This is an old revision of the document!
Today, we are transitioning into talking about energy and are going to start small and examine energy from a few different scenarios. These scenarios highlight the idea of choosing systems and will ask you to analyze each problem from two different systems. This will hopefully help you all compare and contrast these two different approaches to solving energy problems.
Project 7: Learning issues
- Relationship Between Force and Energy - Work
- Choosing a system
- Conservation of Energy
- Potential and Kinetic Energy
Scenario 7a: Captain America slows down a train
There is a train speeding down a track with a constant velocity of 20.9 m/s, which appears to have malfunctioning breaks. Captain America gets in front of the train and slows the train to a stop before the train runs over the cliff. Consider this situation from the time Captain America starts slowing the train down until the train comes to a stop.
Choice 1: System = train + Captain America + Earth Choice 2: System = train
For this problem, stop your analysis after the energy has distributed to Captain America, do not worry where it goes after that.
1. For each choice of system above, answer the following questions:
- Is energy conserved in the system? Why/why not?
- Is work done on the system? Is it positive or negative? How do you know?
- Draw the energy bar charts for the scenario.
2. Pick a system to calculate how much force Captain America must apply to slow the train down in 750 m.
3. Which system did you choose for your analysis? Why?
Scenario 7b: Iron Man speeds up a train
Iron Man encounters a train that has run out of fuel 1000 m before the train station. He decides to put on his blasters (which have a force of 2000 N), and he pushes the train for 500 m to get it up to speed. He's hoping that friction for the rest of the way will slow down the train by the time it arrives at the station.
Choice 1: System = train + iron man + Earth Choice 2: System = train
1. For each choice of system above, answer the following questions:
- Is energy conserved in the system? Why/why not?
- Is work done on the system? Is it positive or negative? How do you know?
- Draw the energy bar charts for the scenario.
2. Pick a system to calculate how fast the train will going when it gets to the station.
3. Which system did you choose for your analysis? Why?
Scenario 7c: Hawkeye Launches Arrow
Hawkeye is standing the edge of a tall building (80 m) and needs to fire an arrow into the sky as a warning to the other avengers. He releases the arrow with an initial speed of 50 m/s at an angle of 60 degrees. Consider this situation from the instant after the arrow is launched.
Choice 1: System = Arrow + Earth Choice 2: System = Arrow
1. For each choice of system above, answer the following questions:
- Is energy conserved in the system? Why/why not?
- Is work done on the system? Is it positive or negative? How do you know?
- Draw the energy bar charts for the scenario.
2. Pick a system to calculate the maximum height that the arrow will reach in the sky.
3. Which system did you choose for your analysis? Why?