Class 49.5: Tuesday, 2/11/25

Warm Up:   A ball rolls down a 2m tall hill in a vacuum, flying off the hill in a horizontal direction and traveling another 3.38m horizontally before hitting the ground, 2m further below.  As the ball rolls down the hill, there is static friction between the ball and the hill, but there is no air resistance during this event. 

1. How would you use energy conservation to find the speed of the ball at the point where it flies off the ledge?

2. How would you use projectile motion strategies to find the speed of the ball at the point where it flies off the ledge.

3.  Why would the two calculated speeds be different?  Which one would be correct? 

4.  Static friction does not reduce the mechanical energy of the ball, but it is an important part of this problem. 

     a.  Why does static friction not do non-conservative work on the ball, converting energy to heat? 

     b.  Why is static friction an important part of this problem?

5.  Something totally different -- mechanical energy is conserved in the system analyzed above.  What object(s) are part of the system?  Is there a problem with this?

Today:

Homework:

  • None
Class 49: Monday, 2/10/25

Warm Up: 

None

Today:  Test

Homework:

  • None
Class 48.5: Friday, 2/7/25

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:  Test review (knot tying on Tuesday)

  • Test
    • Formulas
    • Format:
      • 13 Multiple choice -- various concepts
      • 6 Problems
        • Work by a force in the direction of the displacement
        • Work by a force not in the direction of the displacement
        • Power, Kinetic Energy, and Speed
        • Conservation of Energy with both types of potential energy and non-conservative work -- plus efficiency
        • Conservation of energy without non-conservative work
        • Roller Coaster problem with multiple parts -- frictionless parts with no WNC; parts with WNC; changes in height, speed etc.; and a spring.
    • A few tricky concepts:
      • Calculating work done by a variable force
      • Visualizing changes in motion, position, and energy (like the graphing at the beginning of the packet)
      • Understanding when mechanical energy is and is not conserved, and the implications
      • ways to find k
    • The big idea:  PES0 + PEG0 + KE0 + WNC = PES + PEG + KE
  • "4 minute drill" on p. 20 -- #4 won't be on it. Unit 6 Handout (PDF) Unit 6 Answer Key
  • Energy Conservation Practice Drill, p. 20 -- video from class

Homework:

  • If you want more practice with energy conservation, write the simplest possible equations for the energy scenarios situations PDF.  Here are my equations.  The thing that makes them the simplest is that quantities that change are assumed to be zero on one side of the equation.
Class 48: Thursday, 2/6/25 -- FREEZING RAIN DAY
Class 47.5: Wednesday, 2/5/25

Warm Up: 

1.  How does a hydraulic lift facilitate work with a small input force and a large output force?  In general, how do hydraulics produce so much force?
2.  How could you lift a room full of people by blowing air through a straw?

Today:

  • A1/2 --Notes: #8-9 on p. 12
  • Trucker's hitch challenge clarifications --  (for 2% on Monday's test) -- tie a Trucker's hitch as shown in my videos, explain the physics, and remove it. Here are some details.

Homework:

  • Prepare for the Trucker's Hitch Challenge, tomorrow.
  • Test on Monday
Image result for compound bow drawnClass 47: Tuesday, 2/4/25

Warm Up: 

1.  Where does a compound bow store most of its energy?

2.  How does the pulleys' special design cause the necessary applied force to increase and then decrease as the arrow bow is drawn?

Today:

Homework:

Class 46.5: Monday, 2/3/25

Warm Up: 

Assuming that all three bows are drawn to a distance of 0.5m...

1.  Which bow stores the most energy when it is drawn to this distance?

2.  Estimate the energy stored in each bow.

3.  Why is the compound bow curve so different?

Today:

  • Awards presentation
  • Check/review homework

Homework:

  • Mr. Pennington's Practice Test -- problems section -- p.18-19 Unit 6 Answer Key
  • Test on Thursday
Class 46: Friday, 1/31/25

Warm Up:  None

Today:

  • Skip the homework check.
  • Egg bungee contest -- collect fresh data and compete.
  • Work time
  • Unit 6 Handout (PDF)

Homework:

  • Mr. Pennington's Practice Test, multiple choice only -- p.15-17 Unit 6 Answer Key
  • Finish the previous (class 45.5) homework if you didn't do it.
Class 45.5: Thursday, 1/30/25

Warm Up:  Loop-The-Loop Revisited

A big part of the video focuses on their calculations of the minimum entry speed and the g-force experienced by the driver at the bottom.  If he's going too slow, he will fall off at the top.  If he's going too fast, he will black out from the g-force at the bottom.  They finally calculate 16.1m/s to be the right speed for a 40ft tall loop -- with a normal force of 6g at the bottom.  [**They assume that mechanical energy is conserved!!]

