Day 07 - Drag Forces

Day 07 - Drag Forces#

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Announcements#

  • Homework 2 is due Friday

  • Video recordings will cease; I will try to record my tablet writing next week.

    • Class can still join zoom with password: phy321

  • Updated office hours (Danny-DC; Elisha-EA):

    • Monday 4-5pm (DC) - change?

    • Tuesday 5-6pm (EA)

    • Wednesday 4-5pm (DC)

    • Thursday 5-6pm (EA)

    • Friday 10-12pm (DC then EA); 3-4pm (DC)

Calendar changes and apologies#

  • I’m very behind on class prep. And I’m very distracted right now.

  • The notes for next week will be posted by Friday.

    • If you need anything or I’m missing, just drop me a note. I probably just missed it.

  • There will be no homework 9, and there will be no new material for the last week of class.

    • Instead, that week will be final prep for your projects that will be due Monday of finals week at midnight.

    • More details soon, but we will also use homework and midterms to help you make progress on your final projects.

Seminars this week#

WEDNESDAY, January 29, 2025

  • Astronomy Seminar, 1:30 pm, 1400 BPS, Michiel Lambrechts, Univ. of Copenhagen, Planet formation

  • FRIB Nuclear Science Seminar, 3:30pm., FRIB 1300 Auditorium, Brenden Longfellow of Lawrence Livermore National Laboratory, From Tensor Current Limits to Solar Neutrinos: 8Li and 8B Studies with the Beta-decay Paul Trap

Tomorrow’s Seminar#

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Goals for Week 3#

  • Be able to answer the following questions.

    • What is Mathematical Modeling?

    • What is the process for analyzing these models?

  • Be able to solve “Simple” Motion Problems with Newton’s Laws.

Our man, Reynolds#

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  • The Reynolds number is a dimensionless quantity.

  • It is a ratio of inertial forces to viscous forces.

\[Re = \frac{\rho v L}{\mu}\]

  • \(\rho\) - density of the fluid

  • \(v\) - velocity of the object

  • \(L\) - characteristic length

  • \(\mu\) is the dynamic viscosity

Our man, Reynolds#

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BTW, this is not a photo of Reynolds.

  • This is Stokes.

    • He developed the concept of the Reynolds number.

    • Reynolds “popularized” it according to the Wikipedia.

\[Re = \frac{\rho v L}{\mu}\]

Discussion: What kinds of systems have a high/low Reynolds number?

Clicker Question 6-2#

Assuming a linear model for Air Resistance \(\sim bv\), we obtained this EOM for a falling ball:

\[\ddot{y} = -g + \frac{b}{m}\dot{y}\]

What happens when \(\ddot{y} = 0\)?

  1. The ball stops moving (\(v = 0\)).

  2. The ball reaches a velocity of \(mg/b\).

  3. The ball reaches a terminal velocity.

  4. I’m not sure.