Transcript Chapter 2 - Mrs. Wiedeman

Chapter 2
Motion
Think About It!
How would you describe speed?
 What would you need to know to figure out
speed?
 Do you need to see something move to
know that motion has occurred?
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Motion and Position
A reference point is needed to determine
position
 Motion: occurs when an object changes
position relative to a reference point
 You are moving in your chair
 Relative to the other planets!
 Earth travels 942 billion meters/ year.
How fast is it traveling in mi/hr?
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Distance and Displacement
Distance: how far an object has moved
 km, m, cm, etc.
 Displacement: distance AND direction of
object from starting point
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Distance and Displacement
Speed
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Speed: distance traveled in a certain
amount of time
Constant vs. Changing Speed
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If speed is constant – can calculate speed
over any distance – it’ll be the same!
Usually speed is changing
Average Speed
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Calculate total distance by total time
Instantaneous Speed
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Def: speed at a given point in time.
Speedometers show inst. speed
Constant speed = inst. speed does not change
Practice Problems
Runner travels 200m dash in 22 s. What
is the average speed?
 You had an average speed of 70 mi/hr
from the St. Leon exit to the Harrison exit
on I-74. It took you 10 minutes to get
there. How many miles did you travel?
 You traveled 75 miles at 70 mi/hr from
here to Indianapolis. How long did it take
you in minutes?
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Velocity
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Def: speed of object AND direction
speed : velocity as distance : displacement
Velocity can change even if speed is
constant
Moving Continents
Graphing
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Time on X-AXIS
Distance on Y-AXIS
Constant speed =
straight line
Graphing
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Slope = change in y = rise = distance = speed
change in x run
time
Horizontal line = object at rest
Acceleration
Section 2
Acceleration
Def: rate of change of velocity
 Could be change in speed, direction or
both
 Speeding up  + acceleration
 Slowing down  -- acceleration
 Speed is changing = acceleration
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Acceleration
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+ acc. = going in
same direction as
velocity
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-- acc. = going in
opposite direction as
velocity
Acceleration
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Changing direction is
changing velocity and
acceleration
Speed is constant
Calculations
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If direction doesn’t change velocity = speed
Units: m/s2
Positive Acceleration
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Jet airliner starts at rest and reaches a speed of
80m/s in 20s. Calculate the acceleration.
Has positive slope on graph
Negative Acceleration
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Skateboarder comes to rest in 2s after a speed
of 3 m/s. What is his acceleration?
Has negative slope on graph
Time Calculations
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A car had a negative acceleration of 4
m/s2 when it stopped after having a rate of
40 m/s. How long did it take for the car to
stop?
Final Velocity Calculations
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If a vehicle has an initial velocity of 35 m/s
accelerates at 20 m/s2 for 5 seconds, what
is it’s final velocity?
Motion and Forces
Section 3
What is force?
Def: a push or pull
 Kicking a ball, throwing a ball, etc.
 Chair has a force on you, atmosphere’s
force, etc.
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Changing Motion
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Force causes motion
to change
Balanced Forces
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Net Force: combination of two or more forces
act on an object at same time.
Balanced Forces: forces equal in size but in
opposite direction
Net force = 0
Unbalanced Forces
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Unequal opposite forces - net force will be the
difference between the two and move in
direction of bigger force
Same Direction Forces
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Net force is sum of
the two and moves in
that direction
Inertia and Mass
Inertia: tendency to resist any change in
motion
 Stop a bowling ball vs. ping pong ball
 Greater mass of object  greater inertia
 Newton’s 1st Law of Motion
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Newton’s 1st Law of Motion
Sir Isaac Newton (1642-1727)
 Def: Object moving at constant velocity
keeps moving at that velocity unless an
unbalanced net for acts
 Sometimes called “Law of Inertia”
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Car Collisons
Seat belt vs. No seat belt
 Speed of 50 km/hr collides with wall
 Car stops in .1s
 No belt = person keeps moving at 50
km/hr
 Seat belt = slows person to stop (air bags
help too)
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