Teacher's Manual Features
Below is an excerpt from Unit H of the Teacher's Manual.
|Unit H Conservation Laws
Because the laws of conservation of momentum and the conservation of energy are so fundamental to understanding physics, it is strongly recommended that this unit be covered thoroughly in your physics course.
Summary of Unit H
The unit starts with an examination of inelastic head-on collisions between two free bodies moving in a straight line. Given the masses of the objects, their initial velocities and the coefficient of elasticity, students are encouraged to observe and predict the outcomes of such collisions. It is then shown how the law of conservation of momentum applies to all inelastic collisions and how both momentum and energy are conserved in every elastic collision. The unit continues with a discussion of how potential energy is often converted to kinetic and then reconverted to the original form. The unit concludes by examining the physics of karate and the conservation laws that are involved in this sport.
Suggestions for Presentation
For the most effective learning, students should be allowed to observe actual collisions in the laboratory and encouraged to contrive simulated collisions with the aid of the computer programs. If computers are unavailable, drawing time-position graphs will help in visualizing the concepts. Learning with graphs is not as fast as it is with the computer, but graphs can be made very quickly. Set an example by making them on the blackboard exactly as you expect the students to make them in their notebooks. Graph paper is not necessary, although a ruler is. The basic outline for a collision diagram is three parallel lines equally spaced with a vertical line going through the center. It helps to make the graph on lined loose leaf paper. The time axis is always the same. A scale, chosen for the horizontal axis. should be carefully measured out with a ruler.
Center of Mass
Emphasize that the center of mass of any system never changes its position while any of its particles are colliding. After this concept is developed with observations and diagrams, introduce the algebraic conservation law as a neat, quick way to develop the same answers. Do not spend too much time on the algebraic solution because the algebra does little to increase conceptual understanding of the collision process.
Momentum and Newton’s Third Law
Use the action-reaction law, covered earlier, to lead into another useful way of dealing with forces, the impulse concept. Draw students’ attention to the similarity with the work concept. Here is an excellent opportunity for using a computer to study impulses with an analog to digital converter and a strain gauge to measure the force of a collision.
Stress that both momentum and kinetic energy are conserved in elastic collisions. Again the time-position diagrams show this very well. However, before introducing conservation of kinetic energy, let the students examine elastic collisions from a conceptual level.
Lab H-1: Elastic Deformation and Speed Change (page 189)