Class 20: Friday, 4/3

Warm Up:  Rubber Bands Can Be Perplexing...

1.  Suppose an 8N force is required to stretch a single, ordinary rubber band a distance of x = 20cm.  The band (labeled A in the diagram) is 10cm long before it is stretched.  How far must the other configurations be stretched in order to reach the same tension of 8N? [Assume that the rubber bands behave like ideal springs with a constant k.]

2.  Assuming that the bands are 100% efficient, which one stores the most energy when they are all stretched to a tension of 8N?  Which stores the least?

Configuration: Stretch Distance To Reach 8N (m) Energy Stored In Bands (J)
A.  Standard rubber band -- 2 strands 0.2  
B.  2 rubber bands in "parallel":    
C.  2 rubber bands in "series"    
D. 1 rubber band, cut and laid out as a single strand    

Today:

  • Check/review homework
  • Review the rubber band car contest rules (PDF)
  • Take a look at the car part data sheet.  You can use it to estimate your car's mass before you order the parts.  You can also use it to calculate moments of inertia.
  • Use this form to place your order for frames, wheels, and other materials
  • Here are some more parts that people have asked for -- available to anyone upon request.
  • Finish the lab from last class.
  • Start creating your rubber band motor design.  Now that you know how long your frame will be, you can begin building your motor.  We can measure coefficients of friction so that you have an idea of how much force your motor can apply.

Homework:  

Class 19: Wednesday, 4/1

Warm Up:  Use the 240fps video in Google Classroom to find the average velocity of this car as it crosses the designated "finish interval."

Today:

Homework:  

Class 18: Monday, 3/30

Warm Up:  Use the 240fps video in Google Classroom to calculate the rubber band energy output of a car.

Today:

Homework:  

Class 17: Thursday, 3/26

Warm Up: 

A sphere, a cylinder, a thin hoop, and a frictionless box are released from rest at the top of ramp.  Their masses and heights are identical.  There is no air resistance, and all of the round objects roll smoothly, so there is no kinetic friction. 

1.  Rank the objects according to their arrival times at the bottom of the ramp.

2.  Suppose the bottom end of the ramp is frictionless, and when they reach the bottom, the objects hit a vertical, frictionless wall.  What motions, if any, would continue after impact?

3.  How would the results be different if some objects had more mass or greater size than others?

4.  How would the results be different if the ramp itself were frictionless?

5.  What type of object would roll faster than any of these round objects?

Today:

  • Check/review homework
    •  
  • Unit 6 Handout #2 (pdf):  Torque and Rotational Inertia (a.k.a. MOI).  The goal here is to organize some of the concepts -- since we just "jumped in" to the car calculations.
  • Energy problem practice
  • Finish the MOI lab from last class. 
  • Finish cars.  Add motors.

Homework:  

  •  Momentum test retake option next class.
  • #3 on page 4 of the new handout  Unit 6 Handout #2 (pdf  -- Here's the solution..
Class 16: Tuesday, 3/24

Warm Up: 

Suppose you're trying to balance a meter stick vertically, as shown in the picture.
1.  How might attaching a heavy c-clamp to the meter stick affect your balancing success?
2.  Where should you put the clamp, and where should you not put it (if you want to make balancing easier)?
3.  How/why does this work?

**Listen to the beat demonstrator?

Today:

  • Energy Reretakes are ready
  • Check/review homework
  • Finish the moment of inertia calculations (steps 9-14; skip 6-8, since they don't help calculate MOI)
  • As a group -- calculate your rear and front moments of inertia.  Turn in your calculations to this spreadsheet.
  • Important Dates:
    • Friday, 4/3 -- submit requests for new wheels and frames
    • Tuesday, 4/7 -- assembly of new cars.  Begin group test.
    • Thursday, 4/9 Finish group Test -- predicting your car's top speed
    • Monday, 4/13 -- Contest.  Return group test for corrections
    • Friday, 4/17 -- Individual test

Homework:  

Class 15: Wednesday, 3/17

Warm Up: 

A force is a push or a pull.  A torque is the rotational analog of force.  It's a twisting/rotating force...

1.  What are some other rotational rotational versions of the linear quantities we have been using?

2.  During this unit, sometimes "rad" will just mysteriously disappear from our math.  Some people don't include it at all.  This is because radians is a dimensionless unit.  Why? What does that mean? 

