DAY 44

#Goals: SWBAT...

1. Categorize forces as internal or external

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

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

Warm-Up (5min): Internal vs. External Forces/PE vs. KE/ + - Work

1. Answers to Warm-up A and C are on the board.... read, draw the image, then solve
   1. As the pendulum bob swings to and fro, its height above the tabletop (and in turn its speed) is constantly changing. As the height decreases, potential energyis lost; and simultaneously the kinetic energy is gained. Yet at all times, the sum of the potential and kinetic energies of the bob remains constant. The total mechanical energy is 6 J. There is no loss or gain of mechanical energy, only a transformation from kinetic energy to potential energy (and vice versa). This is depicted in the diagram below, which you should copy into your warm-up.

2. As the 2.0-kg pendulum bob in the above diagram swings to and fro, its height and speed change. Use energy equations and the above data to determine the blanks in the above diagram.
1. ANSWERS:
2. A: h = 0.306 m (6 J = 2 kg *9.8 m/s/s * h)
   1. B: h = 0.153 m (3 J = 2 kg *9.8 m/s/s * h)
   2. C: v = 1.73 m/s (3 J = 0.5 * 2 kg * v2)
   3. D: h = 0 m (0 J = 2 kg * 9.8 m/s/s*h)
   4. E: v = 2.45 m/s (6 J = 0.5 * 2 kg * v2)
   5. F: h = 0.306 m (6 J = 2 kg * 9.8 m/s/s * h)

CLASSWORK

1. #044A: ROLLER COASTERS and ENERGY

• For this activity, you'll work alone, or with one partner
• Goal: investigate the energy relationship between KE and PE_g for a roller coaster car
• LINK to activity.
• Absent today: here's the link to the handout: HANDOUT

2. #044B: PRACTICE PROBLEMS

At Home Learning (HW)

1. Recall: There is a relationship between work and mechanical energy change. Whenever work is done upon an object by an external or nonconservative force, there will be a change in the total mechanical energy of the object. If only internal forces are doing work (no work done by external forces), there is no change in total mechanical energy; the total mechanical energy is said to be "conserved." The quantitative relationship between work and the two forms of mechanical energy is expressed by the following equation:

KEi + PEi + Wext = KEf + PEf

2. Problem Solving:

Use your understanding of the work-energy equation to answer the following questions.

Consider the falling and rolling motion of the ball in the following two resistance-free situations. In one situation, the ball falls off the top of the platform to the floor.In the other situation, the ball rolls from the top of the platform along the staircase-like pathway to the floor. For each situation, indicate what types of forces are doing work upon the ball. Indicate whether the energy of the ball is conserved and explain why. Finally, fill in the blanks for the 2-kg ball.