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Forces And MotionForces-And-Motion

  • Subject: Physical Science
  • |
  • Grade(s): 6-8
  • |
  • Duration: Two class periods

Lesson Plan Sections


Students will understand the following:
1. Gravity is the force of attraction that causes objects to fall toward the center of the earth.
2. Air resistance, or air friction, can slow down the acceleration of a falling object.
3. The area "fronting the wind" affects the amount of air resistance a falling object encounters.
4. Terminal speed is the speed at which the downward pull of gravity is balanced by the equal and upward opposing force of air resistance for a falling object.


Provide the following materials for each group.
Lightweight plastic kitchen garbage-can liners
12 20-inch lengths of light string
3 plastic sandwich bags
3 raw eggs


1. Divide your class into several small groups, and distribute materials to each group.
2. Have students use the following directions to build three "parachutes" for an ordinary chicken egg:
  1. From a lightweight plastic kitchen garbage-can liner, cut out three squares. Make one square 10"x 10", a second square 20" x 20", and a third square 30" by 30".
  2. Make a parachute out of each square by tying a piece of string to each corner of the square, then attaching the other ends of the strings to a plastic sandwich bag.
  3. Place a raw egg in each of the sandwich bags.
3. Ask students to predict which egg has the best chance of surviving a drop from about ten feet from the floor. Students should explain the reasoning behind their predictions.
4. Have students drop each unfurled egg parachute from a height of ten feet, and then determine whether or not their predictions were confirmed.
5. After each group has performed its experiment, ask students to describe the changing forces that acted on the parachutes as they fell and the resulting changes in the parachutes' motion. How did the falls of the larger parachutes differ from the falls of the smaller ones?
6. Review with students that gravity pulled the parachutes downward; air resistance worked as an opposing force to gravity; the parachutes accelerated until the air resistance equaled the gravity, at which point the parachutes reached terminal speed; the bigger parachutes with a larger area fronting the wind created more air resistance than the smaller ones, so the bigger parachutes reached terminal speed earlier.

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Adaptations for Older Students:
Challenge students to express the results of their experiments in mathematical terms.

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Discussion Questions

1. Define the terms speed and acceleration . Explain how the two terms are related.
2. How can creating and analyzing graphs be useful for understanding forces and motion in objects?
3. Construct a position-versus-time graph for the following set of data: ( D1 = 0 m, T1 = 0 sec), ( D2 = 7 m, T2 = 3 sec), ( D3 = 14 m, T3 = 6 sec), ( D4 = 21 m, T4 = 9 sec), and ( D5 = 28 m, T5 = 12 sec). Discuss how you would use this graph to determine the speed of the object being represented.
4. Is the object represented in question 3 moving with constant speed or constant acceleration? Explain how you arrived at your conclusion.
5. Debate what assumptions might be made about the unknown forces that are acting on the object represented by the graph in question 3. Explain the reasons for each of your assumptions.
6. If the object in question 3 continued to move, explain how you might use the graph, its gradient, and the data plotted on its x- and y-axes to determine how far the object would travel in 19.5 additional seconds.
7. From the graph constructed in question 3, calculate the object's speed at three-second intervals, then use this new information to construct a speed-versus-time graph for the object.

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You can evaluate your students on their experiments using the following three-point rubric:
  • Three points: predictions based on sound reasoning, experiment carefully performed, results accurately and completely recorded, explanations clear and logical
  • Two points: predictions based on sound reasoning, experiment performed with sufficient care, results incompletely recorded, explanations acceptable
  • One point: predictions based on guesswork, experiment performed with sufficient care, results incompletely or inaccurately recorded, explanations sketchy
You can ask your students to contribute to the assessment rubric by determining which scientific principles should be covered in the explanations

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Electric Power: Extending the Gradient Concept to a New Context
The gradients (or slopes) of graph lines can generally describe the rate of change of any variable plotted on a y-axis with respect to the change in a related variable plotted on the x-axis. Speed and acceleration are only two of the many kinds of rates that can be determined in this way. Ask your students to use the gradient concept to answer the following questions in a totally different context: The power (P) that we buy from our local electric company is defined as the rate of change of energy (E) with respect to a change in time (T). How many kilowatts of power, therefore, are needed to run your hair dryer if it uses energy at a rate suggested by the following set of data: (E1 = 150 j, T1 = 30 sec), (E2 = 300 j, T2 = 60 sec), (E3 = 450 j, T3 = 90 sec), (E4 = 600 j, T4 = 120 sec), and (E5 = 750 j, T5 = 150 sec). Students should use these figures to create a graph with energy (E) represented on the y-axis and time (T) represented on the x-axis. Then they should calculate the gradient of the resulting line to determine the power (P). Note: Power is measured in kilowatts, energy in joules (j), and time in seconds (sec).

