# W16 Mechanics Week 3

Day 1 (Tuesday): Vectors in the four lenses, proving energy formulas.

1. If students turn their work in as a group, please tell them to put their first and last names on the first page. This helps me a lot!
2. Students need to explain lens choice more and include more pictures.
3. Conceptual understanding is pretty good. One thing is problem 1 on the PS1. Many students confuse total energy with kinetic energy for talking about which one is conserved or converted to heat.
4. I graded the problem set on a 1-3 scale this time rather than letter grades since the homework is for feedback rather than a grade itself. Essentially, A=3, B=2, C=1.

During Class

• Hand back PS#1, answers and discussion posted on main website.
• Vector diagrams for Dynamics Problems, reviewing the string breaking.
• Conserving momentum in the roller coaster problem.
• Collect PS#2, review some difficulties

Day 2 (Thursday): Springs and potential energy graphs.

Before Class

• See the Ted Talk about Body Position and Success
• See Student Project Video: Measuring Speed of Bullet
• Read through problem set #3, posted on the main webpage. Due next Monday.
• 1 problem on LON-Capa for “Springs and Ballistic Pendulum”.
• PS#2 answers and discussion on main class website

During Class

• Big Exam #2 – Midterm #1 is NEXT WEEK Thursday. If you hand in formula sheet stapled to your exam, I’ll check your formula sheet and make sure that everything is correct and relevant. Make sure it reflects your understanding.

Day 3 (Friday): Friction and potential energy graphs PS #3 is posted
Before Class
– Attention! – We don’t have a friction video. If I don’t get one up Thursday at 1:00, please read a few pages in your book.**

• Friction – read section 5.1, but don’t read example 5.1 because we do inclined planes (two dimensional problems) after the first midterm.

Goals:

• Dynamics: Friction is a force = to the product of the coefficient of friction and the normal force between two bodies.
• Dynamics and momentum: Friction acts in the opposite direction of the relative motion of the two bodies exchanging momentum. So say, you are spinning your car’s tires when the light turns green. The tires are moving backwards relative to the ground. This pushes the tires forward and the ground backwards. You can see this especially when some of the road gravel flies backwards.
• Energy: Friction turns kinetic energy or mechanical work into heat energy. In the above example, the tires and road get hot. In drag racing, the cars melt some rubber down on the starting pad by spinning their tires.

During Class

• Discuss Projects

Remember the video project from PS #2 is due on Monday. Please see further description on day 1 of week 4.