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Lesson: Learn about Torque

NFL_TORQUE2(Lesson courtesy of Lessonopoly and NBC Learn). Levels: Grades 5-8. Time Required: One class period. Lesson Pdf

Overview

This lesson introduces students in grades 5-8 to the concept of torque through a short NBC Learn video on the science of the NFL, demonstrations, and a lab activity. Students learn how to calculate torque and determine how it is used in simple machines, everyday life, and in sports.

Objectives

Students will be able to:

  • Define torque
  • Calculate the torque produced if given the force and lever arm
  • List at least 2 examples in which torque is used in daily life
  • Describe how offensive linemen use the concept of torque to their advantage

Standards (California)

Science 6, Investigation and Experimentation

7.0 Scientific progress is made by asking meaningful questions and conducting careful investigations.

Materials

Per group of 2-3 students:

One ruler, one pencil, 20 pennies

Optional Class Demonstrations Materials:

  • Broom handle or equivalent stick, 3-6 lb weight, string
  • Wrench and something with a bolt (preferably larger diameter)
  • 4 foot or greater length of 2 x 4 (or larger cross section) wood, small block or object to be used as a fulcrum

PROCEDURE

Note to teachers: Although the theory of torque is relatively simple (Torque= perpendicular force x lever arm), it is a rather complex task to use it to analyze the dynamics between two opposing football players. There are many variables at play in this interaction (friction, momentum, inertia, center of mass etc.), and the role of torque in any block or tackle depends upon these other variables. In this lesson, torque will be examined under very simplified conditions, and at the end of the lesson there will be a final reflection question relating to football.

Introduction

Ask students what  the following actions have in common: opening a door, playing on a teeter-totter, riding a Ferris wheel, and using a wrench or screwdriver? Answer: In all cases forces are applied to rotate an object. In physics the name for this is “turning force” or torque.

Torque is used dozens of time throughout the day, any time we want to revolve something around a pivot point. Even walking, as our thighs rotate about a ball joint in our hips, our muscles are applying torques to our legs — the teacher can demonstrate by walking and pointing to his or her hip joint.

Every sport involves torques. The short video from NBC the class is about to watch will describe how torque is used by NFL linemen battling on the front lines of a football game.

Part I: Introduction, Term Definitions, Demonstrations

Time required: 15 minutes

1) Show the NBC Learn video on Torque.

2) Have students record on the Activity Worksheet a written description of the following concepts, then discuss:

  • Torque is the tendency of force to produce a rotation
  • Torque= Perpendicular Force x Lever Arm
  • Pivot Point or Axis of Rotation is the point or axis about which an object rotates.
  • Lever Arm is the distance from the pivot point or axis of rotation to where the force is applied.

3) Demonstrate that the amount of torque a given force produces is more than just a question of force; it is essential to consider where that force is applied.

A few good demonstrations of this are the following:

  • Using the door to the classroom, demonstrate how difficult it is to open if you push right next to the hinges. Explain that in this case, the object you are trying to rotate is the door. The pivot point/axis of rotation is at the hinges, and thus, in order to maximize the lever arm, the doorknob where we normally push is located as far as possible from the axis of rotation. This is done to minimize the amount of force needed to “open a door” (aka rotate the body about an axis).
  • Another demonstration involves having one student hold onto one end of a stick with one hand (The stick should be sturdy, like a broom handle or a few meter sticks taped together; the student should be asked to grab it at the very end.) Have the student hold the stick horizontally. Right next to his or her hand, on the stick, hang a textbook or an item that weighs 3-5 pounds. Gradually move the weight to distances further away and the class will see that it becomes nearly impossible for the student to keep the stick horizontal. CAUTION: warn the student that if the stick rotates downward, the object may tend to slide off so be careful to not let the stick fly upwards if this happens.
  • Use a wrench on a bolt, and demonstrate how much more difficult it is if you apply a force on the wrench right next to the bolt versus at the end of the wrench opposite the bolt. You might mention, that a common practice used by mechanics for large bolts, particularly if they are stuck, is to put a metal pipe (aka “cheater bar”) on the end of a wrench in order to produce more torque for a given force.

Part II: Activity: Torque, Levers, and Simple Machines

Time required: 30 Minutes

Discuss with the students the role of torque in a teeter-totter. If two different sized people sit on it at equal distances from the fulcrum what will happen? Of course the larger person will go down (explain that each person is using their weight to apply a torque to the system and the device will rotate if one of the torques is greater. You can also introduce the idea of clockwise and counter-clockwise torque) Ask students if there is a way for 2 people to balance on a teeter-totter with seats that can be adjusted at different distances from the fulcrum? (Answer: If the smaller person is seated further away from the fulcrum, this could produce a torque of equal magnitude in the opposite rotational direction.)

Class Activity

1. Divide students into groups of 2-3, and distribute the supplies to each group:  a ruler, a pencil to act as a fulcrum, and 20 pennies.

2. Have students place the middle of the ruler over the pencil and place a stack of 5 pennies (call this the “load”) on one side of the ruler about with the center of the stack 3 inches from the fulcrum (pencil). The load be taped to this position, as it will not change for this activity.

3. Their goal will be to use the remaining pennies at different locations (with different lever arms) to create applied torques that overcome the torque of the “load” and lifts the load.

Torque demo4. As the groups work on this project, they should record their findings on the Activity Worksheet. Be sure to clarify that the “lever arm” is NOT the reading on the ruler, but that the ruler markings can be used to determine the lever arm, which is the distance from the fulcrum to the CENTER of the stack of pennies.

Lever Arm of Load Load Force Load Torque (counter-clockwise)= Force x Lever Arm
All Trials

Choose 4 different “Lever Arms of the Applied Force”.  For each, find out how many pennies it takes to just lift the load pennies.  Record the information and calculate the applied clockwise torque in each case.

Trial Lever Arm of Applied Force Applied Force Applied Torque (clockwise)= Force x Lever Arm
1
2
3
4

5. When finished, students can start the Post Activity Reflections, or you can assign this for homework and review their answers the following day.

Note: Depending upon the level of your students, you can leave torque calculations unit-less. That is, just multiply the length of the lever arm (the amount of inches or centimeters) by number of pennies. Similarly, you could have them include the units of the weight of the pennies in Newtons or pounds and thus come up with torque units such as inch-pounds or cm-Newtons. (A penny weighs about .006 lbs which is about .027 N)

Closure

Take a 4-foot or greater section of 2 x 4 wood (or larger cross section) and use a small block as the fulcrum. Discuss levers and show that by extending the lever arm on one side of the fulcrum, and shortening it on the other, you get “leverage.” Demonstrate by having a student try to hold down or stand on the shorter lever arm while you easily lift them by applying a much smaller force with a much greater lever arm. (An optional extension would be to discuss and demonstrate class 1, 2, and 3 simple machines.)

Assessment

Informal assessment can be made as students whiteboard and/or present their answers to the Post Activity Reflections.

Extension

This lesson can lead to a more in depth look at class 1, 2, and 3 simple machines. It can also be coordinated with a discussion of the center of mass and related topics such as stability and toppling.

Resources

4 Responses to “Lesson: Learn about Torque”

  1. […] Lesson: Torque in Daily Life […]

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  3. […] Are you a teacher? If so, you might be interested in lesson plans based on these videos, here and here. […]

  4. […] Are you a teacher? If so, you might be interested in lesson plans based on these videos, here and here. […]

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