the input. Therefore, the mechanical advantage of a third-class lever is less than one. Third-class levers multiply distance. A person s forearm is a third-class lever. The elbow is the fulcrum. The biceps muscle attaches to the bone near the elbow. The muscle contracts a short distance to move the hand a long distance. Third-class lever In a third-class lever, the input is between the fulcrum and the output. Interactive Reader 276 Work and Energy
PULLEYS A pulley is another kind of simple machine in the lever family. You may have used a pulley to lift things, such as a flag on a flagpole. As shown below, the point in the middle of a fixed pulley is like the fulcrum of a lever. The rest of the pulley acts like the fixed arm of a first-class lever, because it pivots around a point. The distance from the fulcrum is the same on both sides of a fixed pulley. A fixed pulley has a mechanical advantage of one. Therefore, a fixed pulley does not increase. It simply changes the direction of the. A moveable pulley or a combination of pulleys can produce a mechanical advantage greater than one. Moveable pulleys are attached to the object being moved. Fixed and moveable pulleys can be combined into a single unit to produce a greater mechanical advantage. READING CHECK 5. Describe How does a fixed pulley change the input? = MA =1 = A fixed pulley has a mechanical advantage of one. It changes only the direction of a. = MA = 2 = 75 N A single moveable pulley has a mechanical advantage of two. = = 50 N MA =3 Combining several pulleys produces an even higher mechanical advantage. 6. Infer Of the three pulleys in the figure, which one requires the largest input distance? Explain your answer. WHEEL AND AXLE A wheel and axle is made of a lever or pulley (wheel) connected to a shaft (axle). Bicycle gears, doorknobs, wrenches, and screwdrivers are wheel-and-axle machines. When a small input turns a wheel, the output is multiplied. The output increases because the axle is smaller than the wheel. Therefore, the axle rotates through a smaller distance. The figure at the top of the next page shows a wheel and axle. Interactive Reader 277 Work and Energy
7. Identify What are the input and output s on a steering wheel? A wheel and axle changes a small input into a larger output. READING CHECK 8. Explain How do ramps reduce the needed to move an object? What Are Inclined Planes? Remember that a ramp makes work easier by reducing the needed to move an object. Ramps are examples of simple inclined planes. Simple inclined planes are simple machines with a straight, slanted surface. Inclined planes, like all machines, do not reduce the amount of work needed to move an object. They reduce the you apply by increasing the distance over which you apply it. You do the same amount of work whether you lift something straight up or push it up a ramp. The mechanical advantage of an inclined plane is equal to its length (input distance) divided by its height (output distance). Pushing an object up a long, gradual ramp takes less than pushing the object up a short, steep ramp. In other words, a long, gradual inclined plane has a greater mechanical advantage than a short, steep plane. 9. Apply Concepts A student wants to use one of the two ramps in the figure to move a box of books. If the student wants to use the smallest possible to move the books, which ramp should he use? Explain your answer. A short, steep inclined plane has a small mechanical advantage. A long, gently sloping inclined plane has a greater mechanical advantage. Interactive Reader 278 Work and Energy
WEDGES AND SCREWS A wedge is formed from two inclined planes placed back to back. Ax blades, splitting wedges, and doorstops are examples of wedges. As shown in the figure below, using a wedge is like pushing down on a ramp. In other words, a wedge is a moving inclined plane. A wedge turns a single downward into two s directed out to the sides. It both multiplies and changes the direction of the. 10. Describe How does the wedge affect the input? A wedge is a moving inclined plane. A screw is an inclined plane wrapped around a cylinder. When you tighten a screw with gently sloping threads, you apply a small input over a large distance. The screw then exerts a large output over a small distance. A screw with steeper threads requires more to tighten, because the input is applied over a shorter distance. 11. Identify Relationships How is a screw related to a simple inclined plane? A screw is an inclined plane wrapped around a cylinder. What Are Compound Machines? Many of the things you use every day are compound machines. A compound machine is a machine that combines two or more simple machines. For example, a pair of scissors is made of two first-class levers joined at a common fulcrum. Each lever arm is a wedge. Together, the wedges can cut paper. Bicycles and cars are made of many simple machines, so they are also compound machines. Interactive Reader 279 Work and Energy
Section 2 Review SECTION VOCABULARY compound machine a machine made of more than one simple machine simple machine one of the six basic types of machines, which are the basis for all other forms of machines 1. List What are the six simple machines? 2. Compare How does the input distance of a single fixed pulley compare to the output distance? Explain your answer. (Hint: How are and distance related?) 3. Infer Why are wedges and screws part of the inclined plane family of simple machines? 4. Compare How are second-class levers and third-class levers similar? How are they different? 5. Explain How are fixed pulleys similar to first-class levers? 6. Infer Can a simple inclined plane have a mechanical advantage less than one? Explain your answer. (Hint: Can the height of a ramp ever be greater than its length?) Interactive Reader 280 Work and Energy