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4 To a scientist, work is the use of force that causes an object to move over a distance. Try pushing against a wall. You might say, This is hard work. But is the object moving in the direction of the force applied to it? Is the wall moving in the direction of your push? No matter how much you push, the wall does not move forward. The force applied to the object and the object itself are not going in the same direction. Scientifically speaking, you are not doing any work on the wall.

5 Lift a chair up with your arms. Now you re doing work. The force (lifting) and the object s motion (up) are in the same direction. Work usually involves pushing, pulling, or lifting. When you lift an object up or down, or drag it sideways, making it move in the same direction that you are pushing, pulling, or lifting, you are doing work in the scientific sense. People can make work easier by using machines. A machine is any device that changes the way work is done.

6 A simple machine is a machine that has few or no moving parts and to which you apply only one kind of force. Households are full of simple machines, from scissors to pliers to can openers. One simple machine is a lever. A lever is a bar that pivots (turns) on a fixed point. The fixed point is the fulcrum. To move a big rock, you can use a bar as a lever and a rock as a fulcrum. Place one end of the bar on the ground, underneath the rock. Put a smaller object on the ground underneath the bar so that the bar rests on the object. This smaller object is the fulcrum. When you push down on the other end of the bar, you are applying force. The big rock moves.

7 Different kinds of levers have the fulcrum, applied force, and movement in different places. A broom is a kind of lever. When you hold a broom, your upper hand is the fulcrum, because it doesn t move. Your lower hand applies force in the middle. Movement happens at the bottom where the bristles are. Another kind of lever is a nutcracker. You apply the force at one end. The fulcrum is at the other end. The movement is in the middle.

8 A pulley is a wheel that has a cord around its rim. The cord can be made of string, rope, or chain. The wheel usually has a lip that keeps the line on its rim. Pulling down on one end of the cord makes the other end go up and helps you to lift objects that would otherwise be hard to move. Most pulleys change the direction of force in order to do work.

9 A fixed pulley has a fixed point in the middle. The object you re trying to move, called the load, moves up or down. The pulley doesn t. A single fixed pulley doesn t change how much effort is needed. Like a lever with a fulcrum in the center, the pulley changes the direction of the force, not the amount of force needed. Pulleys can be small or large. They can work in groups. Depending on their arrangement, they can be used to move things that are as light as a book or heavier than a car! Pulleys are used to unload freighters that carry heavy cargoes, and for simple household tasks like opening a window shade.

10 Another machine that uses turning motion to do work is a wheel-and-axle. A wheel-and-axle has a rod (axle) that is attached to a wheel. If you turn the wheel, the axle moves. In a simple wheel-and-axle, the two parts always turn together. If they do not, the wheel and the axle are not part of a simple machine. One of the most common examples of a wheel-and-axle is a doorknob. Think of how hard it would be to open the latch of a door without the knob. The doorknob acts as the wheel, and the rod attached to it acts as the axle. The other end of the rod is connected to the latch inside the door. When you turn the knob, you use a certain amount of force. The axle moves with even greater force and enables you to open the door latch.

11 A fishing rod uses a wheeland-axle for its reel. The crank acts as the wheel. The axle is the post that the crank attaches to. But the fishing rod uses other kinds of simple machines as well. It also uses a lever. The fulcrum is the spot where the hand holds the rod.

12 How would you move a heavy box from the street up into the inside of a truck? You might use a ramp. A ramp is a type of simple machine called an inclined plane. An inclined plane is a slanted surface that changes the way work is done. It is easier to push a box up a ramp than to lift it. The inclined plane allows you to use less force to move the box. But you have to move the box a longer distance.

13 The amount of effort you must use and the distance the load has to travel vary according to the slope of the inclined plane. If the ramp is short and steep, you don t have to move the box as far. But you have to use more force to push the box. The greater the distance the load must travel, the less force is required.

14 How is a screw like a ramp? Both are inclined planes. A screw is a post with threads wrapped around it. As the screw gets turned, objects move up the threaded spiral. Like other inclined planes, a screw makes it possible for you to use less force, but the screw must cover a longer distance as you work it into a wall or other object.

15 Common screws are used to hold wood pieces together. When you turn a screwdriver to drive a screw into wood, you are moving the wood up the ramp of the screw. The more threads the screw has, the greater the distance the wood must travel.

16 An inclined plane is used to move objects to a different height. If you put two inclined planes back-to-back, they make another simple machine: a wedge. Like an inclined plane, a wedge changes the direction of a force. Instead of lifting objects, it pushes them apart. Wedges are often made of metal. Splitting wood would be difficult without using an ax. The person swinging the ax handle uses a downward effort to drive the wedge into the log. That downward force is changed into a force that pushes out on both sides of the wedge. This causes the wood to split.

17 Woodworkers and sculptors often use chisels, another kind of wedge, to carve designs in wood and marble. Cooks use knives with wedge-shaped blades to cut vegetables and chunks of meat. Simple machines help us do work. Levers and pulleys help us lift objects. A wheel-and-axle is used to turn an object. Inclined planes allow us to lift and move objects more easily.

18 fulcrum (FUHL kruhm) The fixed point on a lever (4, 5, 7, 9) inclined plane (in KLYND PLAYN) A simple machine that is a slanted surface (10, 11, 12, 14, 15) lever (LEV er) A simple machine made of a bar that pivots on a fixed point (4, 5, 7, 9, 15) pulley (PUHL ee) A simple machine made of a wheel with a line around it (6, 7, 15) screw (SKROO) A simple machine made of a post with an inclined plane wrapped around it (12, 13) simple machine (SIM puhl muh SHEEN) A machine that you apply just one force to (4, 9, 10, 14, 15) wedge (WEJ) A simple machine made of two inclined planes placed back-to-back (14, 15) wheel-and-axle (weel and AK suhl) A simple machine made of a wheel and an axle that turn together (8, 9, 15) work (WERK) That which is done on an object when a force moves an object over a distance (2, 3, 6, 8, 9, 10, 15)

19 How do simple machines help us do work? Give an example in your answer. How do scientists define work? How are an inclined plane and a screw alike? How are they different? Choose a kitchen item, household tool, or a child s toy, and explain why it is a simple machine. With a friend, walk around the parts of your school that have stairs. Take notes about where ramps could be placed so people in wheelchairs could move around easily. List some of the simple machines in your house. Ask your neighbor if he or she has the same simple machines. Discuss the differences between the simple machines each of you owns.

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