# 6.2.2.2 Forces

6
Subject:
Science
Strand:
Physical Science
Substrand:
Motion
Standard 6.2.2.2

Forces have magnitude and direction and govern the motion of objects.

Benchmark: 6.2.2.2.1 Balanced & Unbalanced Forces

Recognize that when the forces acting on an object are balanced, the object remains at rest or continues to move at a constant speed in a straight line, and that unbalanced forces cause a change in the speed or direction of the motion of an object.

Benchmark: 6.2.2.2.2 Forces & Effect on Motion

Identify the forces acting on an object and describe how the sum of the forces affects the motion of the object.

For example: Forces acting on a book on a table or a car on the road.

Benchmark: 6.2.2.2.3 Contact & Distance Forces

Recognize that some forces between objects act when the objects are in direct contact and others, such as magnetic, electrical and gravitational forces can act from a distance.

Benchmark: 6.2.2.2.4 Mass vs. Weight

Distinguish between mass and weight.

## Overview

Standard in Lay Terms

Minnesota Standard in Lay Terms

When forces acting on an object are balanced, that object will either stay in place or move at a constant speed and in a straight line.  But forces such as gravity, friction, wind, or another object placed in its path can change the motion of the object.  There can be more than one force that affects the object's motion; and some, like magnetic, electrical, and gravitational forces do not have to directly touch the object to influence its motion.

Big Ideas and Essential Understandings

Big Idea

• Forces and Motion:  Motion is a continual change in position of an object as seen by an observer.  A force is any influence that can change the motion of an object, such as its speed or direction.
• Relative Motion:  Motion is always relative to another object.  For example, a car moving at 55 mph is moving that speed in relation to the road.
Benchmark Cluster

MN Standard Benchmarks

6.2.2.2.1:  Recognize that when the forces acting on an object are balanced, the object remains at rest or continues to move at a constant speed in a straight line, and that unbalanced forces cause a change in the speed or direction of the motion of an object.

6.2.2.2.2:  Identify the forces acting on an object and describe how the sum of the forces affects the motion of an object.

6.2.2.2.3:  Recognize that some forces between objects act when the objects are in direct contact and others, such as magnetic, electrical, and gravitational forces can act from a distance.

6.2.2.2.4:  Distinguish between mass and weight.

The Essentials

A NASA video about force, work, and energy.  This can be used to introduce or reinforce the standard.

Correlations
• From the National science education standards. Retrieved May 25, 2011:
• As a result of their activities in grades 5-8, all students should develop an understanding of motions and forces
• The motion of an object can be described by its position, direction of motion, and speed.
• That motion can be measured and represented on a graph.
• An object that is not being subjected to a force will continue to move at a constant speed and in a straight line.
• If more than one force acts on an object along a straight line, then the forces will reinforce or cancel one another, depending on their direction and magnitude. Unbalanced forces will cause changes in the speed or direction of an object's motion.
• Atlas of Science Literacy
• Gravity, Volume 1, pp. 42-43.
• Motion:  Laws of Motion, Volume 1, pp. 62-63.
• Benchmarks of Science Literacy:
• By the end of 8th grade students should know that:
• An unbalanced force acting on an object changes its speed or direction of motion, or both.  4F/M3a
• If a force acts towards a single center, the object's path may curve into an orbit around the center.  4F/M3b

Common Core Standards

• W.6.7.  Conduct short research projects to answer a question, drawing on several sources and refocusing the inquiry when appropriate.
• 6.SP.4  Display numerical data in plots on a number line, including dot plots, histograms, and box plots.
• Math connection:  On other planets, your mass stays the same, but your weight changes.  Have students calculate their weight on other planets in the solar system using this table (Benchmark 6.2.2.2.4).

## Misconceptions

Student Misconceptions

Students tend to think of force as a property of an object ("an object has force," or "force is within an object") rather than as a relation between objects (Dykstra, Boyle, & Monarch, 1992; Jung et al., 1981; Osborne, 1985).  In addition, students tend to distinguish between active objects and objects that support or block or otherwise act passively.  Students tend to call the active actions "force" but do not consider passive actions as "forces" (Gunstone & Watts, 1985).  Teaching students to integrate the concept of passive support into the broader concept of force is a challenging task even at the high-school level (Minstrell, 1989). (AAAS Atlas Vol I)

Students believe constant speed needs some cause to sustain it.  In addition, students believe that the amount of motion is proportional to the amount of force; that if a body is not moving, there is no force acting on it; and that if a body is moving there is a force acting on it in the direction of the motion (Gunstone & Watts, 1985).  Students also believe that objects resist acceleration from the state of rest because of friction-that is, they confound inertia with friction (Jung et al., 1981; Brown & Clement, 1992).  Students tend to hold onto these ideas even after instruction in high-school or college physics (McDermott, 1983). Specially designed instruction does help high-school students change their ideas (Brown & Clement, 1992; Minstrell, 1989; Dykstra et al., 1992. (AAAS Atlas Vol I)

