Transcript 01 Kinematics

Ch 11: Kinematics
M Sittig
AP Physics B
Summer Course 2012
2012年AP物理B暑假班
Describing motion
Scalars
 Distance
 Speed
 …
Vectors
 Displacement
 Velocity
 Acceleration
 Jerk…
Not commonly used.
Practice Problem
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Mr. Ant, standing in an elevator,
moves up 40 m with the
elevator. He then gets out of the
elevator and walks along a
straight hallway for 3 minutes at
a speed 10 meters/minute.
What is the magnitude of Mr.
Ant’s net displacement?
What is the difference between…
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Speed
Average velocity
Velocity
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Velocity
Instantaneous velocity
Acceleration
Practice Problem
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Which of the following cars has a westward
acceleration?
A car traveling eastward and speeding up.
A car traveling westward and slowing down.
A car traveling westward at constant speed.
A car traveling eastward and slowing down.
Solving kinematics problems
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Remember the fundamental kinematics
equations.
Know which quantities each equation has,
and doesn’t have.
Fundamental kinematics equations only
apply for constant velocity.
Use the table of variables – unique to 5S,
and very useful!
Fundamental kinematics variables
Fundamental kinematics equations
* v f  v 0  at
* *  x  v 0 t  1 2 at
2
2
* * * v  v0  2 a x
2
Number of stars = how many exponents each equation has.
Physics Problem Solving
Kinematics Problem Solving
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Step 1: Write out all five variables in a table.
Fill in known values, put “?” next to
unknown values.
Step 2: If 3 or more known values, continue
to Step 3. Otherwise, check your work.
Step 3: Choose the equation that has all of
the knowns and the desired unknown. Solve.
Step 4: Put the correct units on your answer.
Example Problem
Practice Problem
Freefall
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Acceleration due to gravity is a = -10 m/s2
when gravity is the only force on the object.
The horizontal motion of an object will NOT
affect its fall – time to fall depends only on
the vertical position/motion.
Example Problem
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What is the watermelon’s velocity when it hits
the ground?
Practice Problem
downward
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What is the watermelon’s velocity when it hits
the ground?
Projectile Motion
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Break the velocity into vector components.
We can work with the vector components separately
because they are independent of each other.
• The y-component of velocity, vy, equals v(sin θ).
• The x-component of velocity, vx, equals v(cos θ).
• Horizontal velocity is constant.
• Vertical acceleration is g, directed downward.
• Time is the variable that is the same for x and y.
Example Problem
Practice Problem
v
Practice Problem
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5S pg 121 #4
Motion Graphs
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The slope of a position–time graph at any
point is the velocity of the object at that
point in time.
The slope of a velocity–time graph at any
point is the acceleration of the object at that
point in time.
The area under a velocity–time graph
between two times is the displacement of
the object during that time interval.
Describe the motion
Describe the motion
Practice Problem
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The following data shows the
positions of a car at times 0, 2,
4, 6, 8 and 10 s. Match the list
of positions with the graph to
which it corresponds.
0.0, 32.5, 65.0, 97.5, 130.0,
167.5 m.
0.0, 8.0, 32.0, 72.0, 120.0, 168.0
m.
0.0, 55.0, 120.0, 168.0, 168.0,
168.0 m.
Practice Problem
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The graph shows the speed of a car traveling in
a straight line as a function of time.
The value of Vc is 3.80 m/s and the value of Vd
is 5.30 m/s. Calculate the distance traveled by
the car from a time of 1.20 to 5.20 seconds.
Super Challenge Problem
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A student walks into an elevator at rest on the bottom
floor of a building with 4 stories. The elevator then
accelerates upward with an unknown constant
acceleration a during a time interval Δt1 = t, moves at a
constant velocity for Δt2 = 6t, and then decelerates at the
same magnitude a for time interval Δt3 = t. If the
elevator rises a total of h meters, what is the magnitude
of the acceleration a given time t and height h?