www.ck12.org
C HAPTER
13 TE Forces
Chapter Outline
13.1 CHAPTER13: FORCES
13.2 LESSON 13.1 WHATISFORCE?
13.1 Chapter 13: Forces
Chapter Overview
Force is a push or pull acting on an object. The SI unit for force is the newton (N). Combined forces are added or subtracted to yield the net force. Specific forces include friction, gravity, and elastic force, all of which affect motion. Friction is a force that opposes motion between surfaces that are touching. Gravity is a force of attraction between objects that have mass. It causes the acceleration of falling objects, projectile motion, and orbital motion.
Elastic force is the counter force that resists the stretching or compressing of an elastic material.
Online Resources
See the following Web sites for appropriate laboratory activities:
This inquiry lab is based on a simulation at the same Web site. In the lab, students will identify unbalanced forces and predict how they affect an object’s motion. The site provides teacher resources and student worksheets.
• http://phet.colorado.edu/en/contributions/view/3541
These two labs allow students to investigate factors that affect friction, including type of surface and weight.
• http://teachers.net/lessons/posts/3937.html
• http://mypages.iit.edu/~smile/ph9311.html
With the labs at the URLs below, students can explore the effects of gravity on motion, including acceleration due to gravity, projectile motion, and orbital motion.
• http://www.discoveryeducation.com/teachers/free-lesson-plans/gravity-gets-you-down.cfm (acceleration due to gravity)
• http://mypages.iit.edu/~smile/ph9204.html (projectile motion)
• http://phet.colorado.edu/en/contributions/view/3566 (orbital motion)
The lab at the first URL below is an investigation of elastic force using simulated springs. The simulation is provided at the second URL. Students will observe that forces of springs are proportional to the distance they stretch or compress. The scripted version of the lab provides additional teacher resources, including background content, common misconceptions, and a vocabulary list. It provides editable student worksheets.
• http://phet.colorado.edu/en/contributions/view/3543
• http://phet.colorado.edu/en/simulation/mass-spring-lab These Web sites may also be helpful:
The URLs below have links to several flash animations and other online activities relating to forces and motion.
Several of the animations include quizzes that allow students to check their knowledge.
13.1. Chapter 13: Forces www.ck12.org
• http://science.k12flash.com/forceandmotion.html
• http://www.cln.org/themes/force_motion.html
There are many common student misconceptions about forces. Several are listed at the following URL. You can use the list of misconceptions as a true-false quiz to identify any misconceptions that your own students hold.
• http://www.physics.montana.edu/physed/misconceptions/forces/forces.html
You can find links to several videos about the force of gravity at this URL:http://www.neok12.com/Gravitation.htm .
Pacing the Lessons
T
ABLE13.1:
Pacing the LessonsLesson Class Period(s) (60 min)
13.1 What Is Force? 1.0
13.2 Friction 1.5
13.3 Gravity 2.0
13.4 Elastic Force 1.0
13.2 Lesson 13.1 What Is Force?
Key Concept
Force is a push or a pull acting on an object. Examples of forces include friction and gravity. Force is a vector because it has both size and direction. The SI unit of force is the newton (N). The combined forces acting on an object are called the net force. When forces act in opposite directions, they are subtracted to yield the net force.
When they act in the same direction, they are added to yield the net force.
Standards
• SCI.CA.8.PS.2.a, b, c, d, e, f
• MCR.6-8.SCI.10.4
• NSES.5-8.B.2.3
• AAAS.6-8.4.F.3
Lesson Objectives
• Define force, and give examples of forces.
• Describe how forces combine and affect motion.
Lesson Vocabulary
• force:push or pull acting on an object
• net force: overall force acting on an object that takes into account all of the individual forces acting on the object
• newton (N):SI unit for force; equal to the amount of force that causes a mass of 1 kilogram to accelerate at 1 m/s2(kg • m/s2)
Teaching Strategies
Introducing the Lesson
Show students the animated cartoon involving go-carts at the following URL. The animation reviews velocity and acceleration from the previous chapter and introduces the concept of force, which is the focus of this chapter.