Students have difficulty appreciating that all interactions involve equal forces acting in opposite directions on the separate, interacting bodies.  Instead they believe that "active" objects (like hands) can exert forces whereas "passive" objects (like tables) cannot (Gunstone & Watts, 1985).  Alternatively, students may believe that the object with more of some obvious property will exert a greater force (Minstrell, 1992).  Teaching high-school students to seek consistent explanations for the "at rest" condition of an object can lead them to appreciate that both "active" and "passive" objects exert forces (Minstrell, 1982).  Showing them that apparently rigid or supporting objects actually deform might also help (Clement, 1987). (AAAS Atlas Vol I)

Elementary-school students typically do not understand gravity as a force.  They see the phenomenon of a falling body as "natural" with no need for further explanation or they ascribe to it an internal effort of the object that is falling (Ogborn, 1985).  If students do view weight as a force, they usually think it is the air that exerts this force (Ruggiero et al., 1985).  Misconceptions about the causes of gravity persist after traditional high-school physics instruction (Brown & Clement, 1992) but can be overcome by specially designed instruction (Brown & Clement, 1992; Minstrell et al., 1992). (AAAS Atlas Vol I)

## Vignette

Tried and True: Newton's first law: A learning cycle approach.

To demonstrate how Newton's first law of motion applies to students' everyday lives, I developed the following learning cycle series of activities on inertia.  The discrepant event at the heart of these activities is sure to elicit wide-eyed stares and puzzled looks from students, but also promote critical thinking and help bring an abstract concept to life.

I use a demonstration called Newton's Apple for the elicitation phase of the learning cycle.  I begin by embedding the blade of a knife in an apple, just far enough so that the apple will remain stuck to the blade when the apple is lifted.  (A potato will also work, but an apple is more appropriate for a lesson on Sir Isaac.)  Next, I ask students to predict what will happen when I gently tap the back of the knife blade with the blade of a second knife.  After the predictions have been made, I begin tapping.  Following a few taps, the apple is cut in half.  I always hear some oohs and aahs or "That's really cool."  Students record their observations then huddle in their lab groups and attempt to explain them.  Often, students will suggest that the knife simply cut through the apple and that the tapping knife forced it through.  Some groups may not be able to offer any explanation.  But there is usually a group who will surmise that somehow, for some reason, the apple was pushing back on the embedded knife.  I accept all inferences with as little facial expression or body language as possible.

We do not solve the apple problem that day, nor do I introduce the term inertia. Instead, we move into the exploration phase to investigate an object's resistance to motion as well as an object's tendency to remain in motion.

## Resources

Instructional Notes

Suggested Labs and Activities

• 6.2.2.2.1:  Force = Mass x Acceleration.

In this activity, students experiment with cardboard tubes of different lengths to see how far they can blow a marshmallow.

• 6.2.2.2.1:  An object at rest remains at rest, unless acted upon by an outside force.

In this activity, students apply a force to a stack of bricks and observe Newton's Laws of motion take place.

• 6.2.2.2.2:  Forces in Different Directions:

In this activity, students study forces in different directions and apply them to a NASA aircraft.

Instructional Resources

Instructional Suggestions/Options

Demonstrate how Newton's first law of motion applies to students' everyday lives with this learning cycle series of activities on inertia.  This activity is sure to elicit wide-eyed stares and puzzled looks from students, but also promote critical thinking and help bring an abstract concept to life.  Beginning a learning cycle with a discrepant event such as Newton's Apple hooks students' attention and helps them abandon misconceptions and then form and retain the concepts introduced.

6.2.2.2.1 and 6.2.2.2.2:  Newton's Three Laws of Motion Labs:

In these labs, students participate in hands-on activities that introduce and reinforce Newton's Three Laws of Motion.

New Vocabulary

Vocabulary/Glossary

Force: a push or a pull.

Inertia: a property of matter in which a matter maintains a state of rest or a state of motion.

Magnitude: size; a numerical quantity or value.

Mass: the amount of matter in an object.

Newton: the SI unit of force.  A Newton is equal to a force that would give a mass of one kilogram an acceleration of one meter per second.

Reference Point: a point used to compare the position or motion of another object.  Typically a reference point is stationary, but it can be moving.

Weight: the measure of the gravitational force on an object.

Technology Connections

"Explore the forces at work when you try to push a filing cabinet. Create an applied force and see the resulting friction force and total force acting on the cabinet. Charts show the forces, position, velocity, and acceleration vs. time."

