6-8 > Physical Science
 Grade level: 6-8 Subject: Physical Science Duration: One class periods
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Force Counterforce

Students will understand the following:
 1 A force is an agency or influence that, if applied to a free body, results in an acceleration of the body. 2 Gravity is such a force. 3 As the force of gravity on a body increases, the acceleration of the body increases. 4 Air resistance and friction act as counterforces to gravity.

Distribute the following materials to each group of students:
 • 30-inch length of string • Small, freely rolling toy car • Paper clip opened to serve as a hook • A variety of weights that can be attached to the paper-clip hook • Stop watch

 1 Let students know that they will be performing an experiment that will confirm their predictions about the effect of gravity on the motion of a body. 2 Divide the class into small groups, distributing the necessary materials to each group. 3 Instruct students to set up their experiment by tying one end of the string onto the toy car and the other end to the paper-clip hook. 4 Have students place their cars on a flat table or desktop and hang the paper-clip hook over the edge of the table. 5 Calling attention to the variety of weights each group has been given, ask students to predict how the cars will move if different weights are hung from the paper-clip hook and allowed to fall to the floor. Possible predictions might include: (1) as the weight increases, the car will move with increasingly higher constant speeds toward the edge of the table; (2) different weights will have no effect on how the car moves; (3) as the weight increases, the car will move toward the edge of the table with greater and greater acceleration. (Correct prediction: the car will move with greater acceleration as the weight increases.) 6 After students have made their predictions, ask them to support them by creating illustrated diagrams using arrows and labels to indicate the forces and counterforces acting on the car. 7 Once the charts are complete, have students conduct experiments using the cars, weights, and stopwatch to confirm or refute their predictions. Students should record the results of their experiments.

 Adaptations for Older Students:Ask students to identify which of Newton’s laws their experiment illustrates. Challenge them to determine a mathematical relationship between the mass of the car and the weight of the object attached to the paper-clip hook.

 1 Describe in terms of forces and counterforces what causes an object to experience acceleration. 2 Identify the counterforces or forces that bring a soccer ball to rest as it rolls across a field, accelerate a rocket off a launch pad, and act on a leaf as it drifts down to the forest floor. 3 Imagine a few different objects in motion—a punted football, an ice skater, and a satellite. Identify the forces and counterforces acting on them, then debate whether they are moving with constant speed or experiencing acceleration. 4 When an object is in motion, counterforces sometimes seem to get in the way. If gravity didn’t act as strongly on the space shuttle, for instance, launching would be a lot easier. This is not always the case, however. Sometimes, counterforces are invaluable, as when air resistance keeps a skydiver’s parachute in proper working order. Brainstorm a list of examples of useful counterforces from your everyday life. For what situations are they truly invaluable? 5 Analyze the motion of a skydiver falling to the ground. When are the forces and counterforces acting on the skydiver in balance? When are they not in balance? Describe the motion experienced by the skydiver in each situation. 6 From your personal experiences with motion, is it natural for objects to seek a state of balance or unbalance with respect to the forces acting on them? Defend your answer with examples.

 You can evaluate your students on their diagrams and experiments using the following three-point rubric: Three points:diagram clearly designed and labeled, experiment carefully set up and carried out, accurate results yielded by experiment, results of experiment accurately recorded on a well-designed chart Two points:diagram clearly designed and labeled, experiment adequately set up and carried out, accurate results yielded by experiment, results of experiment adequately recorded on a sufficiently clear chart One point:diagram inadequately designed and labeled, experiment carried out, inconclusive results yielded by experiment, results of experiment only partially recorded You can ask your students to contribute to the assessment rubric by determining criteria for a clear and accurate diagram, a well-executed experiment, and a well-designed chart for recording results.