• http://mrhardy.wikispaces.com/Forces.swf
13.2. Lesson 13.1 What Is Force? www.ck12.org Demonstration
Ask two volunteers to help you demonstrate balanced and unbalanced forces with fake arm-wrestling or tug-of-war competitions. Call on other students to go to the board and draw diagrams using force vector arrows to represent each scenario, including the direction in which any resulting motion occurs. Discuss with the class how acceleration occurs in each case.
Activity
Students can explore different types of forces (e.g., tension and torsion) and how they affect real-world structures with the animated simulation activity at the URL below. Students can also investigate the materials and shapes that best withstand different types of forces.
• http://www.pbs.org/wgbh/buildingbig/lab/forces.html Differentiated Instruction
Have visual and English language learners work through the “Balanced and Unbalanced Forces” animated module at the URL below. Students will be able to explore the effects of both types of forces on motion. A simple quiz at the end of the module allows them to self-check their comprehension.
• http://www.engineeringinteract.org/resources/parkworldplot/flash/concepts/balancedandun.htm Enrichment
Some students might be interested in learning how to find the net force when force is applied in more than one dimension, as in this simple vector diagram:
Direct students to the excellent Khan Academy URL below. It shows how to visually add vectors in two dimensions:
• http://www.khanacademy.org/science/physics/mechanics/v/visualizing-vectors-in-2-dimensions?playlist=Phys ics
Science Inquiry
The roller coaster activity at the following URL requires students to explore different combinations of variables that affect the forces and motions of a roller coaster car. The objective of the activity is to find the correct combination of variables that allows the car to make it through the loop and arrive at the stopping point without crashing into the gate at the end of the track. The Web site includes a teacher guide and student worksheet.
• http://www.glasgowsciencecentre.org/forcesatthefunfair.aspx
Common Misconceptions
A common misconception is that forces cause motion (rather than acceleration). Read the background about this misconception at the URL below. Then present students with the problem at the bottom of the Web page. Take a tally of the number of students who agree with each position. Call on students who made the correct choice to explain it.
• http://www.physicsclassroom.com/class/newtlaws/u2l3b.cfm
Reinforce and Review
Lesson Worksheets
Copy and distribute the lesson worksheets in theCK-12 Physical Science for Middle School Workbook. Ask students to complete the worksheets alone or in pairs to reinforce lesson content.
Lesson Review Questions
Have students answer the Review Questions listed at the end of the lesson in the FlexBook® student edition.
Lesson Quiz
Check students’ mastery of the lesson with Lesson 13.1 Quiz inCK-12 Physical Science for Middle School Quizzes and Tests.
Points to Consider
In the next lesson, “Friction,” you will read about the force of friction. You experience this force every time you walk. It prevents your feet from slipping out from under you.
• How would you define friction?
• What do you think causes this force?
13.3. Lesson 13.2 Friction www.ck12.org
13.3 Lesson 13.2 Friction
Key Concept
Friction is a force that opposes motion between two surfaces that are touching. It is greater when objects have rougher surfaces or are heavier. Types of friction include static friction, sliding friction, rolling friction, and fluid friction.
Standards
• SCI.CA.8.PS.2.d
• MCR.6-8.SCI.10.4
Lesson Objectives
• Describe friction and how it opposes motion.
• Identify types of friction.
Lesson Vocabulary
• fluid:substance that can flow and take the shape of its container; either liquid or gas
• friction:force that opposes motion between two surfaces that are touching
Teaching Strategies
Introducing the Lesson
Introduce friction with a friction toy car. The URL below explains the role of friction in winding the spring that powers the car. Challenge students to explain how the car works, and then point out the role that friction plays. Ask a volunteer to try to wind the spring by rolling the car backward on several different surfaces that vary in roughness, such as tile, wood, fabric, and glass. Tell the class they will learn the physics behind the friction car when they read this lesson.
• http://www.ehow.com/how-does_5199953_do-friction-toy-cars-work_.html
Demonstration
Use a simple demonstration to introduce students to the concept of friction as a force that impedes motion when two surfaces are in contact. The URL below describes the demonstration and also provides background information for teachers. The demonstration is a good lead-in to the Science Inquiry activity described below.