"Each activity in the Mass vs. Weight series demonstrates the difference between mass and weight by comparing results with video clips filmed by astronauts performing similar activities onboard the ISS.  Students perform the activities in the classroom, record, analyze, and interpret their data.  Following data analysis, they observe video of astronauts performing similar demonstrations on the ISS.  The activities focus on Newton's Second Law of Motion."

This activity is designed to help students understand the impact of gravity on weight.  An individual enters his/her weight, and the site calculates what the person's weight would be on each planet.  A paragraph of scientific information about the relationship of mass and weight is included at the bottom of the page.

In this simulation, students observe what it means to consider a reference point when describing a motion.

References

6.2.2.2.1:  , creThis lessonated by Project Based Inquiry Science, guides teachers through leading a class discussion with small demonstrations that depict balanced and unbalanced forces.

6.2.2.2.3:  This simulation allows teachers to demonstrate how to read the scales of a triple beam balance when students are learning how to measure the mass of an object.

## Assessment

Assessment of Students

From: Forces, Motion, and Energy. (2002). Austin, Texas: Holt, Rheinart, and Winston:

• "Can a moving object be used as a reference point?  Why or why not?"
• "Explain the difference between balanced and unbalanced forces and how each affects the motion of an object."
• "Explain why your weight would change if you orbited Earth in the space shuttle but your mass would not."

Assessment of Teachers

"When you lift an object, what force opposes your lift? Draw a force diagram of  the situation as the object is being lifted.  Are the forces balanced or unbalanced?"

"When you push an object along the ground, what force opposes your push? Imagine you have two stacked boxes and you push the bottom box. The top box moves along with the bottom box, but no apparent force is applied to it. Why does it move?"

"How did you assess students' understanding of what affects the motion of an object? If you feel your students need more guidance understanding this, what ideas do you have?"

Taken from: this site.

## Differentiation

Struggling Learners

Struggling and At-Risk

Spend time carefully assessing the misconceptions that students may have when they begin this unit of study.  Then use a variety of mediums to present information to that specifically focuses on redirecting misconceptions.  Focus on providing hands-on learning experiences that engage struggling learners.  Allow students to manipulate a variable in those labs and identify and explain the results.

English Language Learners

In this activity, students act out balanced and unbalanced forces as they attempt to break a turkey wishbone.

While developing motion concepts with ELL students make sure to

• Illustrate and diagram concepts with vocabulary on the board during discussion.
• Give step-by-step directions for labs.
• Prepare word walls or glossary sheets with illustrated vocabulary for students to easily access.
• Summarize discussion and learning more frequently.
• In setting up groups, pair non-native with native speakers.
• Make connections to the students' out of school experiences.
• Vary instructional delivery to include picture books, video, etc.

Extending the Learning

Help students find out what the gravitational force on other planets is compared with the gravitational force on Earth.  Then have them write a short story or make a technology-based presentation about what life would be like with different gravitational force acting on their bodies and belongings.  Read Clifford Simak's science-fiction short story "Desertion" for one approach.

Forces, motion, and energy. (2002). Austin, Texas: Holt, Rheinart, and Winston.

Multi-Cultural

Provide students with an opportunity to research a physicist from a culture different from his/her own.  Students might consider researching the physicist's country of origin, family life, educational and professional experiences, and how the work has impacted his/her country.

Consider viewing NASA's "This Week at NASA" podcast.  While the short podcasts may not relate directly to the concepts associated with the benchmark being studied, they do feature a variety of minorities involved in the field of physics.

Special Education

Provide students with a jenga game of stacked blocks.  Prompt students to make observations and sketches of the tower.  (Ex: Are the blocks stationary or moving, balanced or unbalanced, etc.)  Allow the students to take turns playing the jenga game until the blocks fall.  Prompt the students again to record or draw observations of the tower now.  Explain that when the tower was in balance, the forces holding it up were balanced.  When the tower was falling, the forces were unbalanced.

Classroom Observation

Many misconceptions surround the concepts of motion, mass, and weight.  A principal observing teachers instructing these standards should see teachers eliciting prior knowledge from students, and using the prior knowledge or misconceptions to coordinate labs and demonstrations that support students developing a new knowledge set.  Labs and demonstrations should include an opportunity to collect and analyze data that relate to the concepts.  Literature and text-book selections should be introduced after students have had an opportunity to experience the concept.

Parents
• This standard can be explored and discussed with a visit to an arcade and playing a game of air hockey.  The puck has virtually no resistance.  What forces act upon it to make it move and change direction?  How easy is it to start it in motion?  Does it comes to a stop before hitting the wall?
• These concepts can also be explored at the amusement park.  Roller coasters allow students to feel their bodies wanting to travel in one direction, but forces (restraints) safely holding them back.  Magnitude of force and total forces at play can be discussed and analyzed.  Have students examine their favorite ride and analyze the forces at play that make it fun for them.