chapter 2:motion along a straight line translational motion and rotational motion todaylater
TRANSCRIPT
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Chapter 2:Motion along a straight line
Translational Motion and Rotational Motion
Today Later
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Current information
1. Please read carefully all the instructions on the coursewebsite
http://sibor.physics.tamu.edu/teaching/phys201/
2. The problems in the syllabus are the same as in
Mastering Physics on the web.
3. Have a notebook, solve the problems first and then submit
them on Mastering Physics site.
4. There are due dates for the problems on Mastering Physics,
so don’t procrastinate.
5. Enroll in WebAssign, since all tasks for the labs are given
and submitted through this system. Prelab part is due 15 min before
the start of the recitation.
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Modified Mastering Physics Course: schuessler00785
Settings in Mastering Physics:
4 attempts are given (no subtractions within this attempts),
after 4 attempts – no credit. Sometimes need use reset button.
Graded tutorials are due soon after the lecture.
Other problems are due on the date of the respective exam.
Practical hint: perform calculations to 4 sign. figures
and round as needed.
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Describing Motion …
Coordinates
Position (displacement)
Velocity
Acceleration
a) Motion with zero acceleration
b) Motion with non-zero acceleration
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Kinematics in One Dimension: Displacement
0xxΔx
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Average speed and velocity
)_(
)_(_
timetotalt
ntdisplacemetotalxvelocityaveragev
dt
dx
timet
ntdisplacemexvelocityv
t
)(
)(lim0
• Instantaneous velocity, velocity at a given instant
SpeedSpeed is just the magnitude of is just the magnitude of velocityvelocity!! The “how fast” without accounting for the direction.The “how fast” without accounting for the direction.
• Average velocity = total displacement covered Average velocity = total displacement covered per total elapsed time,per total elapsed time,
• Average speed = total distance covered per total elapsed time,Average speed = total distance covered per total elapsed time,
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Average velocity
Δt
Δxv
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Instantaneous velocity
0 ΔtwhenΔt
Δxv lim
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Graphic representation: Velocity
Slope is average velocity during t=t2-t1
Slope is instantaneous velocity at t1
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Ex2-3: Engine traveling on rail
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Average Velocity (example)x (meters)
t (seconds)
2
6
-2
4
What is the average velocity over the first 4 seconds ?
A) -2 m/s D) not enough information to decide.
C) 1 m/sB) 4 m/s
1 2 430
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x (meters)
t (seconds)
2
6
-2
4
What is the instantaneous velocity at the fourth second ?
A) 4 m/s D) not enough information to decide.
C) 1 m/sB) 0 m/s
1 2 43
Instantaneous Velocity
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Acceleration
• We say that things which have changing velocity are “accelerating”
• Acceleration is the “Rate of change of velocity”
• You hit the accelerator in your car to speed up– (Ok…It’s true you also hit it to stay at constant velocity, but
that’s because friction is slowing you down…we’ll get to that later…)
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Average acceleration
Δt
Δva
Unit of acceleration:
(m/s)/s=m/s2
Meters per second squared
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Average Velocity
• For constant acceleration atvv 0 v
t
t
vvav
v0
vv2
1v 0av
The area under the graph v(t)is the total distance travelled
0( ) / 2avx v t t v v
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Kinematics in one dimension
Motion with constant acceleration.From the formula for average acceleration
t
vva 0
atvv 0
atvatvvvvv2
1))((
2
1)(
2
10000
0x xAverage velocity v
t
2000 2
1)
2
1( attvtatvtvxx
We find
On the other hand
Then we can find
20 0
1
2x x v t at
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Recap• So for constant acceleration we find:
atvv 0
200 at
2
1tvxx
a const
x
a
v t
t
t
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Examples
• Can a car have uniform speed and non-constant velocity?
• Can an object have a positive average velocity over the last hour, and a negative instantaneous velocity?
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Conceptual Example
• If the velocity of an object is zero, does it mean that the acceleration is zero?– Example?
• If the acceleration is zero, does that mean that the velocity is zero?– Example?
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Example of graphic solution: Catching a speeder (Example 2.9)
m/s
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Kinematics equations for constant acceleration
atvv 0
200 2
1attvxx
)(2 020
2 xxavv
tvv
xx )2
( 00
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All objects fall with the same constant acceleration!!
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Testing Kinetics for a=9.80m/s2
Experiments on the motion of objects falling from leaning tower of Pisa under the action of the force of gravity
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Acceleration of gravity on different planets, =9.81m/s2
Eartha
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Important to have the correct interpretation of the results! The optimal selection of the reference frame helps.
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Problem: Vertical motion
Stone is thrown vertically upward
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height of the cliff h=50m
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9
t1 = (15 + (15^2 + 4*4.9*50)^0.5)/(2*4.95) = 5.0727 = 5.07 s
t2 = (15 - (15^2 + 4*4.9*50)^0.5)/(2*4.95) = -1.9912 = -1.99 s
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1-D motions in the gravitational field
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When you brake on dry pavement, your maximum acceleration is about three times greater than when you brake on wet pavement. For a given initial speed, how does your stopping distance xdry on dry pavement compare with your stopping distance xwet on wet pavement?a)xdry = 1/3xwet
b)xdry = 3xwet
c)xdry = 1/9xwet
d)xdry = 9xwet
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