• http://www.teachengineering.org/view_lesson.php?url=collection/duk_/lessons/duk_friction_smary_less/duk_- friction_smary_less.xml
Activity
With the simulation “Forces in 1 Dimension” at the following URL, students can explore the forces at work when trying to push a filing cabinet or other object. The simulation allows them to create an applied force and see the resulting friction force and total force acting on the cabinet. The simulation also generates graphs that show forces, position, velocity, and acceleration vs. time.
• http://phet.colorado.edu/en/simulation/forces-1d Differentiated Instruction
Work with students to make a compare/contrast table for static, sliding, and rolling friction. Headings in the table might include “Type of Friction,” “When It Occurs,” Example,” and “Relative Strength.”
Enrichment
Friction can be used to start a fire, not only with a match but without a match as well, because friction always generates heat. Ask a few students to investigate the physics behind starting a fire without a match (first URL below) and then to learn about some of the materials and methods that can be used to start a fire with friction (second URL below). Have them present their investigation to the class in an oral report and demonstrate how to start a fire with a fire bow or other friction-based method. (For safety reasons, students should not actually start a fire in the classroom.)
• http://dsc.discovery.com/fansites/mythbusters/articles/friction-start-a-fire.html
• http://adventure.howstuffworks.com/survival/wilderness/how-to-start-a-fire.htm Science Inquiry
Have groups of students do the Associated Activity “Sliding and Stuttering” at the URL below. They will measure frictional force and experiment with different types of surfaces to determine which surfaces generate more or less friction.
• http://www.teachengineering.org/view_lesson.php?url=collection/duk_/lessons/duk_friction_smary_less/duk_- friction_smary_less.xml
Real-World Connection
Students can investigate and report on friction in their daily lives with the activity at the following URL. The Web site provides background information, a student worksheet, teacher notes, and a sample report.
• http://teacher.scholastic.com/dirtrep/friction/index.htm
13.3. Lesson 13.2 Friction www.ck12.org
Reinforce and Review
Lesson Worksheets
Copy and distribute the lesson worksheets in theCK-12 Physical Science for Middle School Workbook. Ask students to complete the worksheets alone or in pairs to reinforce lesson content.
Lesson Review Questions
Have students answer the Review Questions listed at the end of the lesson in the FlexBook® student edition.
Lesson Quiz
Check students’ mastery of the lesson with Lesson 13.2 Quiz inCK-12 Physical Science for Middle School Quizzes and Tests.
Points to Consider
A skydiver like the one in the figure above falls to the ground despite the fluid friction of his parachute with the air.
Another force pulls him Earthward. That force is gravity, which is the topic of the next lesson.
• What do you already know about gravity?
• What do you think causes gravity?
13.4 Lesson 13.3 Gravity
Key Concept
Gravity is traditionally defined as a force of attraction between two masses, and it is measured by weight. Newton’s law of universal gravitation states that gravity acts between all objects in the universe and the strength of gravity depends on mass and distance. Einstein’s theory of gravity states that gravity is an effect of curves in space and time.
Gravity causes falling objects to accelerate, and it also causes projectile and orbital motion.
Standards
• SCI.CA.8.PS.2.d, f, g
• MCR.6-8.SCI.10.1
• NSES.5-8.D.3.3
• AAAS.6-8.1.A.4; AAAS.6-8.4.B.3; AAAS.6-8.4.F.4; AAAS.6-8.4.G.1, 2; AAAS.6-8.12.D.9
Lesson Objectives
• Define gravity.
• State Newton’s law of universal gravitation.
• Explain how gravity affects the motion of objects.
Lesson Vocabulary
• gravity:as traditionally defined, force of attraction between things that have mass
• law of universal gravitation:law stating that gravity is a force of attraction between all objects in the universe and that the strength of gravity is greater when masses of objects are greater or distances between objects are shorter
• orbit: path of one object around another, such as the path of the moon around Earth or of Earth around the sun
• projectile motion:motion of an object that has initial horizontal velocity but is also pulled down toward Earth by gravity
13.4. Lesson 13.3 Gravity www.ck12.org
Teaching Strategies
Introducing the Lesson
Because of the ubiquity of Earth’s gravity, it can be difficult to appreciate its effects on us. Introduce the lesson by helping students realize the effects of gravity. First remind them that Earth’s gravity pulls all objects toward the center of Earth. Then have them try to imagine what it would be like to live on Earth if it had no gravity. Ask each student in turn to describe at least one thing that would be different (e.g., if you jumped up you would keep going up and not come back down). Tell students they will learn more about gravity in this lesson.
