2 Simple Machines

Name Class Date

CHAPTER 13 Work and Energy SECTION 2 Simple Machines

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 READING TOOLBOX machines. In physics, a machine is a mechanical device Compare As you read that changes the motion of an object. Remember that this section, make a chart machines make work easier by changing the way a force showing the similarities and is applied. Many machines, such as cars and bicycles, differences between the six simple machines. Describe are complicated. However, even the most complicated how each machine affects machine is made from a combination of just six simple input and output forces machines. Simple machines are the most basic machines. and distances. Include the Scientists divide the six simple machines into two fam- mechanical advantage each machine provides. ilies: 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

Simple lever Pulley EHHDBG@K Wheel and axle 1. Infer What do you think The is the reason that the wedge inclined and the simple inclined plane plane are in the same family of simple machines? family Screw Simple inclined Wedge plane

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.

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CLASSES OF LEVERS Scientists divide levers into three main classes based on where the fulcrum, input force, and output force are. In a first-class lever, the fulcrum is between the input and output forces. The mechanical advantage of a first- class lever depends on the position of the fulcrum. EHHDBG@K First-class lever 2. Identify What part of the Output Input hammer acts as the fulcrum force force when the hammer is used to remove a nail?

FulcrumFulcrum In a ﬁrst-class lever, the fulcrum is between the input force and the output force. In a second-class lever, the output force is located between the fulcrum and the input force. 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 force. Second-class lever EHHDBG@K Output 3. Compare How do the force directions of the input and output forces in a second- class lever compare? Input Fulcrum Input Fulcrum forceforce In a second-class lever, the output force is between the fulcrum and the input force. In a third-class lever, the input force is located between the fulcrum and the output force. The output force of a third-class lever is less than the input force. Therefore, the mechanical advantage of a third-class lever is less than one. 8g^i^XVaI]^c`^c\ Third-class levers multiply distance. A person’s fore- 4. Apply Concepts How arm is a third-class lever. The elbow is the fulcrum. The does the input distance of a third-class lever compare to biceps muscle attaches to the bone near the elbow. The the output distance? muscle contracts a short distance to move the hand a long distance. Third-class lever Output force

FulcrumFulcrum InputInput force

In a third-class lever, the input force is between the fulcrum and the output force.

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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 force. It simply changes the direction of the force. READING CHECK A moveable pulley or a combination of pulleys can 5. Describe How does a produce a mechanical advantage greater than one. ﬁxed pulley change the input Moveable pulleys are attached to the object being moved. force? Fixed and moveable pulleys can be combined into a single unit to produce a greater mechanical advantage.

Input force = Input 150 N force = 75 N Output EHHDBG@K force = Input 150 N force = 6. Infer Of the three 50 N MA =1 Output pulleys in the ﬁgure, which force = Output one requires the largest 150 N MA = 2 force = 150 N MA =3 input distance? Explain your A ﬁxed pulley has a A single moveable Combining several answer. mechanical advantage of pulley has a mechanical pulleys produces an one. It changes only the advantage of two. even higher mechanical direction of a force. advantage.

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 force turns a wheel, the output force is multiplied. The output force 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.

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EHHDBG@K Output 7. Identify What are the force input and output forces on a steering wheel?

Input A wheel and force axle changes a small input force into a larger output Fulcrum force.

What Are Inclined Planes? Remember that a ramp makes work easier by reducing the force 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 force you apply by increasing the distance over which you apply it. You do the same amount of work whether READING CHECK you lift something straight up or push it up a ramp. 8. Explain How do ramps The mechanical advantage of an inclined plane is equal reduce the force needed to to its length (input distance) divided by its height (out- move an object? put distance). Pushing an object up a long, gradual ramp takes less force 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.

Output force

Input EHHDBG@K force 9. Apply Concepts A student wants to use one of A short, steep inclined plane has a small the two ramps in the ﬁ gure mechanical advantage. to move a box of books. If the student wants to use the smallest possible force to Output move the books, which ramp force should he use? Explain your answer. Input force A long, gently sloping inclined plane has a greater mechanical advantage.

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WEDGES AND SCREWS A wedge is formed from two inclined planes placed back to back. Ax blades, splitting wedges, and door- stops 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 force into two forces directed out to the sides. It both multiplies force and changes the direction of the force. Input force

EHHDBG@K 10. Describe How does the wedge affect the input force?

Output force 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 force over a large distance. The screw then exerts a large output force over a small distance. A screw with steeper threads requires more force to tighten, because the input force is applied over a shorter distance.

EHHDBG@K 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 com- bines 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.

Copyright © by Holt, Rinehart and Winston. All rights reserved. Interactive Reader 279 Work and Energy Name Class Date Section 2 Review SECTION VOCABULARY compound machine a machine made of more simple machine one of the six basic types of than one simple machine 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 force 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?)