Forces and Motion

Forces and Motion

Forces and Motion

Level: 8th Grade
Time Frame: 1 Quarter
Description: Friction, inertia, and momentum are examined by students using a variety of labs and experiments. Balanced and unbalanced forces as well as actions and reactions are illustrated. Work and mechanical advantage are examined and students also measure various forces using simple instruments.

Overview
Activity List
Learning Objectives
Standards
Resources

Unit Overview
In middle school, it is important that students develop qualitative understandings of forces and motion concepts through concrete experiences. For some students, this unit on forces and motion will be the last time they receive instruction in this “physics” topic, and for the purposes of “scientific literacy” they need an everyday, real-world understanding of the basic principles and applications of science. For students who will go on to take high school and college physics courses, it is also important to develop such understandings as a solid foundation for the transition to math-based physics. As is explained in the quote below from the benchmarks, it is important to keep in mind that many students find equations confusing rather than clarifying; and that other students who are good at plugging numbers into formulas may have no real understanding of what the equations mean.

The text chapters on forces and motion were clearly written with some of the information below in mind. The primary text contains no equations and emphasizes activities that aid students in developing qualitative understandings. Quantitative information is relegated to the source book section, which serves as a source of background information for the teacher as well as enrichment for the students who desire this. Also, specific difficulties with forces and motion concepts, as identified in the Benchmarks below, are addressed in the text. For these reasons, many of the lessons in the unit are based to a great extent on reading and discussing the text information and doing the included activities.

From the National Science Education Standards:
“The study of motions and the forces causing motion provide concrete experiences on which a more comprehensive understanding of force can be based in grades 9-12. By using simple objects, such as rolling balls and mechanical toys, students can move from qualitative to quantitative descriptions of moving objects and begin to describe the forces acting on the objects. Students' everyday experience is that friction causes all moving objects to slow down and stop. Through experiences in which friction is reduced, students can begin to see that a moving object with no friction would continue to move indefinitely, but most students believe that the force is still acting if the object is moving or that it is "used up" if the motion stops. Students also think that friction, not inertia, is the principle reason objects remain at rest or require a force to move. Students in grades 5-8 associate force with motion and have difficulty understanding balanced forces in equilibrium, especially if the force is associated with static, inanimate objects, such as a book resting on the desk.”

From the Benchmarks for Science Literacy:
“The (middle school) benchmarks for understanding the motion of objects and repeating patterns of motion do not demand the use of equations. For purposes of science literacy, a qualitative understanding is sufficient. Equations may clarify relationships for the most mathematically apt students, but for many students they are difficult and may obscure the ideas rather than clarify them. For example, almost all students can grasp that the effect of a force on an object's motion will be greater if the force is greater and will be less if the object has more mass—but learning a=F/m (which to many teachers seems like the same thing) is apparently much harder.

Newton's laws of motion are simple to state, and sometimes teachers mistake the ability of students to recite the three laws correctly as evidence that they understand them. The fact that it took such a long time, historically, to codify the laws of motion suggests that they are not self-evident truths, no matter how obvious they may seem to us once we understand them well. Much research in recent years has documented that students typically have trouble relating formal ideas of motion and force to their personal view of how the world works.

These are three of the obstacles:

A basic problem is the ancient perception that sustained motion requires sustained force. The contrary notion that it takes force to change an object's motion, that something in motion will move in a straight line forever without slowing down unless a force acts on it, runs counter to what we can see happening with our eyes.
Limitations in describing motion may keep students from learning about the effect of forces. Students of all ages tend to think in terms of motion or no motion. So the first task may be to help students divide the category of motion into steady motion, speeding up, and slowing down. For example, falling objects should be described as falling faster and faster rather than just falling down. As indicated earlier, the basic idea expressed in Newton's second law of motion is not difficult to grasp, but vocabulary may get in the way if students have to struggle over the meaning of force and acceleration. Both terms have many meanings in common language that confound their specialized use in science.
Like inertia, the action-equals-reaction principle is counterintuitive. To say that a book presses down on the table is sensible enough, but then to say that the table pushes back up with exactly the same force (which disappears the instant you pick up the book) seems false on the face of it.

What is to be done? Students should have lots of experiences to shape their intuition about motion and forces long before encountering laws. Especially helpful are experimentation and discussion of what happens as surfaces become more elastic or more free of friction.”