Activity
Students can explore the concept of gravity on their own by doing the “What in the World Is Gravity?” Web quest at the following URL. Students are presented with the task of creating a brochure to explain Earth’s gravity to aliens that have just landed on their roof. The Web site includes a teacher page.
• http://www.zunal.com/webquest.php?w=50254 Using Visuals
Have students compare the figures in the lesson that illustrate projectile motion of a cannon ball and orbital motion of the moon. Ask them to identify similarities between the two types of motion. (In both cases, an object follows a curved path because of forces acting on it in two different dimensions.) Explain that if a cannon ball were shot out of a cannon at the correct angle and with great enough force, it could orbit Earth like the moon.
Differentiated Instruction
Have the class do a gallery walk relating to the effects of gravity on motion. First, post at least one critical thinking question about each of the following topics on different walls of the classroom: acceleration due to gravity, projectile motion, and orbital motion. Beside each set of questions, place a large sheet of blank paper. Divide the class into groups, and have groups move around the room from one topic to the next, discussing and recording answers to the questions on the large sheets of blank paper. Each group should also record any comments they have about the answers posted by other groups. (Have each group record their answers and comments with a different color of ink so they will be easy to distinguish.) After the gallery walk, discuss the answers and comments as a class. Ask students to identify common themes and also any misconceptions.
Enrichment
Some students may be interested to learn more about Einstein’s theory of gravity, including how it fits in with his theory of general relativity and how recent NASA discoveries provide support for the theory. Direct these students to the URLs below as starting points in their investigation. Ask them to create a Web site, poster, or other visual presentation as a way to share what they learn with the class.
• http://www.pbs.org/wgbh/nova/physics/relativity-and-the-cosmos.html
• http://raja-suvendra.com/Suvendra/node17.htm
• http://theory.uwinnipeg.ca/mod_tech/node60.html
• http://news.nationalgeographic.com/news/2011/05/110505-einstein-theories-confirmed-gravity-probe-nasa-sp ace-science/
Science Inquiry
Have students do the experiment described at the URL below. They will test the acceleration due to gravity of objects with different masses. The experiment will give them a chance to apply many basic science skills.
• http://mypages.iit.edu/~smile/phma1700.htm
Common Misconceptions
Several common student misconceptions about gravity are listed below. Discuss the misconceptions with your students and explain why they are incorrect.
Gravity exists only on Earth so there is no gravity in space.
• [Students often think that there is no gravity in space because they have seen astronauts appear weightless in movies and in pictures. The astronauts are not really weightless. They only appear so because the space shuttle and the astronauts inside of it are in a constant state of free fall around Earth.]
Gravity is selective and acts differently on some matter.
• [Gravity acts in the same way on everything. Only the strength of gravity varies.]
Planets far from the Sun have less gravity.
• [Gravity depends not only on the distance between objects. It also depends on the mass of the objects.]
Gravity can push as well as pull.
• [Students have learned that a force is a push or a pull. However, gravity is an attractive force only. It pulls objects together but does not push them apart.]
Weight and mass measure the same thing.
• [Mass is the amount of matter in an object. Weight is the force with which gravity pulls the object. Weight depends not only on the object’s mass but also on the force of gravity. This explains why the weight of a given mass is less on the moon than on Earth.]
Biology Connection
Discuss how Earth’s gravity is related to life on Earth. Explain how, throughout the history of Earth, life has evolved to survive changing conditions, such as changes in the climate and habitat. One of the few constant factors in evolution since life first began on Earth is the force of gravity. As a consequence, all living things have adapted to the ever-present force of gravity and even small variations in this force can have a significant impact on the health and function of organisms. For example, plant roots grow toward the pull of gravity and plant shoots grow away from the pull of gravity. For animals, there is a limit on how large they can become because of the pull of gravity. If they were to become too large, they would be crushed by their own weight.