topic i.. i. branch of mechanics that deals with motion without regard to forces producing it....
TRANSCRIPT
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TOPIC I.
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I.
• Branch of Mechanics that deals with motion without regard to forces producing it.
A. Distance and Displacement1. Distance: the total length of the path that an object travels.
a. A scalar quantityb. SI Unit is the meter (m)c. length, width, and height
are all distances!
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2. Displacement: the change in position of an object.
a. A vector quantity because it has a both magnitude and direction
b. “As the crow flies” (magnitude is a straight line from initial to final
positions)
A B
C
Displacement Look!!!
A Right Triangle!!!
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c. SI unit for displacement is the meter (same as for distance)
d. magnitudes for displacement and distance are NOT usually the same!
10 m
7 m
7 m10 m
10 m • What is the
distance traveled?
Start
End
44 m
Example: Displacement
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• What is the displacement?
10 m
7 m
7 m10 m
Start
End A Right Triangle has
been formed!
Use the Pythagorean Theorem!
(10 m)2 + (14 m)2 = c2100 m2 + 196 m2 = c2296 m2 = c2
17.2 m = c
10 m
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B. Speed and Velocity1. Speed: the distance an object moves
per unit timea. Speed is a scalar quantity b. SI units are meters per second
(m/s)c. other units for speed:
• kilometers per hour (km/hr or kph)
d. Formula for average speed (v):
d = distance (m)
t = time (s)
_
t
dv
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Example: Speed Conversion!
If you were traveling 170 km/hr, what is your speed in meters per second (m/s)?
Answer: 47.2 m/s
What is this speed in MPH? Would you get a ticket?
Answer: 105.6 mph
YES!!
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2. Velocity: the time rate of change of an object’s displacement
a. A vector quantity since it adds direction to speed
b. Units: (m/s) but with a direction attached
c. It is possible that two objects can have the same speed, but different velocities
+ = velocity
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d. Finding Velocity Mathematically (using formulas)
1. Basic formula:
Reference Tables!!
2. Also:
t
dv
2fi vv
v
f
i
v
v
t
d
v average velocity
displacement
time
initial velocity
final velocity
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Examples: Velocity
What is the average velocity of a car that travels 2450 meters to the east in 1 minute?
How long does it take a jet to fly 1 kilometer if its velocity is 250 m/s North?
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C. Acceleration1. Definition: is the rate of change of velocity.
• “how fast something is speeding up or slowing down”
• Example: gas pedal = increasing speed
(accelerator)
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2. Acceleration is a vector quantity! Always has a direction attached!
3. Formula:
t
va
timet
velocityinchangev
onacceleratia
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4. Units for acceleration using dimensional analysis:
t
dv
t
va
ssm
2s
m
s
m
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5. Finding Acceleration Mathematicallya. Basic formula for “change in
velocity per unit time”:
• since Δv = vf – vi
substitute into the above equation:
t
Δva
t
vva if
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• solve for final and initial velocities:
atvv if
atvv fi Speed
(m/s
)
Time (sec)
iv
fva
t
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Example: Acceleration
The space shuttle starts from rest and speeds up to 10000 kilometers per hour in 90 seconds. What is the acceleration of the shuttle?
A truck speeds up at a rate of 10 m/s2. If the truck was initially travelling 15 m/s, how fast would it be travelling after 20 seconds?
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b. Acceleration with displacement or distance (d):
• Now d can be expressed as:
2
2
1attvd i
Example: Solve for a
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Example: Find Displacement
A car decelerates rapidly from 26.94 m/s and comes to rest in 3.25 s. The deceleration provided by the brakes is 8.3 m/s2. How far does the car travel while stopping? Assume the car was traveling South.
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Warm Up #6 //14
A school bus slowly drives through Carrollton at 35 mi/hr.
What is the bus speed in meters per second?
What how far will the bus travel in 10 seconds?
What is the acceleration of the bus if it comes to a stop in 5 seconds?
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Example: Find a
In a drag race, two beat up, old, Honda Civics with need to cover 500 meters. The cars start from rest and reach top speed in 8 seconds. What is the acceleration of the cars?
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advv if 222
d. Bottom Line:
any piece of unknown information can be found if 3 variables in any situation are known:d, vi , vf , t , and a
c. If time is NOT known:
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Example: Find Final Velocity
A car accelerates from 10 m/s at a rate of 5 m/s2 over the course of 100 meters. What is the car’s final velocity? Assume the car was traveling West.
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Example: Find Distance
A sled moving at 5 m/s decelerates to rest at a rate of 2 m/s2. How far did the sled travel while it was coming to a stop?
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e. Many times, objects start from rest, causing initial velocity (vi) to be zero!
• This makes equations easier!
t
va f
t
vva if
2fi vv
v
2fv
v 2
2
1attvd i
2
2
1atd
advv if 222
adv f 22
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D. Graphing Motion1. Distance vs. Time
a. Constant Speed
Time
Dis
tan
ce
Positive Direction
Negative Direction
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b. Changing Speed (acceleration)
Time
Dis
tan
ce
Increasing Speed
Time
Decreasing Speed D
ista
nc
e
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c. No Movement
Dis
tan
ce
Time
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2. Speed Versus Time Graphs
Speed
(m/s
)
Time (sec)
a. Constant or Uniform Acceleration
(speed is increasing at a steady rate)
Speeding UP!!
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Speed
(m/s
)
Time (sec)
b. Constant or Uniform Deceleration
(speed is decreasing at a steady rate)
Slowing DOWN!!
Speed vs. Time
NEGATIVE Acceleration
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Speed
(m/s
)
Time (sec)
Speed vs. Time
c. NO Acceleration: Velocity is constant!
Steady Speed!
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3. Slopes of Motion Graphs
run
riseslope
x
yslope
or
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Practice:
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• Calculate the slope of the graph… (units!)• What was the velocity at 2.5 seconds?• What time was the object moving at
25m/s?• What does the slope of the graph mean?
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4. Positive and Negative Motion Graphs
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D. Freely Falling Objects1. In a space all objects will fall or accelerate toward the most dominant source of gravity (usually a large mass).
2. Earth causes falling objects accelerate at a constant 9.81m/s2 toward the planet’s center.
symbol g
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4. ALL Free falling objects are instantaneously accelerated or decelerated at a rate of g the moment they are released
3. The equations for motion are usable in the cases of freely falling objects: replace a with g.
2
2
1gttvd i gtvv if gdvv if 222
• note what happens to these equations when an object is dropped “from rest”
Cat