3.  When people don't include radians at all, what units do they use for angular velocity?

3.  One rotation = _____ radians

Today:

  • Return tests and retakes -- retake videos are in Google Classroom.  Anyone who wants to take the Energy, Gravity, Circular motion test can have one more try, but you need to set up a time to do it, outside of class time, before the end of the quarter.
  • Unit 6 Handout #1 -- Rotational Motion and Rubber Band Cars  (PDF)
  • In Block B1/2, spend 30 minutes on "Class Notes Version" of the homework.  Then stop and assemble cars.

Homework:  

Class 14: Thursday, 3/16

Warm Up:  None

Today:

  • Test -- Momentum and Impulse
  • If there's time -- Assemble rubber band practice cars
    • Look at this Parts Diagram
    • Watch this assembly video
    • Some of the actual parts look a little different than those in the video.  Here's how...
      • Colors of 3D printed parts are different -- everything is red, because the library only had red
      • The pin harness and rubber band harnesses look different, because they're 3-D printed.  They actually look like this...
    • If anyone wants some design files (to do your own CAD design and modifications), they're in this folder.

Homework:  

  •  None
Class 13: Thursday, 3/12

Warm Up: 

1.  Solve this simple problem... a 100% efficient 1kg toy car uses its motor to accelerate from rest across level ground.  If the motor uses 0.5J of energy during this process, what is the car's final speed?

 

2.  Does the entire 0.5J of energy get transferred to the car, or does some of it also go to the Earth?  Do we need to recalculate the car's speed to account for the energy that is transferred to the Earth?

 

Today:

Homework:  

  •  Momentum and Impulse test next class
Class 12: Tuesday, 3/10

Warm Up:  None

 

Today:

  • Test retake: circular motion, gravity,

Homework:  

 
Class 11: Friday, 3/6

Warm Up:  Suppose I place some foam on my table top, and then I shoot it with the two darts in the picture, using the same Nerf ® gun.  Compare the effects of the two darts impact on the motion of the foam.

Today:

  • Check/review homework
  • Finish the notes (#16 on p.3)
  • Choose Rubber Band Car Groups -- so that I can gather sufficient materials
  • Questions about the circles and energy test retake?
  • Other stuff?
    • Force plate
      • Bouncing
      • Jumping
      • Dropping
    • Gauss Gun
    • Newton's Cradle -- how it works (partly)
  • Start Momentum and Impulse practice test

Homework:  

  • Test retake next Tuesday
  • P. 7-8 (practice test) due next Thursday Momentum and Impulse Handout (pdf)  Handout Key -- The multiple choice questions are probably trickier than the questions on the real test.
  • Optional:  there are more practice problems in the handout that I don't plan to assign for homework.  Try them if you want more practice.

 
Class 10: Wednesday, 3/4

Warm Up: 

1.  What happens when I hold a tennis ball on top of a basketball and drop them to the floor together?

2.  The momentum formula is p = mv  (momentum is "p").  Can you explain the balls' behavior in terms of the momentum formula?

3.  How could this concept be applied to towel snapping?

Today:

Homework:  

  •  Problems 1-3, 10, and 13 on p. 4-6.  Handout Key
  • Test retake next Tuesday
    • If you want more roller coaster problem practice, watch this video.  Then print out a fresh copy of page 8 from the last handout.

 
Class 10: Thursday, 2/19

Warm Up: 

There is a "pith ball" hanging next to the Van de Graaff generator.  The pith ball is foam that is covered with a conductive, metallic paint.  What do you think will happen when the Van de Graaff generator builds up a strong negative charge?  Why?

Today:

Homework:  

  •  Have a great break!
Class 9: Tuesday, 2/17

Warm Up: 

Today:

  • Test

Homework:  

  • None
Image result for compound bow drawnClass 8: Friday, 2/13

Warm Up: 

How and why does a compound bow change the nature of W=Fd?

Today:

  • Check/review homework
  • Test review (test on Tuesday)
  • If there's time -- start some static electricity demos

Homework:  

  •  Test prep
Class 7: Wednesday, 2/11

Warm Up:  None

Today:

  • Try out the sled park feature
  • Work time

Homework:  

  •  Continue practice test
Class 6: Monday, 2/9

Warm Up: 

1.  What is the point of having a variety of gears on a bicycle? (or a car, motorcycle, etc.)

2.  If you ride as fast as possible in one gear, how does your acceleration change over time?

3.  How does changing to a higher gear affect the F and d components of your work (e.g. Fd vs Fd)?   Consider changes to F and d where your foot meets the pedal and where the tire meets the road.

