Unit II: Mechanics

 

Introduction

This unit should be about six weeks. It should introduce the students to the concepts of mechanics. Mechanics can be broken down into linear motion, nonlinear motion, Newton’s laws of motion, momentum, energy, rotational motion, gravity, and satellite motion. There is an enormous amount of information within this unit. Demonstrations and experiments are important facets in understanding all of the concepts packed into mechanics. Students should become more aware of the mechanics in nature that they face every day from driving a car to the motion of a satellite.
 

Intended Learning Outcomes
 
 
General Instructional Objectives (and corresponding Specific Learning Outcomes) Unit Content
Knows terms

Understands the meaning of terms

speed, velocity, acceleration, inertia, free fall, momentum, impulse, projectile, mass, weight, force, friction, impulse, angular momentum, work, power, potential and kinetic energy, torque, centripetal and centrifugal force, circular and elliptical orbits, efficiency, equilibrium, spring and neap tide, weightlessness, black hole, big bang, satellite, ellipse
Understands scientific concepts motion is relative, velocity is a vector quantity vs. scalar quantity, free fall, elastic vs. inelastic collision, projectile motion, parabola, Newton's Laws, potential vs. kinetic energy, conservation of momentum and energy, center of mass and gravity, Kepler's Laws, law of universal gravitation, inverse-square law, gravitational field, escape speed
Knows all laboratory procedures

Prepares a plan for an experiment

Participates in classroom activities

Respects the scientific process

Displays a scientific attitude

Demonstrates skills in laboratory work

All content

 

Instructional Foci

Experiments: motion with a constant and a nonconstant velocity, the pendulum, Newton’s second law, work and power, friction; demonstrations: free fall, and centripetal force; class discussions, quizzes, homework, readings, questions for thought from The Flying Circus of Physics by Jearl Walker
 
 

Teaching Strategies

For linear motion the teacher should begin with a general description of motion. Speed, velocity, and acceleration should then be introduced. The concept of free fall (acceleration due to gravity, g) can be introduced with a discussion of Galileo and the Leaning Tower of Pisa. Perhaps you could have your own leaning tower experiment by dropping different object from standing on top of the desk. This could then lead to a discussion about Aristotle and his theories in contrast to Galileo Galilei and his theories—noting the public dissatisfaction with Galileo’s leaning tower experiment and Galileo himself.

Nonlinear motion should begin with the concept of relative motion. Then the concept of vectors versus scalars should be introduced, distinguishing velocity as a vector quantity as opposed to speed, which is a scalar quantity. From here the teacher should increase in complexity to projectile motion and circular motion. The space shuttle’s projectile motion could be explored to make the concept relevant to the student, expanding on what they are already familiar with.

Building on the information presented in linear and nonlinear motion, the teacher should explore Newton’s Laws of Motion. Many in class demonstrations and discussions should be utilized to ensure students understand the concepts presented here. The difference between mass and weight should be discussed. Force, inertia, and the conservation of energy are other key element to be taught in this section, also.

Next, the idea of combining inertia and motion to get momentum should be introduced. To produce a change in momentum either the velocity or the mass must be manipulated. Most commonly the velocity is changed due to a force acting on the object for a given time which is know as an impulse. Impulses such as the brakes of a car, the swing of a golf club or baseball bat, or the bouncing of a ball can all be discussed and demonstrated. The conservation of momentum should then be presented by discussing collisions—both elastic and inelastic—using examples of cars and billiard balls as common examples.

Then we begin the study of energy. Energy is difficult to define because it is a "thing" as well as a process. Everything has energy, but we only observe it when it as it is being transformed. In other words, we see it in forms—in the form of the food we eat, in the form of electromagnetic waves we feel as heat from the sun, in the form of photosynthesis for plants, etc… These aspects of energy should be introduced before exploring related concepts, like work. Mechanical energy, such as potential and kinetic energy should then be compared and contrasted. Of course one of the fundamentals of physics, the concept of the conservation of energy, should then be taught, giving examples of how cars consume more fuel when the air conditioner, lights, and radio are on—nothing is free. Then the teacher can discuss energy used for machines and energy used for life—what are the similarities and differences?

There is much more to nonlinear motion and Newton’s Laws than previously discussed. Rotational motion builds on these principles to explain the rotation of an object about an axis. The teacher should discuss rotational inertia and how torque effects that inertia. The center of mass and center of gravity as well as centripetal and centrifugal force should be demonstrated. The classic demonstration of a bucket full of water rotating around your arm (swinging it) is ideal in demonstrating centripetal and centrifugal force. And of course the exploration of this section should end with a discussion of the conservation of angular momentum.

To introduce this section on gravity the teacher should discuss Johannes Kepler and Kepler’s laws of planetary motion. He/she should then go on to discuss Newton’s law of universal gravitation. Then weight and weightlessness could be introduced with a discussion on taking a scale into an elevator (what does the scale read when stationary, when moving, and when free falling) and astronauts. Following this, the ocean tides should be explained. Gravitational fields and black holes should be taught as a precursor to the next section on satellite motion.

Current satellites—their forms and functions—should be presented as an introduction to the final section of mechanics (at last!), satellite motion. Newton’s ideas of the falling apple and "falling moon" should be studied. Basic satellite motion and escape speed should also be explored. Finally, what would a section of mechanics be without a discussion of energy conservation and satellite motion?
 
 

Special Notes

1. All labs should be preceded by an in class demonstration by the teacher of the proper procedures. After viewing the demonstration, students’ homework should be to prepare a plan for the experiment. Due to the time constraints of high school classes it is imperative that students have thought through the experiment before class. You, the teacher, need to know that they are aware of proper equipment set-up and safety precautions as well as appropriate hypothesis development, variable controls, and observation and measurement procedures.

2.  Labs can be found in Laboratory Manual in Conceptual Physics by Bill Tillery.

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