|Physical Science||Machines and Work|
|Science in Society||Societal Needs 4.4 d|
CONTENT STANDARD: Physical Science
CONTENT TOPIC: Machines and Work
CONCEPT: Simple and compound machines make work easier
CONTENT OBJECTIVE: 751AI.00 To understand simple and compound machines and how they are used to do work
INSTRUCTIONAL OBJECTIVES: The learner will:
TN COMPONENT OF SCIENCE: Science In Society
BENCHMARKS: Technology cannot always provide successful solutions for problems or fulfill every human need.
How strong are you? (response) Can you lift a car? (response) Can you drive this screw into this board with your fingers? (response) Can you lift the lid off this paint can with only your fingers? (response) Can you pull the nail out of this board with just your fingers? (pause) No, of course you can't do these things alone, but you can do all of them if you use a machine to help you. What machine could you use to help you lift a car? (Jack) What machine would help you drive the screw into the board? (Screwdriver) (Have a student use the screwdriver to drive the screw into the board). What machine would you use to get the lid off the can of paint? (Screwdriver) (Have a student do this.) What machine would you use to pull the nail out of the board? (Claw hammer) (Have a student do this.) Each of you have worked and you used a machine to make your work easier. Today we are going to learn more about work and machines.
1. An inclined plane is simply a ramp - a flat surface with one end higher then the other. The inclined plane doesn't move when it is used. Using an inclined plane and a small force you can do work that needs a lot of force without the inclined plane. An inclined plane multiplies force. Heavy furniture can be moved more easily into a truck using an inclined plane. All public buildings are now required to have ramps for people in wheelchairs or on crutches. It is easier to walk or drive up a long, gentle slope than up a short, steep one. The force needed is less even though the distance is greater. When you climb stairs what weight are you moving? (Your own) When you walk up an inclined plane? (Maybe half your weight, it depends on the amount of the slope). (Have students do Activity One under Active Participation.)
2. An inclined plane does not have to be a straight surface - it can curve. A screw is simply a spiral inclined plane. It has to move up or down in a material. When you turned the screw in the block of wood with the screwdriver, what resistance did the screw have to overcome? (Friction) The screw acts as a winding inclined plane and multiplies the force you used and makes it great enough to push the screw into the wood. We said that on an inclined plane the longer the slope, the less force is needed. Now look at different types of screws. (Have a variety of screws in class for students to see.) Examine the threads. Trace one thread from the top of the screw to the point. Which would be easier to push into wood, a screw with many threads or one with few threads? (Many threads) The thread of the screw acts like a ramp. The inclined plane thread increases the force. (Have students do Activity Two under Active Participation.)
3. If you put two inclined planes back to back you would have another simple machine called a wedge. A wedge is a movable inclined plane that has to move through an object to get work done. A wedge is usually used in splitting or separating. Some often used wedges are: ax, needle, knife, nail and chisel. A nail and a need I a have a different shape, they are inclined planes with round surfaces that come to a point. However, they are still wedges because the nail forces wood apart and a needle forces material apart. The ax and chisel have sloping sides. Which would multiply your effort more - a wedge that is thicker or one that is thinner? (pause) The thinner wedge because the inclined plane is less steep. (Have students do Activity Three under Active Participation.)
4. A lever is a simple machine that consists of a bar supported by a fulcrum; it is used for raising or moving a weight at one end by pushing down on the other end. A lever you are very familiar with is a seesaw. You probably could not lift another person into the air in your arms. But with a lever you could. A person could sit on one end of the lever with a fulcrum placed about center and you could push down on the other end. The person moves upward and work is being done. The claw hammer you used to pry the nail out of the wood is a good example of a lever. The support, or fulcrum, is very close to the wood as you pry up the nail. A bottle opener is another example of a lever. The fulcrum of the bottle opener is on top of the cap. (Have a student lift the cap off a bottle of pop using a bottle opener.) Other levers you have used are: crowbars, pliers, and nutcrackers. The nutcracker and pliers are made of two levers joined at the fulcrum. (Have a student crack a nut to see how the two levers exert greater force on the nut then just using the hand.) (Have students do Activity Four under Active Participation.)
5. One modification of a lever is a pulley. A pulley consists of a wheel with a grooved rim in which a rope can run. One end of the rope is tied to an object. The rope runs over the wheel and someone pulls down on the other end of the rope. As the person pulls down on the rope the object rises. Since the pulley stays in one place this is called a fixed pulley. The fixed pulley only changes the direction of the force but cannot multiply the force. The pulley at the top of a flag pole is a fixed pulley. As a person pulls down on one end of the rope hanging over the pulley the other end of the rope with the flag attached to it goes up. Venetian blinds operate with a fixed pulley. You pull down on the cord and the blind goes up. There is also a moveable pulley. In this type, the pulley itself is attached to the object so when the object moves, the pulley moves. This kind of pulley increases force as well as direction. Both pulleys help us lift heavy objects but a moveable pulley makes the lifting easier because it increases the effort. You have seen construction workers use pulleys called cranes to lift bricks and steel beams. Farmers use pulleys to lift bales of hay and bags of grain. (Have students do Activity Five under Active Participation.)
