PULLEYS AND GEARS

 

Introduction:

Pulling a weight downwards feels easier than lifting it up. A pulley changes a downward pull to an upward lift allowing a small force to be applied over a long distance. Lifting a heavy object is much easier if you attach the object to one end of a rope and sling the rope over a wheel fixed to a high beam. You can then use your own weight to pull down the other end of the rope and raise the heavy weight.

This type of simple machine is called a single pulley. Running the rope around more pulley wheels allows you to do less work! Two wheels halve the amount of work, and three wheels cut the work to one third. Cranes have pulleys that lift heavy weights using small motors, and sailing boats have ropes with pulleys to help the crew overcome the force of the wind on the sails.

Materials:

• sturdy shoe box with lid
• plastic cup
• pencil (cut or broken in half)
• paper clip
• scissors
• 2- thread spools
• 2 - wood screws (long enough to pass through the center of the spools)
• marbles or weights
• kite string
• tape
• Phillips screw driver
• piece of wood (around the width of a spool of thread and 1/3 meter in length)
• book

Classroom Management:

Students should build their pulleys in small groups of 3-5 students. Teachers should nail the spools to the piece of wood for the students. If it is too difficult to obtain the materials, one pulley system can be made for demonstration to the entire class.

Activity Steps:

Design challenge : Making and Testing Pulley Power

 

• An adult should screw two spools to a piece of wood as shown. The spools should be able to turn easily.

 

• Push the pencil into the lower spool to act as a handle. Cut and tape a meter of string to this spool.

• Cut a hole in the shoe box lid so that the wood sticks out at an angle.
• Cut a short piece of string about 30 centimeters long. Tape the ends to the top of the cup like the handle on a bucket.

• Loop the string over the top spool. Hold the string taut while you wind it onto the bottom spool.

• Open the paper clip and bend it into a hook. Tie it to the other end of the string.

• Fill the cup with marbles and hook it to the paper clip. Then wind the handle and lift the cup.

You might think that it would be easier to lift this cup without a pulley. But now imagine that the marbles are bowling balls or cars. Why is it better to use a pulley?

Discuss with your group and then write a few sentences describing the relationship between force and distance while using pulleys.

 

Extensions:

Take a turn with gears: When steam engines came along, engineers turned to gears to help turn the wheels of trains, cars, and other machines. The teeth of gear wheels link together so that they do not slip. When one wheel is turned, it will turn another. Design and build (K'NEX can be used if available) a gear system that attaches to a waterwheel that could be used to turn a windmill.

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