Given the other constraints of this problem, how would changing the radius affect the g-force felt at the bottom of the loop?  What if the radius were increased?  What if it were decreased? Solution

Today:

  • Check/review homework.
  • Continue Egg Bungee Lab --
    • collect force data and make a prediction. Predict what mass will stop at a height of 5cm if you drop it from 10cm below ceiling height.  You may drop this mass from this height more than once, but you are not allowed to test other masses.
    • Modify your measuring process and model, if necessary, before tomorrow's egg bungee drop.

Homework:

Class 45: Wacky Wednesday, 1/29/25

Warm Up:  A spring is hanging from the ceiling.  A 500g mass is hooked onto the spring without initially stretching the spring, as shown, and released from rest.  Ater bobbing for a long time, the mass comes to rest 37cm below its release point.

1.  What would a graph of energy vs. time look like, during the first full bob down and back up? Include all forms of mechanical energy.  For simplicity, ignore non-conservative work and OE.graph -- without OE

2. Stop assuming 100% efficiency, because the apparatus clearly isn't 100% efficient.  During this process (release to rest), how much mechanical energy is converted to OE?

 

Today:

Homework:

Class 44.5: Tuesday, 1/28/25

Warm Up: 

For an ideal spring, the applied force is directly proportional to the stretch distance.

The spring constant, k, is a ratio of force to stretch* distance.  The units we will use for k are N/m. 

Suppose a screen door spring has a spring constant k = 40N/m.

1.  What is the tension in the spring when it is stretched 1m?

2.  What is the tension in the spring when it is stretched 20cm?

3.  How much work is required to stretch the spring from a stretch distance of 0m to a stretch distance of 1m?

*In the case of a compression spring, x is the compression distance.

 

Today:

Homework:

Class 44: Monday, 1/27/25

Warm Up:  Consider the event you examined -- which begins as heavier block A starts to fall, causing a lighter block B to rise -- and which ends just before block A hits the table

1.  Populate graph on the right with bars showing the distribution of energy at the beginning and the end of this event.

2.  How does non-conservative work fit into this model?

 

 

Today:

Homework:

  • P.7-8, work and energy practice problems 1-5. 
  • Answers:  1a. 40J  1b. 400W  1c. 21.7m/s  2a. 161,040J  2b. 0.49 Snickers Bars  3a.17.8N   3b. 535J   3c.17.7m/s   4. 0.67hp   5a. 604,747J   5b. 201,582W   5c. 270hp   5d. 43.8   5f. 65.7m
  • Unit 6 Answer Key 
Class 43.5: Friday, 1/24/25

Warm Up: 

A compact car (1,500kg) and a fully-loaded dump truck (36,000kg) are traveling at the same speed on level ground...

1.  Compare the distances that they will travel up a "runaway truck ramp" before coming to a stop. 

2.  Compare the distances that they will slide if they both lock up their wheels and skid to a stop.  Assume that their coefficients of friction are equal.

 

 

Today:

Homework:

  • Finish the practice problems on p. 6-7 (#4-7 and #7-11).  Ignore the headings; the problems are mixed up. Unit 6 Answer Key
Class 43: Thursday, 1/23/25

Warm Up: 

1. What will this energy vs. time graph look like?

2. What will it look like with friction?

 

Today:

Homework:

  • Finish the roller coaster problem (#6, page 8) -- if it wasn't finished in class.
Class 42.5: Wednesday, 1/22/25

Warm Up:  How can you make the PhET skate boarder do a continuous, clockwise loop-the-loop?

Today:

Homework:

Class 42: Tuesday, 1/21/25

Warm Up:  The apparatus on the right is called an Atwood Machine.  Assume that the pulleys and strings are massless and frictionless, and that the masses start at rest.

1.  What kind(s) of energy does each mass have in the beginning?

2.  How does the energy of each mass change?

3.  What kind(s) of energy will each mass have just before the 2kg mass hits the surface?

4.  What will happen to the overall energy of the system?

Today:

Homework:

  • None
Link to Semester 1