The textbook and this unit are organized to support this approach to teaching about forces and motion. Students do exploration activities and develop experience-based ideas about forces and motion before the formal instruction on concepts and laws. A fairly extensive review of friction precedes other sections on the laws of motion because the typically overlooked effect of the force of friction is what makes the first law of motion seem counterintuitive.

Most of the lessons in this unit are based on the primary text section of the textbook; but a few lessons based on the sourcebook section are included as well. It should be recognized, however, that only the high academic level students are likely to truly understand the quantitative material. These students can be encouraged to explore math-based physics in greater depth for “extra credit.” Assessment for the unit is based primarily on qualitative understanding of the concepts.

Activity List

Lesson 1(10:1): Forces at Work (Text pp. 216-221)
Lesson 2 (10:2): Different Kinds of Forces (Text pp. 222-225)
Lesson 3 (10:3): What Makes Things Fall? (Gravity) (Text pp. 226-230)
Lesson 4 (11:1): Measuring Forces: Tools to Use (Text pp. 233-239)
Lesson 5 (11:2): Estimating the Size of a Force (Text pp. 240-243)
Lesson 6: Motion: Definitions and Descriptions (Text pp. S72-S78)
Lesson 7 (12:1): Friction (Text pp. 250-256)
Lesson 8 (12:2): Causes and Consequences of Friction (Text pp. 257-263)
Lesson 9: (13:1-2): Motion and Forces ( Text pp. 264-274)
Lesson 10 (13:3): Inertia (Text pp. 275-279)
Lesson 11 (13:4): Forces in Pairs (Action/Reaction) (Text pp. 280-283)
Lesson 12: Momentum (Text p. S82)
Lesson 13: Work, Mechanical Advantage, and Simple Machines
Structures Extension/Integration
Lesson 14 (14:1): The Science of Structure (Text pp. 295-303)
Lesson 15 (14:2): The Response of Structures to Force (Text pp. 304-313)
Lesson 16 (14:3): Engineering Structures (Text pp. 314-325)
Lesson 17 (15:1-2): The Art of Design (Text pp. 336-345)

Learning Objectives
Students will know and/or observe:

A force is a push or pull exerted by one object on another
A force can change the shape or motion of an object
Forces have size and direction
There are many types of forces, including buoyant, magnetic, electrical, gravitational, elastic, and frictional.
Buoyant, elastic, and frictional are contact forces. Magnetic, electrical, and gravitational forces are non-contact forces (can act over a distance).
Every object in the universe exerts a gravitational force on every other object
The size of a gravitational force depends on the masses of the objects involved and the distance between them.
The weight of an object changes with its location in the universe, but the mass of an object remains constant
Elastic materials that stretch uniformly when masses are added to them can be used to measure force
The unit of force, the Newton, is approximately equal to the weight of 100 grams on the Earth’s surface
The size of the force needed to lift something can be estimated based on knowledge of the relative weights of objects
The size of a force can be measured in newtons with a force meter (spring scale).
Mass and weight are measured with different kinds of scales and in different units
Motion is a change in position of an object when compared with a reference point
Motion can be started, stopped, or changed by forces
Speed, or the rate at which an object moves, refers to the distance that is covered in a unit of time
Acceleration is a change in the speed or direction of motion
Friction opposes a starting or continuing motion
Friction is greater when motion is starting than when it is continuing
Friction varies with the kinds of surfaces involved
Friction results when surfaces that are moving past each other interlock
Friction may be reduced by smoothing opposing surfaces, lubricating them, or separating them with ball bearings.
If balanced forces are acting on an object, its motion will not change.
An unbalanced force will cause an object to speed up, slow down, or change direction, and will cause an object at rest to start moving.
When balanced forces act on an object at rest, it will remain at rest
An object in motion with balanced forces acting on it will continue in a steady, straight motion
Inertia is the tendency of moving objects to continue in the same direction at the same speed and of resting objects to remain at rest (Newton’s First Law)
The amount of an object’s inertia depends on its mass
If object A exerts a force on object B (an action), then object B exerts a force on object A that is equal in size but opposite in direction (a reaction) (Newton’s Third Law)
Both action and reaction forces can affect the motion of the objects on which the forces are applied
Momentum is a characteristic of a moving object that can be thought of as “pushing power” or “bashing power.” The momentum of a moving object increases with the mass and speed (velocity) of the object
Simple machines are used to modify the amount and/or direction of the force needed to move an object

(Structures Extension)