4.  At what point should you change gears?

Today:

Homework:  

  •  Practice Test -- due on Friday  Answers
  • Test on Tuesday
Class 5: Thursday, 2/5

Warm Up: 

In this video, a driver supposedly enters a loop-the-loop at a speed of 36mph (16.1m/s).  The driver supposedly experiences 6g at the bottom and approximately 0g at the top.  They say the loop is 40feet high, so the radius is approximately 6.1m. 

1.  In all of the practice problems we have done so far, how many more gs are experienced at the bottom of a loop-the-loop, compared to the top?

2.  Why isn't that the case here?

3.  Does the driver really experience 6g at the bottom, or is it closer to 5g?

*This would be a good context for a bonus problem.

Today:

  • Check/review homework
  • The real sled launch will be next Wednesday.  Tuesday was just data collection, so we can figure out how to do next Wednesday safely.  Bring warm stuff again.
  • Work on the roller coaster problem

Homework:  

  • At least #1 and #2 (and #3 if you want a little more practice) of the Sledding Energy Problems (pdf)  Solutions
  • Optional -- think about how we should design a jump to give everyone a comfortable landing.  We have some control over speed, but not super-precise control.  The current dimensions of the snow pile that we have to work with are (very roughly) about 15m long x 3m wide x 1m high.  I made this spreadsheet to let you easily visualize the flight paths of 3 jumpers.  You can make a copy and adjust the values.  I haven't reviewed all of the video from Tuesday, but our velocities ranged from about 18mph to 35mph.
Class 4: Tuesday, 2/3

Warm Up: 

We're going to use the law of conservation of energy to find out how many pullers it takes to accelerate the sled to a target speed -- for any occupant mass.

 

1) What does this equation look like for the 15m (approximately) over which the sled accelerates?

2) How are we going to use this equation to find the number of pullers required to accelerate the sled to a target velocity?

3) What data do we need to collect?

Today:

  • Check/review homework
  • Sled Accelerator Data Collection -- there's a google spreadsheet in Classroom.
  • Homework work time

Homework:  

  •  P.10-11 (#1-4)
Class 3: Friday, 1/30

Warm Up: 

1.  Why do we have tides?

2.  Why is there a high tide on the opposite side of the Earth from the Moon?

3. Which object is excerting a greater gravitational force on you right now, the Moon or the Sun?

4.  How are tides related to black holes and spaghettification?

 

Today:

Homework:  

Class 2: Wednesday, 1/28

Warm Up: 

We don't have to answer all of these.

1.  What is the net force acting on the jogger in the video?  What is exerting this force?

2.  Approximately how fast is the jogger in this video moving?

3.  If the jogger turned around and jogged the other way, would he feel any different?  What if he ran faster? 

4.  What if the floor didn't have any friction (and no drag in the air)?

5.  What must move in order for the person to experience simulated gravity... the space station, the person, neither, or both?  What does "move" mean in outer space?

6.  There are essentially two ways to simulate 1g of gravity in "outer space."  What are they?  How are they similar?  How are they different?  How do they compare to real gravity?

 

Today:

  • Check/Review Homework
  • Newton's Law of Gravitation -- and practice problems.  Video is on the class playlist.
  • Sled accelorator data collection on Tuesday (Friday is too cold and too soon).  Wear warm clothes and footwear with good traction.  Bring a helmet if you want to wear yours.

Homework:  

  • Four Problems
    • P.6, #16
    • P.7, #3
    • P.8, #4
Class 1: Thursday, 1/22

Warm Up:  What's happening to this guy?  Why? 

Today:

  • Return exams
  • Are there any more completed course recommendation sheets out there?
  • Mid-Year Mop-Up -- tentative plan --  2 classes each of: centripetal motion/gravity, energy/work, momentum/impulse.  Then a practice test and a test.
  • This new unit is interesting, but it doesn't have to be hard.  There are just three new things... 1) Net force = mv2/r   2) Net force is centripetal   3) We have a new way to calculate weight/Fgravity
  • Unit 4 Handout:  Centripetal Acceleration and Gravity (PDF)  Answer Key
  • Derivation of ac = v2/r (direction and magnitude)
  • Work through Handout:  VIDEO of today's notes
    • Change the name of the handout to "Unit 4."
    • Get rid of the last part of the problem on the first problem #4 (p.3)
    • Work on practice problems 1, 2, and 4, on p. 3-4.

Homework:  

  • 3 problems on 2 different pages:
    • Page 3, #3 and #5

Link to 1st Semester