Work and Machines

Discuss the images from Avenelle's grandfather's text book.

From the student worksheet:

Avenelle and her friend Keziah were visiting Avenelle’s grandfather. Avenelle’s grandfather shared his old science textbook with the girls, and they were especially interested in this old illustration. Even though the text was in French, the girls could tell it said something about how pulleys can make it possible for humans to lift objects that they could not possibly lift on their own.

At school they worked with single pulleys, but the system here with two pulleys confused them. “Why bother with the extra pulley?” wondered Avenelle.

Together they investigated.

Students now embark on a second investigation by creating two pulley systems. On this page they are asked to consider how much force they must apply to lift the load off the table or floor.

Materials required include 2 pulleys, string, and a load to lift.

The two images show possible materials and how to attach the pulleys on a stiff piece of cardboard or wood. The load in the image is a binder clip with papers attached. The load can be anything that you can clip on or tie a string around. A coffee mug weighs enough and has a handle to tie. If you don’t have stiff cardboard or wood to hang the pulleys, you can use lab stands. Equally effective is to put a chair on top of a table and hang the pulley from a bar or slat. For the second system, the two pulleys should be at least 30 centimeters apart.

Read the instructions together and then be sure that students understand that the first task is to feel how much force they need to apply in order to lift the load in each of the 2 systems. You may ask them to predict before they construct the systems which will require the greater force.

​

Read the instructions and emphasize that students should pull the string exactly 30 centimeters. Challenge them to find a way to accurately measure how far they have pulled the string. If students have difficulty devising a method, ask them to describe where on the string they will measure when they begin. Can they mark the string? Also, ask whether it is ok if the string is loose before they begin pulling.

Once the string has been pulled 30 centimeters, a lab partner must measure how high the load has traveled.

Challenge students to write at least two complete sentences to answer the prompt on the bottom of the page. Students should observe that pulley system #1 required more force than #2, and that the load moved 30 centimeters. System #2 required less force than #1 and the load moved about 15 centimeters. Of course, not all measurements will be accurate, but in the summary students should state that with two pulleys less force was required but the load was lifted about half as far as the string was pulled.

Sample response

Our group pulled both strings 30 centimeters. We found that for system #1, more force was needed. More force times 30 centimeters will equal a greater amount of work. For system #2, less force was needed to raise the load. Less force times 30 cm means that the work for system #2 is less than #1 and the load won’t go up as far. For system #1, the load moved up 30 centimeters. For #2 the load went up only 15 cm so for system #2, the work must be less than the work exerted for system #1.