6 . Another modification of a lever is a wheel and axle. A wheel and axle consists of two different-sized wheels put together. The larger one is the wheel and the smaller one, the axle. Turning the axle makes the wheel turn. You use a wheel and axle every time you open a door with a door knob, turn the knob on your radio or television, turn the steering wheel of a car or ride a bicycle. A wheel and axle makes work easier by increasing the force and by multiplying the speed also. For instance, when you ride a bicycle you apply force through the chain to the axle of the rear wheel. The axle turns the wheel - speed is multiplied. Anytime force is applied to the axle, speed increases and if the force is applied to the wheel, the force increases. A doorknob is an example of the letter. (Have students do Activity Six under Active Participation.)
Often we use machines in combination. Two or more simple machines are often combined to make a compound machine. A shovel, for example, is a compound machine made up of a lever (Handle) and a wedge (Blade). Scissors are made up of two levers. A meat grinder is a wheel and axle turning a large screw. A crane uses a wheel and axle and a pulley. Even very complex machines like trucks, tractors, bulldozers, duplicating machines, and typewriters are just a number of simple machines put together. It is hard to imagine any job that does not involve machines. Men used to build highways using picks and shovels. He kept improving his tools until today he uses bulldozers and the like to help build highways. Machines are inventions of man to help make his work easier. People have expressed fear that machines will "put man out of work". What do you think? (response)
2. Have students (in small groups) experiment turning screws with different number of threads into a block of wood using a screwdriver. Predict beforehand which screws will require more force. (Remember what you learned about the relationship of slope to force - the longer the slope the less the force.) What resistance must the screw overcome? (Friction, the force that resists motion.)
3. Have students investigate a variety of wedges: (Be sure to closely supervise these activities)
a. Have one student cut through an apple with a knife.
b. Have one student sew a piece of material using a needle and thread.
c. Have one student drive a nail into a block of wood with a hammer.
d. Have one log. (If in these student top two or more wooden shingles (Wedges) into a crack at the top of a small the log does not have a crack you need to cut one about two inches deep.) Then hammer wedges until the log splits. You may need to go outside for this activity.
4. Place the box of books in the middle of one of the four-foot boards. Have two students at each end of the board carefully lift the load of books. Now place a fulcrum about center under the board and place the box of books on one end of the board. Have one student push down on the opposite end of the board to lift the load of books. Which took less effort? (Using the lever) Experiment by moving the fulcrum closer to the load and farther away from it. Which makes it easier to lift the load of books? (When the fulcrum is nearer the load.)
5. Tie a strong rope around the box of books securely so that the rope can lift it. Tie another length of rope (approximately six feet) to the rope around the box. Fasten a pulley to a solid support above the box (a strong bar like a cost rack). Place the rope over the pulley and ask several students to pull down on the loose end of the rope until the box is lifted. This is how a fixed pulley works. Now demonstrate a moveable pulley by taking the six-foot rope and tying one end of it to the support bar. Tie the pulley to the box of books. Run the end of the rope through the pulley and have several students pull up on the loose end of the rope. Which way takes less effort? (Moveable pulley) Remember the fixed pulley helps us lift because it changes the direction but the movable pulley makes the lifting easier because it multiplies the effort used. If possible go outside to the flagpole and have students raise and lower the flag. Also, a service station could demonstrate a tow truck, if possible.
6. Have students take a doorknob apart. If possible have them remove the knob from a doorknob on one of the school doors (get permission first). Have them try to open the door just by turning the axle. They will find it not easy to do without the knob (the wheel). Have students (in small groups) make a top by cutting a circle from construction paper and pushing a sharpened pencil through the exact center of the circle. Spin the pencil (the axle) between your finger and thumb. When the axle (the pencil) turns, the paper (the wheel) starts spinning. Which has more speed the axle or the wheel? (Wheel) A wheel and axle is a machine that makes work easier by multiplying speed.
force - anything which causes or changes motion in a body
inclined plane - a ramp; a flat surface with one end higher than the other
lever - a bar supported by a fulcrum that is used for raising or moving a weight at one end by pushing down on the other end
machine - an instrument that serves to modify force and motion to help us do work.
pulley - a wheel with a grooved rim in which a rope can run
screw - a spiral inclined plane
simple machine - a single tool to help us do work
wedge - a movable inclined plane that has to move through an object to get work done
wheel and axle - two different sized wheels put together and turning on the same axis.
work - when a force moves an object through a distance
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