Stosh’s Laws

Introduction

My high school physics teacher stole this from one of his professors. To this day, I recall him saying, “I’m sure one of you will steal this from me.” I vowed to be the student that stole it from him. They used to be Paquet’s Laws[1]. I’ve made some updates and slapped my own name on them. My students have always found them useful. I hope you will too!

How to Solve Any Physics Problem

Honestly, this works for pretty much every physics problem. I got through graduate course work using this approach.

  1. Read the problem. I mean really dig in and read it.
  2. Record all given information, including units… Units!! Units!!
  3. Draw a picture representing the physics.
  4. Identify what the problem is asking for
  5. Link what you have to what you need. This is usually an equation.
  6. Put your information into the link
  7. Plug and Chugalug
  8. Inspect your answer. Does it make sense? Do your units work out?
  9. If 8 checks out you’re finished. If it doesn’t, go back to 1.

How to solve a Vector Addition Problem

Vector addition problems are a sub-set of physics problems. Most everything we deal with (forces, angular momentum, etc) are vectors. Understanding how to break them up and combine them will be critical.

  1. Make an array of all vectors listing the vector, magnitude, and angle.
  2. Using trigonometry, find all of the x and y components of each vector.
  3. Calculate: $x_r = \Sigma x$ and $y_r = \Sigma y$. This gives you the x and y components of the resultant vector.
  4. Use $c = \sqrt{x_r^2 + y_r^2}$ to find the magnitude of the resultant vector.
  5. Use $\theta = arctan(y_r/x_r)$ to find the angle of the resultant vector.

Important notes:

  1. All angles are measured from the +x-axis as being zero degrees.
  2. Since there are two tangents that have the exact same value, inspect your final direction. Ensure that you have it
    going the correct way.

Creating Study Guides

When studying physics you need a study guide. They keep you organized while codifying new information. A good study guide gives you a high level idea of what’s in the chapter. It provides a list of important equations and definitions. Writing out concepts in your own words ensures you understand them.

  • Record the title of the Chapter
  • Record the section heading
  • While reading the section, record any bold face terms and equations. Don’t forget the meanings of variables!
  • Note anything that confused you. Look it up in a different book, or speak with your professor to clear up these issues.
  • Continue to repeat this process until you reach the end of the chapter.
  • When you are finished make a crib sheet of all the important equations. Use the crib sheet to work on your homework.

  1. [1]
    J. Paquet, Paquet’s Laws, Private Communication. (2002).

One Reply on “Stosh’s Laws”

  1. The assumption that V(xf), V(yf) is incorrect. Clearly, as the projectile descends, it gains momentum. Based on the presentation, have the initial and final Y velocities set to zero implies no change in momentum. We know this to be false. If one does the problem as a vector problem, one will find that indeed the final Y velocity is -19.8 m/s. However, the Y motion is not linked to the X motion. Only by the convenience of the 45 degree angle, does this give the initial velocity in X the same value with the opposite sign. Actually, the final velocity is 28 m/s. Indeed, demonstrating that the momentum MV has increased due to the work done by the gravitational force! Remember, that the initial X velocity is conserved as is the momentum in X. Not so for Y hence the increase in velocity. As stated, the problem violates conservation of momentum…..

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