Defining Terms
Have your students write definitions for the following terms and explain how they are related to each other: acceleration, constant speed, terminal speed, balanced forces, unbalanced forces, force, opposing force.

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Suggested Readings

Experiments with Motion
Robert Gardner. Enslow, 1995.
Easy-to-do experiments better illustrate the text and allow a student to understand Newton's Laws of Motion and their application to space flight as well as to the movement of animals and vehicles. Diagrams, explicit lists of materials needed, answers to puzzles, and a bibliography contribute to the usefulness of this work.

Mechanics Fundamentals
Robert W. Wood. Learning Triangle Press/McGraw-Hill, 1996.
Important principles of physics, specifically relating to the effect of forces on objects at rest or in motion, are explained through the simple-to-perform experiments in this book. Line drawings illustrate all experiments and a glossary explains new terms.

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Describing Motion with Position vs. Time Graphs
Tom Henderson's physics students at Glenbrook High School, have access to some of the best physics curriculum on the web. Your students are invited to learn about kinematics through the analysis of Position to Time graphs with this excellent example of a multimedia and interactive physics text.

Describing Motion with Velocity vs. Time Graphs
The next step to understanding the study of moving things ("kinematics") is a tour of the chapter on Velocity to Time graphs at Tom Henderson's multimedia physics text web site.

Virtual Motion Graphs Lab
An interactive virtual graphing lab includes a printable guide that allows the student to vary the magnitude and direction of velocity data and instantly see what happens to the shapes of the position to time and acceleration to time graphs.

Free Body Diagrams or FBD's
Learn to predict whether or not a body will move with constant velocity or acceleration, by constructing Free Body Diagrams that identify of all the forces acting on the body and their net effect on motion.

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Click on any of the vocabulary words below to hear them pronounced and used in a sentence.

speaker    accelerate
Definition: To move faster or gain speed.
Context: A car will accelerate when the forces propelling it forward are stronger than the forces attempting to slow it down.

speaker    gradient
Definition: Change in the value of a quantity with change in a given variable and especially per unit distance in a specified direction.
Context: On a distance-versus-time graph for a moving object, the gradient is equal to the speed of the object.

speaker    graph
Definition: A diagram that represents the variation of a variable in comparison with that of one or more other variables.
Context: The motion of a car can be represented on a graph with position plotted on the y-axis and time on the x-axis.

speaker    terminal speed
Definition: The speed at which the downward pull of gravity is balanced by the equal and upward opposing force of air resistance for a falling object.
Context: Not long after she jumped out of the airplane, the skydiver reached her terminal speed.

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This lesson plan may be used to address the academic standards listed below. These standards are drawn from Content Knowledge: A Compendium of Standards and Benchmarks for K-12 Education: 2nd Edition and have been provided courtesy of theMid-continent Research for Education and Learningin Aurora, Colorado.
Grade level: 6-8, 9-12
Subject area: science
Understands motion and the principles that explain it.
Benchmark 6-8:
Knows that an object's motion can be described and represented graphically according to its position, direction of motion, and speed.
Benchmark 6-8:
Understands effects of balanced and unbalanced forces on an object's motion (e.g., 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 such as friction will cause changes in the speed or direction on an object's motion).
Benchmark 9-12:
Knows that laws of motion can be used to determine the effects of forces on the motion of objects.

Grade level: 9-12
Subject area: mathematics
Understands and applies basic and advanced properties of functions and algebra.
Understands properties of graphs and the relationship between a graph and its corresponding expression (e.g., maximum and minimum points).

Grade level: 9-12
Subject area: mathematics
Understands and applies basic and advanced properties of the concepts of measurement.
Solves problems involving rate as a measure (e.g., velocity, acceleration).

Grade level: 9-12
Subject area: mathematics
Understands and applies basic and advanced properties of the concepts of geometry.
Uses synthetic (i.e., pictorial) representations and analytic (i.e., coordinate) methods to solve problems involving symmetry and transformations of figures (e.g., problems involving distance, midpoint, and slope; determination of symmetry with respect to a point or line).

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Ted Latham, physics and science/technology teacher, Watchung Hills Regional High School, Warren, New Jersey.

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