Stuff you asked about:

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1 Stuff you asked about: I'm so excited to walk to Loomis in -25 degree weather. I can't wait. I might not even fall asleep tonight out of anticipation. Oh centripetal acceleration, my old nemesis. Will we have to memorize the formulas, or will they be provided for exams? I don't care anymore. I need sleep. i stopped caring after the first homework assignment. Don't worry. I got a lot of stuff right. but sleep is the priority right now Eh. It's really not that bad. I don't like Math Warm ups or whatever they're called though. I thought it was very nicely explained When the dog and human move toward each other, how the vector equation works there. I am still not quite sure why a secured object will follow the path tangential to the circle when the string/cord breaks. Otherwise, the lecture explained things very well. So far so good! (~._.)~ I would like to go over the adding of the vectors where two objects are moving right at each other. Also, I think that centripetal acceleration can be tricky, so it would be beneficial to me to go over it in class. I really have to pee but I'll hold it until I finish this prelecture! I'd like to have more clarity on the different reference frames and see more problems like the dog on the treadmill How many theoretical physicists does it take to screw in a lightbulb? Two. One to screw in the lightbulb and one to rotate the universe.

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3 Homework Deadlines: Don t forget there is a second deadline in a week for 80% credit on whatever you didn t finish yet. Common issue: Rounding Your answer must be within 1% of the correct answer in order to be judged correct. Rounding errors pile up during any calculation so carry lots of sig. figs along the way. Common issue: Developing Good Habits Draw a good picture and label your axes. Read & understand problem don t just plug stuff into equations. Do algebra with variables and only put in the numbers at the very end.

4 Clicker Tips Use frequency AA (old clickers default to this) If you are using a clicker with a screen it should say PHYS 211 when you turn it on in this classroom. You know you vote counted if: - Old clicker or + : green LED - V2 clicker: checkmark If you get a red light or don t get checkmark, just vote again.

5 Physics 211 Lecture 3 Today s Concepts: a) Relative motion b) Centripetal acceleration

6 Relative Motion I don't really know how to break down the relative motion problems into math problems. I'm finding that I just have to use logic to solve it. What is Mike s velocity relative to the ground? What you just did v v v ac ab bc v v v Mike, Ground Mike, Train Train, Ground

7 Your Problem Everything is clear, but more examples help a lot. You are flying east on an airplane, that is traveling 150 m/s due east w.r.t. the air. There is a 30 m/s wind that is traveling due west w.r.t the ground. You carry your food tray toward the back of the plane with a speed of 3 m/s w.r.t. the plane. How fast are you moving relative to the ground? A) 183 m/s B) 177 m/s C) 123 m/s D) 117 m/s V you,ground = V you,plane + V plane,air + V air,ground V you,ground = -3 m/s m/s -30 m/s = 117 m/s East

8 Prelecture 3, Question 1 (Red s POV) 0 + I still am having a really hard time with the velocity comparison graphs, like from the prelecture, can we go over those again?

9 Prelecture 3, Question (Blue s POV) 0 + Red Blue Blue A B C D

10 Checkpoint A girl stands on a moving sidewalk (conveyor belt) that is moving to the right at a speed of 2 m/s relative to the ground. A dog runs on the belt toward the girl at a speed of 8 m/s relative to the belt. What is the speed of the dog relative to the ground? v dog,belt = 8 m/s v belt,ground = 2 m/s A) 6 m/s B) 8 m/s C) 10 m/s Im still confused on the moving walkway scenario..

11 What is the speed of the dog relative to the ground? v dog,belt = 8 m/s v belt,ground = 2 m/s A) 6 m/s B) 8 m/s C) 10 m/s +x v dog, ground = v dog, belt + v belt, ground = (-8 m/s) + (2 m/s) = -6 m/s Understanding reference frames?? Especially with the dog and girl; how do you put that into the equation?

12 Checkpoint A girl stands on a moving sidewalk (conveyor belt) that is moving to the right at a speed of 2 m/s relative to the ground. A dog runs on the belt toward the girl at a speed of 8 m/s relative to the belt. What is the speed of the dog relative to the girl? v dog,belt = 8 m/s v belt,ground = 2 m/s A) 6 m/s B) 8 m/s C) 10 m/s

13 What is the speed of the dog relative to the girl? v dog,belt = 8 m/s v belt,ground = 2 m/s A) 6 m/s B) 8 m/s C) 10 m/s A) V dog,girl = V dog,belt + V belt,girl = = -6 B) V dog,girl = V dog,belt + V belt,girl = = -8 C) V dog,girl = V dog,belt + V belt,ground = = 10

14 Clicker Question A man starts to walk along the dotted line painted on a moving sidewalk toward a fire hydrant that is directly across from him. The width of the walkway is 4 m, and it is moving at 2 m/s relative to the fire-hydrant. If his walking speed is 1 m/s, how far away will he be from the hydrant when he reaches the other side? A) 2 m B) 4 m C) 6 m D) 8 m The stuff with the swimming. That always trips me up.!

15 If the sidewalk wasn t moving: Time to get across: Dt = distance / speed = 4m / 1m/s = 4 s v man, sidewalk

16 Just the sidewalk: v sidewalk, hydrant

17 Combination of motions: v v v m an, hydrant m an, sidew alk sidew alk, hydrant

18 D D = (speed of sidewalk) (time to get across) = (2 m/s) (4 s) = 8 m

19 Clicker Question Three swimmers can swim equally fast relative to the water. They have a race to see who can swim across a river in the least time. Relative to the water, Beth swims perpendicular to the flow, Ann swims upstream at 30 degrees, and Carly swims downstream at 45 degrees. Who gets across the river first? A) Ann B) Beth C) Carly Ann Beth Carly y x

20 Look at just water & swimmers Time to get across = D / V y D A B 30 o 45 o C y x V y,beth = V o V y,ann = V o cos(30 o ) V y,carly = V o cos(45 o )

21 Clicker Question Three swimmers can swim equally fast relative to the water. They have a race to see who can swim across a river in the least time. Relative to the water, Beth swims perpendicular to the flow, Ann swims upstream at 30 degrees, and Carly swims downstream at 45 degrees. Who gets across the river second? A) Ann B) Carly C) Both same Ann Carly y x V y,ann = V o cos(30 o ) V y,carly = V o cos(45 o )

22 A girl twirls a rock on the end of a string around in a horizontal circle above her head as shown from above in the diagram. If the string brakes at the instant shown, which of the arrows best represents the resulting path of the rock? Checkpoint (& Demo) REVIEW ROCK ON A STRING A B C D Top view looking down Wine glass demo

23 Centripetal Acceleration The equations behind centripetal acceleration and angular velocity felt very rushed through and I feel like I didn't have enough time to see them come from each other. That was something I'm interested in that I'd love to see again..

24 Centripetal Acceleration w v = wr Keys: w = dq/dt v = wr I think it would be convenient to cover the 'centrifugal force' thing, in terms of proving it wrong so that the real cause for the circular motion is clear

25 Keys: w = dq/dt v = wr Where this comes from: dq R v dt =R dq dq v R dt =w v = Rw

26 Earth s Rotation When standing on the equator, what is the direction of your acceleration due to the rotation of the earth? A) East B) West C) Toward Center of Earth D) Away from Center of Earth (toward sky) E) North

27 Earth s Rotation a c due to earths rotation is really small compared to g so we can ignore it