Day 07 - Drag Forces

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

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

  • The Reynolds number is a dimensionless quantity.
  • It is a ratio of inertial forces to viscous forces.

  • - density of the fluid
  • - velocity of the object
  • - characteristic length
  • is the dynamic viscosity

Our man, Reynolds

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.

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

Clicker Question 6-2

Assuming a linear model for Air Resistance , we obtained this EOM for a falling ball:

What happens when ?

  1. The ball stops moving ().
  2. The ball reaches a velocity of .
  3. The ball reaches a terminal velocity.
  4. I'm not sure.

Clicker Question 6-3

For the system of Linear Drag in 1D, we found a solution for the velocity as a function of time, with at .

where .

CQ 6-3

Which sketch could be correct for the velocity of the ball?

Clicker Question 6-4

For the system of Quadratic Drag in 1D, we found a solution for the velocity as a function of time, with at .

where . Do the units make sense? What are the units of ?

  1. Yes, the units for are ;both sides have the same units.
  2. No, the units for are m/s; each side has different units.
  3. Yes, the units for are unit-less; both sides have the same units.
  4. No, the units for are unit-less; each side has the different units.

Clicker Question 6-5

For the system of Quadratic Drag in 1D, we found a solution for the velocity as a function of time, with at .

where . What happens when ?

  1. The object stops moving.
  2. The object travels at a constant velocity.
  3. The object travels at an increasing velocity.
  4. The object travels at a decreasing velocity.
  5. I'm not sure.