Spring 16 Mechanics Week 3

Day 1: See my response to your feedback statements on the main webpage.

The vector sum of the Forces = mass * acceleration.

Before Class

  1. The Elevator Dynamics Problem
  2. Bozeman Science FBDs
  3. Read chapter 3.4-4.7 about dynamics
  4. Finish Problem Set #2 (posted on main page)
  5. Check out PS #3 (just posted Monday at noon)
  6. See reflection from feedback posted on main class page.

During Class
What is equilibrium? How do we find the resultant force? We introduce the “dynamics protocol” (Quiz on this tomorrow):

“The Protocol for Dynamics”:

0) Identify that this problem is about forces => It’s a dynamics problem

1) “oh shit, I don’t know anything”… there’s no formula for a dynamics problem!

2) “but I do know that (vector sum) F = ma”

3) I can draw a force (free body) diagram labeling all the forces at the point of action.

4) I can ask myself “is it in equilibrium?”

=> yes – then the sum of the forces is zero and the vectors must close on themselves

=> no – then I ask another very important question, “which way is it accelerating?” and know that the forces must add to be in the same direction as the acceleration.

5) I can make a Vector Sum of the Forces diagram showing how the forces add to give a net force …. this net force must be in the same direction as the acceleration.

During Class

  • Vector diagrams for Dynamics Problems, reviewing the string breaking.
  • Conserving momentum in the roller coaster problem.
  • Collect PS#2, review some difficulties

Day 2 : Vectors in the four lenses, proving energy formulas.

See comments from graders regarding Big Exam! #1, which may help you for today’s Big Exam!:

Problem 1 – Incorrectly labeled dynamics and kinematic lenses. Students that stated kinematics would be the best method to finding the final speed concluded that all blocks ended with different speeds, instead of using energy to reason out the final speed would be the same.

Problem 3 – Many students said that kinetic energy of Pete and Beverly must be equal in order for them not to move after collision, rather than the momentum. The students that correctly stated that the momentum must cancel concluded the mass of Pete is twice as large. However, only about half of those students that found mass correctly showed that kinetic energy is not equal. There is a confusion about the conservation of momentum and the conservation of energy. For example, how can momentum be conserved with a block increasing velocity on a ramp? and how can energy not be equal between Pete and Beverly, but energy is always conserved?

Problem 5 – I did not grade the graphs because there were many issues. Drawing position, velocity, and acceleration graphs needs review.

On every problem, there was a lack of diagrams. Many just had paragraphs of words and a hard to find answer to which lens they were looking through.

This is how problem sets are graded:
(Optional- grade only work that is complete and do not doc points for work that is missing, but do comment generously)
3 points- A/B quality (all requirements complete, work readable, might have small errors but not large conceptual misunderstandings)
2 points- B/C quality (occasionally missing a requirement, one or two misunderstandings that can be explained via grader comments)
1 point- C/D quality (consistently missing requirements, work does not make sense, misunderstandings are severe, refer to office hrs)

During Class: Big Exam #2

Day 3 : Springs and Review

Before Class

  • See video about: Springs!
  • See Student Project Video: Measuring Speed of Bullet
  • Watch solutions for PS#2: Throwing Box Off Cliff
  • I posted Big Exam! #2 on the main class website and also added this question to PS#3 on the main class website.
  • Read about the video project. The link is on the main page.

During Class

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

Day 4 : Friction and potential energy graphs. Midterm #1 is next week Wednesday (I had it wrong before)

Before Class
– Attention! – We don’t have a friction video. 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.


  • 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 Energy Diagrams

Remember study for midterm #1 over weekend – I give you a link to last quarter’s midterm on next week’s schedule