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Unformatted text preview: The first question in the prelecture. The iim more of a quantum physics kinda guy. m I've learned all of this before, so it's fairly simple for me. I've The most confusing concept was question number 2 which involved the The moving walkway and the dog. Relative motion. moving It was stated we will not use accelerated frames of motion, but a single example It to demonstrate the difficulty of dealing in those frames would be interesting to see, if only once for my first semester in college.. I do not 100% understand how you know that the acceleration vector in a case of do centripetal acceleration is pointed towards the center of the circle of it's rotation. please talk about how you get the formula for centripetal acceleration. Could you please spend some more time dicussing reference frames. Could I would like to talk about the velocity vectors and how to set up your velocity equation. It is difficult for me to put the velocity in terms of one of the other objects involved in the equation,. For example, what is the difference between the velocity of the dog with respect to the girl and the velocity of the dog with respect to the ground. respect Box number 5 is not clear. Box Stuff you asked about: Oops – sorry – should say “Please Explain” Stuff you should know
When you need help you should go to Office Hours: When http://online.physics.uiuc.edu/courses/phys211/fall10/office_hours.htm Exam Weeks Mode stuff you should know
If you have trouble with your clicker: If 1) Replace all 3 batteries. 1) all 2) If that doesn’t fix it bring it to Mats. Physics 211
Le cture3
Today's Concepts:
a) Relative Motion b) Centripetal acceleration Physics 211 Le cture3, S 5 lide How familiar are you with the concepts of relative motion and centripetal acceleration from your high school course. and A) I already know this stuff A) already B) It seems familiar, but I need a review B) It C) We didn't learn this in high school C) We About 3/4 of you don’t feel very About confident about this material. confident Re lativeMotion What you just did r r r vac = vab + vbc Physics 211 Le cture3, S 7 lide Pre flight
A girl stands on a moving sidewalk that is moves to the right at 2 m/s girl relative to the ground. A dog runs toward the girl in the opposite direction along the sidewalk at a speed of 8 m/s relative to the sidewalk. along What is the speed of the dog relative to the ground? Vdog,belt = 8m/s 8m/s dog,belt Vbelt,ground = 2m/s 2m/s belt,ground A) 6 m/s B) 8m/s C) 10m/s Physics 211 Le cture3, S 9 lide What is the speed of the dog relative to the ground? Vdog,belt = 8m/s 8m/s dog,belt Vbelt,ground = 2m/s 2m/s belt,ground A) 6 m/s B) 8m/s C) 10m/s
+x +x V(dog, ground) = V(dog, belt) + V(belt, ground) V(dog, = 8m/s + 2m/s = 6m/s Physics 211 Le cture3, S 10 lide 10 Pre flight
A girl stands on a moving sidewalk that is moves to the right at 2 m/s girl stands relative to the ground. A dog runs toward the girl in the opposite direction along the sidewalk at a speed of 8 m/s relative to the sidewalk. along What is the speed of the dog relative to the girl? Vdog,belt = 8m/s 8m/s dog,belt Vbelt,ground = 2m/s 2m/s belt,ground A) 6 m/s B) 8m/s C) 10m/s About 55% of you got this right – lets try it again. Physics 211 Le cture3, S 11 lide 11 What is the speed of the dog relative to the girl? Vdog,belt = 8m/s 8m/s dog,belt Vbelt,ground = 2m/s 2m/s belt,ground A) 6 m/s B) 8m/s C) 10m/s A) Because the girl is actually moving and the two vectors are opposite, so together they make 6 m/s so B) Because the girl is not moving relative to the belt, and the dog is going 8 m/s relative to the belt, the dog is also moving 8 m/s relative to the girl.. C) The dog and girl are running towards each other so when you add the C) two velocities together it would be 8+2. two
Physics 211 Le cture3, S 12 lide 12 What is the speed of the dog relative to the girl? Vdog,belt = 8m/s 8m/s dog,belt Vbelt,ground = 2m/s 2m/s belt,ground A) 6 m/s B) 8m/s C) 10m/s B) Because the girl is not moving relative to the belt, and the dog is going 8 m/s relative to the belt, the dog is also moving 8 m/s relative to the girl.. Using the velocity formula: Using V(dog, girl) = V(dog, belt) + V(belt, girl) = 8m/s + 0m/s 8m/s = 8m/s
Physics 211 Le cture3, S 13 lide 13 Tractor De o (m m oving cardboard)
Which dire r t Which ction should I point thetoy bulldoze to ge it across the cardboard faste st? cardboard A) To thele ft B) S traight across C To theright ) ABC Physics 211 Le cture3, S 37 14 lide 14 Act
A man starts to walk along the dotted line painted on a moving sidewalk man 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 firehydrant. If his and m/s walking speed is 1 m/s, how far away will he be from the hydrant when he m/s how reaches the other side? reaches 1 m/s A) B) C) D) 2m 4m 6m 8m Moving sidewalk 2 m/s 4m Physics 211 Le cture3, S 15 lide 15 1 m/s If the sidewalk wasn’t moving: Time to get across: ∆ t = distance / speed = 4m / 1m/s =4s 4m r vman , sidewalk 1 m/s Just the sidewalk: 2 m/s 4m r vsidewalk ,hydrant 1 m/s Combination of motions: 2 m/s 4m r r r vman ,hydrant = vman , sidewalk + vsidewalk , hydrant 1 m/s D 2 m/s 4m D = (speed of sidewalk) x (time to get across) (speed (time = (2 m/s) x (4 s) = 8 m AC T y x Three swimmers can swim equally fast relative to the water. They have Three 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 Beth 30 degrees, and Carly swims downstream at 30 degrees. Carly Who gets across the river first? Who A) Ann A) B) Beth C) Carly C) Ann Beth Carly 42 Look at just wate & swim e r m rs
Time to get across = D/Vy B A
30o 30o y x D C V V =V = V cos(30) y,Beth = V0 cos(30) V AC T AC y x Three swimmers can swim equally fast relative to the water. They have Three 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 Beth Ann 30 degrees, and Carly swims downstream at 30 degrees. Carly Who gets across the river second? Who A) Ann A) B) Carly C) Both same C) Ann Carly 42 Pre flight (& De o!) m
A girl twirls a rock on the end of a girl 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? rock? A B C D After the string breaks, the rock will have no After force acting on it, so it cannot accelerate. Therefore, it will maintain its velocity at the time of the break in the string, which is directed tangent to the circle. directed Top view looking down
Physics 211 Le cture3, S 23 lide 23 Show Prelecture Physics 211 Le cture3, S 24 lide 24 Physics 211 Le cture3, S 25 lide 25 v = ωR
ω is therateat which theangleθ change s: dθ ω= dt
θ Once around:
v = ∆ x/∆ t = 2π R/T ω = ∆θ /∆ t = 2π /Τ ∆θ Physics 211 Le cture3, S 26 lide 26 v = ωR
Anothe way to se it: r e vdt =Rdθ dθ R dθ v=R dt v = Rω Physics 211 Le cture3, S 27 lide 27 Wecan ignorethis acce ration dueto Earth's rotation sinceits sm le all Physics 211 Le cture3, S 28 lide 28 ...
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This note was uploaded on 01/27/2011 for the course PHYS 211 taught by Professor Liss during the Fall '08 term at University of Illinois at Urbana–Champaign.
 Fall '08
 Liss
 Quantum Physics

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