Pulleys and Force

This lesson focuses on the concept of force and the use of pulleys to reduce required force. Students learn about different uses for pulleys, the impact of multiple pulleys, and identify pulley use in school and their community. Students test the ability to move weights using one, two, and four pulleys in a series.

Learn about pulleys and pulley systems.
Learn how using multiple pulleys can dramatically reduce required force.
Learn how pulley systems are used in machines and impact everyday life.
Learn about teamwork and problem solving in groups.

Age Levels: 8-11

Lesson Plan Presentation

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Materials & Preparation

Build Materials (For each team)

Required Materials

Twine/String
Two 2″ or larger pulleys
Weight (1 small plastic water or soda bottle filled with liquid or sand)

Teacher Resource (to assist students with pulley designs)

Single Pulley Set Up – The direction of force exerted to lift the bottle is the opposite of the motion of the bottle – you pull down to lift the bottle up.
- Step One: Tie one pulley to a door knob, drawer pull, or some fixed object that can withstand the weight of the bottle.
- Step Two: Tie a length of twine to the neck of the bottle and thread through the pulley.
- Step Three: Pull down on the twine and notice the amount of force needed to lift the bottle. Observe that the direction of force (down) is the opposite of the resulting bottle movement (up).
Two Pulley Set Up – Another option is to do the set up using two pulleys. This second pulley should reduce the amount of force needed to lift the bottle by one half. Additional pulleys should further reduce the force needed.
- Step One: Tie one pulley to a door knob, drawer pull, or some fixed object that can withstand the weight of the bottle.
- Step Two: Affix the neck of the bottle with string to the second pulley.
- Step Three: Tie string to the bottom of the top pulley. Wind the string through the bottom pulley, then back up and around the wheel of the top pulley, then back down through the wheel of the bottom pulley for the second time, and back up through the wheel of the top pulley. Now the string has wound twice through the two pulleys.
- Step Four: Pull on the string and compare the force needed to lift the bottle to the one pulley system described previously.

Engineering Design Challenge

Design Challenge

You are a team of engineers who has been given the challenge of operating a single pulley to lift a container.

Criteria

Can use only 1 pulley
Pulleys can be tied to door handles, drawer pulls, or other stable objects with twine/string
Twine/string may be used to run through the pulleys and tied to the neck of the bottle.

Constraints

Use only the materials provided

Activity Instructions & Procedures

Break class into teams of 2-4.
Hand out the Pulleys and Force worksheet, as well as some sheets of paper for sketching designs.
Discuss the topics in the Background Concepts Section.
Review the Engineering Design Process, Design Challenge, Criteria, Constraints and Materials.
Show students the filled container and ask them to develop a plan to lift the container using a single pulley. Ask the students to think about the direction they would be pulling (or exerting force) and how, when using one pulley, it is the opposite direction of the intended work (lifting the container).
Give students some time to brainstorm a plan.
Ask teams to demonstrate how they would use one pulley to lift the container.
Ask students to add another pulley to their team’s system and determine how adding the second pulley impacts the amount of force needed to lift the container. Does it require less force to move the container?
Combine two student teams so that four pulleys are now available. Have the new, larger group design a new system for lifting the container using all four pulleys. The team should predict what will happen to the required force needed to lift the container.
As a class, discuss the student reflection questions.
For more content on the topic, see the “Digging Deeper” section.

Student Reflection (engineering notebook)

Do you expect the force you will have to apply to move the bottle will be reduced? By how much?
Do you think that adding ten more pulleys would make a difference? Why, or why not?
Do you think the size of the pulley impacts how much force is needed to lift the bottle? Why, or why not?
Do you think the smoothness of the rope or twine pulley impacts how much force is needed to lift the bottle? Why, or why not?
Did you find you had to pull the rope or twine further the more pulleys you added to your system?
Can you think of examples of three machines that incorporate pulley systems?
Can you think of engineering problems that were solved through the use of a pulley or a pulley system?
Can you find any examples of pulleys in your school, home, or community?

Extension for Older Students

Students work in teams to develop a pulley system that would allow a student weighing 60 pounds to lift an adult that weighs 180 pounds or three times the student’s weight.

Time Modification

The lesson can be done in as little as 1 class period for older students. However, to help students from feeling rushed and to ensure student success (especially for younger students), split the lesson into two periods giving students more time to brainstorm, test ideas and finalize their design. Conduct the testing and debrief in the next class period.

Engineering Design Process

Background Concepts

What is Force? and What are Pulleys?

Force

By pushing or pulling on an object we give it energy and cause it to move, stop moving, or change direction. For example, when we lift a water bottle we exert force on it to cause it to lift in the air. Similarly, the liquid in the bottle exerts a force on the wall of the water bottle. The force may produce motion of the body or may cause the body to deform. Energy may be expended in the process, or the applied force may be balanced by an opposing force so that no energy is expended.

Sir Isaac Newton

Sir Isaac Newton (1642-1727) was the first to state the basic laws of motion of bodies. He presented three fundamental principles:

Newton’s first law states that every object will remain at rest or in uniform motion in a straight line unless compelled to change its state by the action of an external force. This is normally taken as the definition of inertia.
The second law explains how the velocity of an object changes when it is subjected to an external force. The law defines a force to be equal to change in momentum (mass times velocity) per change in time.
The third law states that for every action (force) in nature there is an equal and opposite reaction. In other words, if object A exerts a force on object B, then object B also exerts an equal force on object A

What is a Pulley?

A pulley is a rotating wheel with a curved convex rim which is mounted on a hook or base for stability. A rope, belt, or chain can move along the wheel’s rim to change the direction of a pulling force. Examples are a flagpole and a curtain rod. A “single-fixed pulley” is a pulley that is attached to an object. A “single-moveable pulley” is a pulley that is attached to a wire or rope so that it can move with the wire or rope. A “single-fixed pulley” gains nothing in force, distance or speed, but it changes the direction of the force. A system of pulleys may be used to improve leverage in lifting weights, thereby reducing the force required to move an object.

