The Norwood Science Center Forces Grade BACKGROUND INFORMATION: Pulleys are wheels, just like gears. However, while the gear has teeth along the circumference the pulley has a groove. This groove is a track for a string that applies or transfers force to the pulley system. There are two types of pulleys. The first type is a fixed pulley. This pulley does not move from a fixed point. The load being lifted moves up or down. An example of a fixed pulley is the pulley at the top of the flagpole in front of many of the school buildings in Norwood. The pulley at the top of the flagpole does not move up or down the pole when the flag is raised or lowered. A cartoon of a fixed pulley is shown at the right. 1 The most interesting aspect of a fixed pulley is that it does not give any mechanical advantage; it is not easier or harder to move the load. It does, however, change the direction of the force being applied to move the load. If a person sitting on top of a flagpole raised the flag, they would expend as much energy moving the flag up the pole as someone who remained on the ground and raised the flag via the pole top pulley. Fixed pulleys are for convenience. It is also easier to pull down rather than up against gravity. Mechanical advantage in pulleys occurs when a moveable pulley is employed. A moveable pulley is attached to the load being moved so the pulley moves along with the load - hence the clever name of moveable pulley. A cartoon of a moveable pulley is shown to the left. Imagine a teacher wants to bring a 0 pound bucket filled with junk food up to her classroom on the second floor of a school. For the fun of it let us say the room is feet from the ground. The bucket is too heavy to carry up the stairs, so she will lift it up from the ground into her room with a rope. All she has to do is lift feet of rope, with the bucket of junk food attached, and she will have 1 http://www.the-office.com/summerlift/pulleybasics.htm Pulleys Gr. Page 1 of 6
accomplished the task. However, the bucket is too heavy to carry; it is too heavy to lift. She secures one end of the rope to the window sill, sends the rope down to the bucket, passes the rope through a pulley attached to the handle of the bucket and sends the free end of the rope back up to the window. If she goes to the window and pulls on the free end of the rope, she will raise the pulley along with the bucket. With a moveable pulley the bucket will seem like it weighs only half as much compared to lifting it directly with the rope. The hungry teacher will use only half of the force to lift the bucket off the ground to the window. But, with mechanical advantage, if the force is decreased the distance it applied is increased. She may use half the force only if she will move twice the rope, or 40 feet of rope. In our study of pulleys we will be using a frame made of PVC pipe. The feet of the frame, the part of the frame that rests upon the desk, will be referred to as the base. The piece that is parallel to the desk, but 18 inches above the base, will be referred to as the crossbar. The fixed pulley will be attached to the crossbar where it will be used to move a load up from the desk into the air. Pulleys Gr. Page 2 of 6
TITLE: PULLEYS PURPOSE: Why would you use a fixed pulley? MATERIALS: (per three students) One Pulley Frame Pulley Worksheets (3) One pulley tool kit Pulley with pipe cleaner (1) s (1 x g, 1 x g, 1 x g) Length of string with cups on either end (1) PROCEDURE: 01. Assemble students in groups of three around one desk. 02. Distribute Pulley Frames to the groups. Teachers may assemble prior to lesson. 03. Distribute worksheets to every student. Of course they remembered to bring a sharpened pencil with them when they moved to be with their group. Pulleys Gr. Page 3 of 6
04. Students are to assemble a fixed pulley. They are to hold the pulley below the cross bar and twist the pipe cleaner around the cross bar. Students are to be sure that the string is in the groove. 0. Students are to identify the s. g g g 06. Students place g of mass into one cup. This will be the cup. The cup is always to start on the desk. Pulleys Gr. Page 4 of 6
07. The students are to place gram cubes, one at a time, into the empty cup. This empty cup will be the cup. They add gram cubes to the cup until the cup has moved down and rests on the desk. 08. Students record the (gram cubes) need to lift the -gram. 09. Students add grams of mass to the cup. The cup is moved to the desk.. Students are to add gram cubes to the cup in order to lift the -gram off the desk. They do not need to empty the cup prior to doing this, but many will. 11. Upon completion of the worksheet students return all the materials to the Pulley tool kit. Conclusion: 01. Simple machines make the feel lighter. Does a fixed pulley make the feel lighter? 02. When would someone use a fixed pulley? Pulleys Gr. Page of 6
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