Physics 2211: Lecture 6 Todays Agenda

1
Physics 2211 Lecture 6Todays Agenda

• Reference frames and relative motion

2
Inertial Reference Frames

• A Reference Frame is the place you measure from.
• Its where you nail down your (x,y,z) axes!
• An Inertial Reference Frame (IRF) is one that is
  not accelerating.
• We will consider only IRFs in this course.
• Valid IRFs can have fixed velocities with respect
  to each other.
• More about this later when we discuss forces.
• For now, just remember that we can make
  measurements from different vantage points.

3
(No Transcript)
4
(No Transcript)
5
Relative Motion...

• The ball moves vertically in the IRF attached to
  the train
• (Kims IRF)

Train
6
(No Transcript)
7
(No Transcript)
8
(No Transcript)
9
Relative Motion...

• But suppose the train is moving to the left in
  the IRF attached to the ground.
• Vkj is the velocity of train (Kims IRF) w.r.t.
  the ground (i.e. Joes IRF).

Train
Vkj
10
Relative Motion...

• From the grounds IRF, the balls motion exhibits
  projectile
• motion

11

RECALL x and y components of motion are
independent.
Reference frame on the moving train.
Reference frame on the ground.
12
(No Transcript)
13
(No Transcript)
14
(No Transcript)
15
Lecture 6, Act Relative Motion

• You are swimming across a 50m wide river in which
  the current moves at 1 m/s with respect to the
  shore. Your swimming speed is 2 m/s with respect
  to the water. You swim across in such a way that
  your path is a straight perpendicular line across
  the river.
• How many seconds does it take you to get across
  ?(a) (b)(c)

50 m
2 m/s
16
(No Transcript)
17
(No Transcript)
18
(No Transcript)
19
(No Transcript)
20
(No Transcript)
21
Relative Motion

• Consider a problem with two distinct IRFs
• An airplane flying on a windy day.
• A pilot wants to fly from Champaign to Chicago.
  Having asked a friendly physics student, she
  knows that Chicago is 120 miles due north of
  Urbana. She takes off from Willard Airport at
  noon. Her plane has a compass and an air-speed
  indicator to help her navigate.
• The compass allows her to keep the nose of the
  plane pointing north.
• The air-speed indicator tells her that she is
  traveling at 120 miles per hour with respect to
  the air.

22
Relative Motion...

• What is the velocity of the plane in an IRF
  attached to the ground?
• Vp,g is the velocity of the plane w.r.t. the
  ground.

Vp,g
23
Relative Motion...
Tractor

• Vp,g Vp,a Va,g Is a vector equation
  relating the airplanes velocity in
  different reference frames.

Va,g
Vp,a
Vp,g
24
(No Transcript)
25
(No Transcript)
26
(No Transcript)
27
(No Transcript)
28
(No Transcript)
29
Recap for today

• Reference frames and relative motion.
• Examples, Act
• For next time Review Chapter 3 in Tipler.