All structures must be designed to withstand destructive forces
Problems of design and structure have a variety of solutions
Valuable information about designing structures can be learned from studying structures that have failed and those that have survived.
Forces applied to structures may be classified as either tensile, compressive, or shear
Structures are compressed by their own weight
Materials that respond to the removal of stress by returning to their original shape are said to behave elastically
In an elastic response, tensile force caused an extension that is proportional to the applied force
One measure of strength is the ability of a given material or structure to withstand force without breaking
A truss framework adds strength and support to a structure
For the same amount of material, an I-beam has greater rigidity and strength that does an ordinary beam
A cantilever is a beam that is fixed at one end.
An arch provides more strength against a vertical load than does a simple beam; an arch transforms vertical loads into lateral ones.
Bridges are classified according to their structures: arch, cantilever, suspension, or girder beam

Standards

Illinois Learning Standards (Middle School) As a result of their schooling students will be able to:

Know and apply concepts that describe force and motion and the principles that explain them:
12.D.3a Explain and demonstrate how forces affect motion (e.g., action/reaction, equilibrium conditions, free-falling objects).
12.D.3b Explain the factors that affect the gravitational forces on objects (e.g., changes in mass, distance).

Know and apply the concepts, principles and processes of scientific inquiry:
11.A.3a Formulate hypotheses that can be tested by collecting data.
11.A.3b Conduct scientific experiments that control all but one variable.
11.A.3c Collect and record data accurately using consistent measuring and recording techniques and media.
11.A.3d Explain the existence of unexpected results in a data set.
11.A.3e Use data manipulation tools and quantitative (e.g., mean, mode, simple equations) and representational methods (e.g., simulations, image processing) to analyze measurements.
11.A.3f Interpret and represent results of analysis to produce findings.
11.A.3g Report and display the process and results of a scientific investigation.

Know and apply the concepts, principles and processes of technological design.
11.B.3a Identify an actual design problem and establish criteria for determining the success of a solution.
11.B.3b Sketch, propose and compare design solutions to the problem considering available materials, tools, cost effectiveness and safety.
11.B.3c Select the most appropriate design and build a prototype or simulation.
11.B.3d Test the prototype using available materials, instruments and technology and record the data.
11.B.3e Evaluate the test results based on established criteria, note sources of error and recommend improvements.
11.B.3f Using available technology, report the relative success of the design based on the test results and criteria.

Know and apply concepts that describe the interaction between science, technology and society.

13.B.3b Identify important contributions to science and technology that have been made by individuals and groups from various cultures.

13.B.3c Describe how occupations use scientific and technological knowledge and skills

State Assessment Framework :
12.7.63 Understand the concept of force as any influence that tends to accelerate an object. Know that a force, for example, can speed up an object, or slow it down, or change its direction. Understand that forces can be measured in various ways. Understand how to calculate the acceleration of an object.
12.7.64 Identify and understand Newton’s laws of motion. The first law of motion states that things at rest or in motion tend to stay at rest or continue in motion unless some force is applied to them. Newton's second law of motion (force = mass × acceleration) shows how force, mass, and acceleration are related. The third law states that for every action there is an equal and opposite reaction.
12.7.65 Understand the concept of work. A force acting through distance is work. Recognize applications of simple machines (wedge, lever, inclined plane, pulley, screw, and wheel and axle) in common tools.
12.7.67 Understand that the gravitational force between two bodies decreases as the bodies get farther apart from each other. Know that the gravitational force between two bodies decreases as their masses decrease.
12.7.68 Understand how to calculate average speeds, given the distance traveled and the time taken.
12.7.69 Distinguish between mass and weight. Know that the mass of a body remains the same regardless of where it is but that the weight of it depends on how strong the force of gravity is in its current location.

National Science Education Standards: Physical Science

CONTENT STANDARD B: 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.

Benchmarks for Science Literacy By the end of 8^th grade, students should know that

An unbalanced force acting on an object changes its speed or path of motion, or both. If the force acts toward a single center, the object’s path may curve into an orbit around the center
Every object exerts gravitational force on every other object. The force depends on how much mass the objects have and on how far apart they are. The force is hard to detect unless at least one of the objects has a lot of mass.

Resources

Untitled Document

Best Viewed in Internet Explorer 7 and Firefox 2.0 it is strongly encouraged to upgrade if you are using previous versions

For problems, issues, concerns, constructive criticism
and compliments, please email webmaster2@champaignschools.org

Champaign Community Unit School District #4 * Mellon Administrative Center
703 South New Street * Champaign, IL 61820 * 217.351.3800