Changing Velocity non uniform motion...... (seconds) 1 2 3 4 5 6 7 8 5 10 15 20 ... his motion...
Transcript of Changing Velocity non uniform motion...... (seconds) 1 2 3 4 5 6 7 8 5 10 15 20 ... his motion...
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Chapter 3: Accelerated Motion
* objectives: 1) define acceleration 2) recognize acceleration as a vector quantity3) relate velocity and acceleration to the motion of an object
uniform motion = straight line on a graph
"unchanging"
time t (seconds)
1
2
3
4
5
6
7
8
5 10 15 20
position d
(meters)
This chapter analyzes motion with a changing velocity
0 8mm
0m 8m
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Changing Velocity (velocity is speed in a direction)
uniform motion non uniform motion
no increase or decrease in
speed
no change in direction
something about velocity is changing
speed or
direction
AND
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time
(s)
velocity
(m/s)
0.00 0.00
1.00 5.00
2.00 10.0
3.00 15.0
4.00 20.0
you get a straight line = the car is speeding up at a constant rate
the slope of the line = the rate at which the car's velocity is changing
calculate the slope of this line
rise y vf virun x tf ti
==
position
(m)
time (s)
1020304050
6070
80
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slope of vt graph = acceleration
the rate at which an object's velocity changes
when velocity changes at a constant rate,
the object has constant acceleration
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constant velocity vector arrows are all the same length
in the second example, the vector arrows are longer between each one second interval, indicating that the object's velocity is increasing
1 second between each dot
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pick two time points and find the slope
slope = vf vitf ti
1s 2s 3s 4s 5s 6s 7s
acceleration
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10s 11s 12s 13s 14s 15s
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draw a motion diagram for the graph, include velocity vectors
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draw a motion diagram for the graph, include velocity vectors
What is the instantaneous acceleration at 2.0 seconds?
at 8.5 seconds?
What is the average acceleration for the 9.5 seconds?
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velocity
(m/s)
time (s)
2 4 6 8 10 12 14 16 180
1.0
0.5
1.5
0s 2s 14s 18s
motion of the elevator is always upward
acceleration is positive when acceleration is upward
acceleration is negative when acceleration is downward
In this problem
vf vi vf vi
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positive acceleration does NOT always mean an object is speeding up and negative acceleration does NOT always mean an object is slowing down
ex: Let's make right positive and left negative
begin end
is this a positive or negative acceleration?
is this a positive or negative acceleration?
begin end
beginend
is this a positive or negative acceleration?
beginend
is this a positive or negative acceleration?
+
+
is this positive or negative velocity? speeding up or slowing down?
is this positive or negative velocity? speeding up or slowing down?
is this positive or negative velocity? speeding up or slowing down?
is this positive or negative velocity? speeding up or slowing down?
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Joseph is skateboarding along a level surface into a very strong wind, which slows him and eventually moves him backward. As his motion changes direction, what is his velocity? Is his acceleration as his motion changes positive, negative or zero?
time (s)
+ velocity
velocity
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3.0m/s
4.5 m/s
Known
vi = 3.0 m/s
vf = 4.5 m/s
t = 2.5 s
unknowna
formulaa = vf vi
tf tia = 4.5 m/s (3.0 m/s)
2.5 s
a = 7.5 m/s2.5 s = 3.0 m/s2+
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At main engine cutoff (MECO), the Space Shuttle is at an altitude of 113 km (70 miles), traveling 7600 m/s (17,000 mph) relative to the earth. This occurs 7 minutes 40 seconds into the mission. Determine the magnitude of the average acceleration experienced by the shuttle astronauts from lift off to MECO.
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When ejection seats were being developed, it was not known if a human could survive the intense acceleration needed to clear a jet fighter in an emergency. In 1954, US Air Force Colonel John Stapp was strapped into the seat of a rocket sled and blasted across the New Mexico desert at 282 m/s (632 mph) to examine the physiological effects of high speed ejection. The sled traveling at eighttenths the speed of sound, a land speed record at that time, was then guided into a large trough of water, stopping it in a mere 1.4 s. Determine the magnitude of the average acceleration during the critical portion of this experiment. (Colonel Stapp subjected himself to several extreme acceleration experiments and survived all of them relatively unharmed.)
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3.2 Motion with Constant Acceleration
1) rearrange to solve for final velocity
2) What is the acceleration of a car traveling at a constant velocity?
3) A skateboard is rolling downhill with an acceleration of 4.50 m/s2 . It began moving at 1.3 m/s and had a final velocity of 3.5 m/s. How long did it take to go down the hill?