 Acceleration vs. Constant Speed Ask your students to make a list of five different moving objects they have observed in the past 24 hours. For each item on their list, they should identify the forces and counterforces that were acting on it, and then determine whether the object was experiencing constant speed or acceleration. (Acceleration is positive when the forces acting on an object are stronger than the counterforces and negative when the reverse is the case. Negative acceleration can be thought of as deceleration.) Using words such asforce,counterforce,balanced, andunbalanced, students should explain why some of the objects they observed were moving with constant speed while others were accelerating. Air Resistance and Acceleration Ask your students to use a stopwatch to measure the time it takes a freely rolling toy car to roll from the top to the bottom of a slightly inclined board. Instruct them to record their times. Ask students if the car appears to be rolling with a constant speed or accelerating. Then have students cut three squares out of a stiff piece of cardboard, making one square 5” x 5”, one 10” x 10”, and one 20” x 20”. Have them fasten the 10” x 10” square onto the front of the toy car so that the plane of the square is perpendicular to the forward motion of the car. Next, have students measure and record the time it takes their “sail car” to roll from the top to the bottom of the same inclined board. Instruct students to make a diagram of each part of the experiment, representing each force and counterforce acting on the car with a labeled arrow indicating the force’s probable direction. Students should use their diagrams to explain the difference they observed in the times they recorded, both with and without the “sail.” Students can then use similar diagrams to predict what will happen when they replace the first cardboard “sail” with the other two and conduct experiments that will test their hypotheses.

 Fatal Forces Nick Arnold. Scholastic, 1997.Let science be fun as this college professor explains physical forces so that even nonscientists can understand them. Lots of humor and lots of cartoons make this an entertaining, informative volume. Movement Sally and Adrian Morgan. Facts on File, 1993.Animals have adapted over many thousands of years to move successfully; man has studied and put to use the design of animals to make machines that move. The experiments provided in this book allow a better understanding of both animals and machines and the forces that affect their movement. An index, glossary, and answers to questions are included.

 Experiments and Demonstrations with Air Pressure Hands on activities that use Bernoulli's Principle to lift heavey weights, syphon water, crack boards, crush soda cans and much more. Principles of Aeronautics At a beginner, intermediate or advanced level, learn how aeronautical principles are applied to the study of the atmosphere and weather, vehicles, sports equipment, animals, and even mythological characters. Is Air Something? Fifteen lessons about air pressure with hands on activities that involve parachutes, bubbles, paper airplanes, balloons, whirling wonders, wind vanes and more. www.grc.nasa.gov/WWW/K-12/Summer_Training/Elementary97/IsAirSomething.html Aeronautics Learning Laboratory for Science Technology and Research An online Aeronautical tutorial includes multimedia presentations on the history and principles of flight on three different levels of difficulty. FoilSim Virtual Wind Tunnel In this interactive virtual wind tunnel students analyze the effects on the motion of an airplane wing or baseball by changing their position and shape, by moving slider controls that vary the parameters of airspeed, altitude, angle of attack, thickness and curvature.

 Click on any of the vocabulary words below to hear them pronounced and used in a sentence. Definition:To move faster or gain speed. Context:If the forces acting on an object do not cancel each other out, the object might change speed and accelerate. Definition:A counterbalancing of weight, force, or influence. Context:An object is said to be in balance when all of the forces and counterforces acting on it cancel each other out. Definition:An opposing or corresponding force. Context:Air resistance provides a counterforce to gravity as a skydiver falls out of an airplane. Definition:An agency or influence that, if applied to a free body, results chiefly in an acceleration of the body and sometimes in elastic deformation and other effects. Context:Gravity is one of the fundamental forces that acts on an object in flight.

 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 Standard: Understands motion and the principles that explain it. Benchmarks: 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 of 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 (e.g., objects change their motion only when a net force is applied; whenever one object exerts force on another, a force equal in magnitude and opposite in direction is exerted on the first object; the magnitude of the change in motion can be calculated using the relationshipF=ma,which is independent of the nature of the force). Grade level:9-12 Subject area:mathematics Standard: Understands and applies basic and advanced properties of the concepts of geometry. Benchmarks: Benchmark: Understands the characteristics and uses of vectors (e.g., representations of velocity and force). Benchmark: Uses geometric constructions (e.g., the parallel to a line through a given point not on the line, line segment congruent to a given line segment) to complete simple proofs, to model, and to solve mathematical and real-world problems.

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