AP Test Week #1 so we had to move a little slower.

PhySP18SS6.5.1: It took Tatum awhile. The icing looks like a nebula.

PhySP18SS6.5.2: F&F6.1

PhySP18SS6.5.3: F&F6.1

PhySP18SS6.5.4: F&F6.3

PhySP18SS6.5.5: F&F6.6

PhySP18SS6.5.6: We talked about the reversing of the Earth’s Magnetic Field

PhySP18SS6.5.7: We REALLY need our magnetic field to protect us from the Solar Wind.

PhySP18SS6.5.8: The problem is, what happens while the field flips?

PhySP18SS6.5.9: The way it is going to be (in your lifetime)

PhySP18SS6.5.10: “Work” leads to changes in Kinetic Energy.

PhySP18SS6.5.11 Work-Kinetic Energy Equation

PhySP18SS6.5.12 Dot products in general give you a number which represents how much the two vector arrow directions agree with each other.

PhySP18SS6.5.13: So work is a dot product between Force vector and displacement vector. The number tells you how much the kinetic energy of your system will increase or decrease.

PhySP18SS6.5.14: . . .

PhySP18SS6.5.15: Another big suitcase.

PhySP18SS6.5.18

PhySP18SS6.5.19

PhySP18SS6.5.21 Let the cosine ONLY tell you whether the force does positive work or negative work over the displacement. If the angle bewtten the Froce vector and the Displacement vector is in the 1st or 4th quadrant that force will do positive work and will increase the kinetic energy of the system. It the angle between force and displacement is in the 2nd or 3rd quadrant then the force will do negative work on the system and the system will lose kinetic energy and therefore slow down.

PhySP18SS6.5.22 That top equation is a good one to use for many different situations.

PhySP18SS6.5.23 I found this girl and her boyfriend hanging out on campus. I asked her if I could take her picture balancing on the tightrope. So, why do you think she holds her arms out to help her balance. (Hint: It has to do with moment of inertia (her mass distribution from her potential axis of rotation)

PhySP18SS6.5.24 The work the normal force from the chair does on me as I stand up on the chair at a constant rate. Overall there was NO work done on the system. The normal Force did positive work, but while I am rising with no acceleration, there was no overall work being done. The normal force does positive work and gravity (mg) does negative work. ∑W = 0 Nm.

PhySP18SS6.5.25 Dropping a tennis ball, gravity (mg) does positive work since cosine =0 and it speeds up the ball. Gravoty ios pouring energy into the system.

PhySP18SS6.5.26 Pulling the cart. No work by gravity (mg) because it is 90° to the displacement vector (∆x). THe force of pull does positive work (cosine 0 = 1) and the friction does negative work (cosine 180° = (-1)).

PhySP18SS6.5.27 Doesn’t matter which direction you push on the wall. As long as it doesn’t move, the work done by the push will be zero.

PhySP18SS6.5.28

PhySP18SS6.5.29

PhySP18SS6.5.30

PhySP18SS6.5.31

PhySP18SS6.5.32 Work done by the participating forces for an object being pulled at an angle up a hill) Notice gravity force (mg) is at an obtuse 2nd quadrant angle so cosine = (-1)

PhySP18SS6.5.33: An object moving DOWN an incline at a constant velocity.

PhySP18SS6.5.35 Sliding into base. The only force doing work is the force of sliding friction (fk)

PhySP18SS6.5.36 Flight attendant pulling on his/her flight bag.

PhySP18SS6.5.37

PhySP18SS6.5.38

PhySP18SS6.5.39 From Friday: Dot products of unit vectors.

PhySP18SS6.5.40 When dotting two vectors given in i j format it is real easy since F O I L turns into F L.

PhySP18SS6.5.41 Example of “Stack and Mult”

PhySP18SS6.5.42 The force acts THROUGH a distance in a dot product. So the work is a measure of how much energy the force added to or took away from the system. Stack & Mult is a really easy way to determining whether energy is flowing into a system or pouring out.

PhySP18SS6.5.43 . . .

PhySP18SS6.5.44 . . .

PhySP18SS6.5.45 . . .

PhySP18SS6.5.46 We’ll discuss more Monday

PhySP18SS6.5.47 We’ll discuss more Monday

PhySP18SS6.5.48 got some good candidates in the contest.

PhySP18SS6.5.49 We’ll discuss when we run stadium steps.