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3.2 Motion with Constant Acceleration
1) What is the acceleration of a car traveling at a constant velocity?
2) A bus that is traveling at 30.0 km/h speeds up at a constant rate of 3.5 m/s2. What velocity does it reach 6.8 s later?
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3.2 Motion with Constant Acceleration
acceleration = change in velocity in a time interval
a = vf vit
this can be rearranged to solve for any of the variables
when acceleration is constant, a is also the instantaneous acceleration
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A vt graph contains information about an object's displacement
remember: v = dt
so d = v tv
t
v
t
area = base x height
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area of a rectangle = base x height
area of a triange = 1/2 base x height
total area under line = bh + 1/2 bh
d = tf vi + 12 a tf
2
a = vt
no acceleration
v = a t
tfvi 1/2 tfvfdf di
df = tf vi + 12 a tf
2di +
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page 68
another variation d = 1/2 (vf vi) t
this is just the triangle under the VT graph line
same thing as 1/2 a t2
any equation can be rearranged to solve for alternative variables
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An airplane accelerates down a runway at 3.20 m/s2 for 32.8 s until is finally lifts off the ground. Determine the distance traveled before takeoff.
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You start your bicycle ride at the top of a hill. You coast down the hill at a constant acceleration of 2.00 m/s2 for 6.0 s. When you get to the bottom of the hill, you are moving at 18.0 m/s and you pedal to maintain that speed. If you continue at this speed for 1.00 min, how far will you have gone from the time you left the hilltop?
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The highest speed achieved by a standard nonracing sports car is 3.50 x 102 km/h. Assuming that the car accelerates at 4.00 m/s2, how long would it take for this car to reach its maximum speed if it is initially at rest? Calculate the total distance the car travels during this time.
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In 1993, Ileana Salvador of Italy walked 3.0 km in under 12.0 min. Suppose that during her walk, Salvador is observed to steadily increase her speed from 4.20 m/s to 5.00 m/s in 25.0 s. What is the distance traveled by Salvador during the time of acceleration?
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Two part motion motion problems must be broken apart whenever the acceleration changes
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3.3 Free Fallmotion of a body when air resistance is
negligible and the action is due to gravity alone
acceleration due to gravity : studied by Galileo, but he couldn't get the object to move slow enough for his primitive equipment to gather the info
conclusion: all objects in free fall have the same accelerationindependent of mass or height the object is dropped from
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Acceleration due to gravity : "g"
on Earth averages 9.80 m/s2
on moon averages 1.67 m/s2
the force of gravity is determined by:
a) mass of the object causing the gravity
b) distance from the center of the object causing the gravity
we use the average because the actual number varies slightly as distance from Earth' center varies
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Is "g" positive or negative?the magnitude of "g" on Earth is 9.80 m/s2
the direction of "g" is indicated by the +/
it all depends on the coordinate system YOU decide on
if you decide "up" from Earth's surface is the positive direction, then g is negative because gravity pulls things "down" to Earth.
if you decide "down" to Earth's surface is the positive direction, then g is also positive because it is pulling thing "down" to Earth
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Ex: A flower pot falls from a third story window.
If you decide UP is positive direction, then g is negative
If you decide DOWN is positive direction, then g is positive
motion diagram
open to page 72 in text
2 groups : find time for an object falling 2.0 m from rest
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pt graph vt graph
d (m)
t(s) t(s)
v(m/s)
10
20
10
20
1 2 3 4 00
20
20
1 2 3 4
a ball tossed straight up in the air and then it falls back into your hand
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Acceleration due to gravity (9.80 m/s2) is still acting on that ball when it is traveling at 0.0 m/s at the top of its peak.
If there were zero acceleration at the peak, then the object would remain at 0.0 m/s and hover in the air rather than accelerating toward Earth (falling).
At the peak.... is the ball accelerating?
That is only an instantaneous velocity. It is changed in the very next instant. So velocity IS changing, there IS acceleration.
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Plug "g" into your formulas including accelerated motionWhat are the 3 main formulas from this chapter?
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Mrs Stowell accidentally drops her paint brush when she's restoring the ceiling of the Sistine Chapel.
a) What is the velocity of the paint brush after 2.50 s?
b) How far does the paint brush fall during this time?
c) Conceptually neglecting air resistance, if Mrs. Stowell has 120x the mass of the paintbrush and she falls from the ladder, what is her velocity after 2.50 s?
Sep 27:23 AM
Practice Problem 45 P 74A tennis ball is thrown straight up with an initial speed of 22.5 m/s. It is caught at the peak of its flight.
a) How high does the ball rise?
b) How long does the ball remain in the air?
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