Pulley Application: The Elevator

Pulley systems are used in many machine designs and especially for moving and lifting very heavy equipment and goods. The elevator is an example of a pulley system designed to lift weights. Most elevators use counterweights which equal the weight of the elevator plus about 40% of the maximum weight it will carry. The counter-weight reduces the amount of weight the motor must pull. In a lifting drum installation, a hoist cable runs from a drive drum attached to the motor, runs around a very large pulley attached to the top of the elevator, then runs up to a second pulley attached to the roof of the elevator shaft, and then runs down to the counterweight.

Vocabulary

Constraints: Limitations with material, time, size of team, etc.
Criteria: Conditions that the design must satisfy like its overall size, etc.
Engineers: Inventors and problem-solvers of the world. Twenty-five major specialties are recognized in engineering (see infographic).
Engineering Design Process: Process engineers use to solve problems.
Engineering Habits of Mind (EHM): Six unique ways that engineers think.
Force: Pushing or pulling on an object to give it energy and cause it to move, stop moving, or change direction.
Iteration: Test & redesign is one iteration. Repeat (multiple iterations).
Prototype: A working model of the solution to be tested.
Pulley: A rotating wheel with a curved convex rim which is mounted on a hook or base for stability.

Dig Deeper

Internet Connections

Wikipedia Pulleys

Recommended Reading

Using Pulleys and Gears (Machines Inside Machines) (ISBN: 1410914453)
New Way Things Work, by David Macaulay (ISBN: 0395938473)
Moving Heavy Things, by Jan Adkins (ISBN: 0937822825)

Writing Activity

Write an essay or a paragraph describing how pulleys are used in a marina or shipyard.

Curriculum Alignment

Alignment to Curriculum Frameworks

Note: Lesson plans in this series are aligned to one or more of the following sets of standards:

U.S. Science Education Standards (http://www.nap.edu/catalog.php?record_id=4962)
U.S. Next Generation Science Standards (http://www.nextgenscience.org/)
International Technology Education Association’s Standards for Technological Literacy (http://www.iteea.org/TAA/PDFs/xstnd.pdf)
U.S. National Council of Teachers of Mathematics’ Principles and Standards for School Mathematics (http://www.nctm.org/standards/content.aspx?id=16909)
U.S. Common Core State Standards for Mathematics (http://www.corestandards.org/Math)
Computer Science Teachers Association K-12 Computer Science Standards (http://csta.acm.org/Curriculum/sub/K12Standards.html)

National Science Education Standards Grades K-4 (ages 4-9)

CONTENT STANDARD B: Physical Science

As a result of their activities, all students should develop an understanding of

Properties of objects and materials
Position and motion of objects

CONTENT STANDARD E: Science and Technology

As a result of activities, all students should develop

Abilities of technological design

CONTENT STANDARD G: History and Nature of Science

As a result of activities, all students should develop understanding of

Science as a human endeavor

National Science Education Standards Grades 5-8 (ages 10-14)

CONTENT STANDARD B: Physical Science

As a result of their activities, all students should develop an understanding of

Properties and changes of properties in matter
Motions and forces
Transfer of energy

CONTENT STANDARD E: Science and Technology

As a result of activities, all students should develop

Abilities of technological design
Understandings about science and technology

CONTENT STANDARD G: History and Nature of Science

As a result of activities, all students should develop understanding of

Science as a human endeavor
History of science

Next Generation Science Standards Grades 3-5 (Ages 8-11)

Motion and Stability: Forces and Interactions

Students who demonstrate understanding can:

3-PS2-1. Plan and conduct an investigation to provide evidence of the effects of balanced and unbalanced forces on the motion of an object.

Next Generation Science Standards Grades 3-5 (Ages 8-11)

Engineering Design

Students who demonstrate understanding can:

3-5-ETS1-1.Define a simple design problem reflecting a need or a want that includes specified criteria for success and constraints on materials, time, or cost.
3-5-ETS1-2.Generate and compare multiple possible solutions to a problem based on how well each is likely to meet the criteria and constraints of the problem.
3-5-ETS1-3.Plan and carry out fair tests in which variables are controlled and failure points are considered to identify aspects of a model or prototype that can be improved.

Next Generation Science Standards Grades 6-8 (Ages 11-14)

Motion and Stability: Forces and Interactions

Students who demonstrate understanding can:

MS-PS2-2. Plan an investigation to provide evidence that the change in an object’s motion depends on the sum of the forces on the object and the mass of the object.

Engineering Design

Students who demonstrate understanding can:

MS-ETS1-1 Define the criteria and constraints of a design problem with sufficient precision to ensure a successful solution, taking into account relevant scientific principles and potential impacts on people and the natural environment that may limit possible solutions.
MS-ETS1-2 Evaluate competing design solutions using a systematic process to determine how well they meet the criteria and constraints of the problem.

Standards for Technological Literacy – All Ages

Technology and Society

Standard 5: Students will develop an understanding of the effects of technology on the environment.
Standard 7: Students will develop an understanding of the influence of technology on history.

Design

Standard 9: Students will develop an understanding of engineering design.
Standard 10: Students will develop an understanding of the role of troubleshooting, research and development, invention and innovation, and experimentation in problem solving.

Abilities for a Technological World

Standard 13: Students will develop abilities to assess the impact of products and systems.

Student Worksheet

Engineers use pulleys in all sorts of applications. Here’s your challenge — design a plan that uses two pulleys to life up a soda bottle. Draw your plan in the box below.