Physics 135A

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Physics 135A

• Physics 135A
• Lecture 4 2 Dimension Motion
• Dr. Z. Gong

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Physics 135A
Administrative Information

• Homework Set 1 due Today
• Homework Set2 is assigned today, and is due next
  Thursday, Sept. 12
• Homework and Exam show your work!
• Class note will be available after each class

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Physics 135A
Review -1

• Vector - with direction, magnitude and unit
• Scalar - with magnitude and unit

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Physics 135A
Review -2

• Vector Addition - Tail to tip
• Vector Addition - Parallelogram

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Physics 135A
Review -3

• Vector Addition Add component

y
?A
?B
x
o
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Physics 135A
Two Dimension Motion

• Vectors can be projected onto both horizontal and
  vertical axes and treated separately
• Two dimensional motion can be separated into 2
  one dimensional motions
• Rules and methods developed in one dimensional
  motion can apply

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Physics 135A
Two Dimension Motion

• In a projectile motion the projected horizontal
  motion and vertical motion share the same time
  tAx tAy

tAx
o
tAy
A
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Physics 135A
Projectile Motion
t1
t2
t3
t4
t0


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Physics 135A
General Equations

• If ax 0, ay -g -9.80m/s2

x component
y component
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Physics 135A
Example -1

• Walk through Example 3-4 in p.62
• What is given?
• vo and the angle ? gt vox and voy
• What else do we know?
• vox vx ax 0 ay -g y0 y1 0
• vty 0 at the highest (top) point
• What we want to know?
• max height time velocity at max height
• acceleration at max height

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Physics 135A
Example -1

• Walk through Example 3-4 in p.62
• y direction object falling due to gravity

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Physics 135A
Example -1

• Walk through Example 3-4 in p.62
• x direction constant velocity motion

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Physics 135A
Relative Motion

• When we talked about vectors, we always need to
  keep the frame of reference in mind

WRT ground, the cowboys velocity is vcg
30km/h WRT the train, his velocity is vct
-100km/h Because the trains velocity wrt ground
is 130km/h (suppose they are moving on the same
direction)
vtg
vcg
vct
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Physics 135A
Relative Motion

• If we denote the relative velocity of object A
  with respect to reference frame B as
• then

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Physics 135A
Relative Motion
Example 3-12 the velocity of a plane wrt air is
200km/h due north, the wind velocity is 100km/h
wrt ground from northeast to southwest, whats
the velocity of the plane wrt ground?
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Physics 135A
Relative Motion

• If the displacement and time is known, then you
  can always figure out the velocity for that
  motion and use it in your calculation
• If you dont know the time, but know 2
  displacements plus 1 velocity, then the other
  relative velocity can also be calculated