DAY 43

#Goals: SWBAT...

1. Categorize forces as internal or external

2. Understand how internal vs. external forces affect the TME

2. Answer medium difficulty energy problems with conservative and non-conservative forces

NOT CHECKING HW TODAY. WILL CHECK THE EXTRA CREDIT IF YOU HAVE IT DONE. JUST COME UP. LAST DAY FOR THIS EXTRA CREDIT IS MONDAY

Warm-Up (4min): Internal vs. External Forces/PE vs. KE/ + - Work

1. Does physics make happy?
2. A tennis ball with a mass of 58 grams is launched from a trebuchet with an initial speed of 6.8 m/s and an initial height of 1.3 meters. Assume level ground, and no air resistance (friction). Consider the system to just be the ball. Does the trebuchet apply an internal or external force on the ball?
3. Considering only the flight (after launch, before impact with the ground) is energy conserved?
4. The Trebuchet converts PE to KE to launch the ball. After the ball is launched, how does the ball's KE change?
5. Does the trebuchet do positive or negative work on the ball?

CLASSWORK

1. #043A: "Introductory Conservation of Mechanical Energy Problem using a Trebuchet" Notes

• Introductory Conservation of Mechanical Energy Problem using a Trebuchet - EDpuzzle

2. 043B: Relating Work from External Forces to TME

• Whenever work is done upon an object by an external force, there will be a change in the total mechanical energy of the object (energy not conserved).
• If forces are only internal, energy is conserved
• The relationship between work and mechanical energy is expressed by the following equation:

• TMEi + Wext = TMEf
• The equation states that the initial amount of total mechanical energy (TMEi) plus the work done by external forces (Wext) is equal to the final amount of total mechanical energy (TMEf). A few notes should be made about the above equation. First, the mechanical energy can be either potential energy (in which case it could be due to springs or gravity) or kinetic energy. Given this fact, the above equation can be rewritten as
• KEi + PEi + Wext = KEf + PEf

3. #043C: Examples and Practice Problems

• LINK

4. Quiz grading

At Home Learning (HW)

1. Monday we'll be solving some pretty complex TME problems. Complete the three practice problems from 043C, which we began in class, as well as the stopping distance activity immediately below it.

2. #043D: TME Practice with Flipping Physics: Watch/take notes/complete edpuzzle questions on the following:

1. (4:58) Introduction to Mechanical Energy with Friction - EDpuzzle