Chapter 3
Motion in a line (linear motion)
Motion Vocabulary
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A cat moves a distance of 10 meters (how
far?) in 5 seconds. What information do you
KNOW from this? What can you calculate?
Speed (technically average speed) = dist/time
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Most common units: meters/sec (or miles/hour)
Velocity – speed AND direction
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This is the quantity scientists use. Velocity tells us
how fast things are moving with extra detail
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direction.
Speed/velocity questions
• Questions: a car moves 20 meters/sec.
How far does it go in:
– 3 seconds?
– 10 seconds?
– 1 minute?
– 1 hour?
_______________
_______________
_______________
_______________
• Describe how something could move with
constant speed but CHANGING velocity.
When you’re sitting completely still,
are you moving?
1. Yes
2. No
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Debrief
• Talk to your neighbors:
– Did you get it right?
– Why or why not?
– Make sure you understand.
Motions
• Daily motion – Earth’s rotation once per __
– Speed varies: ___ for Santa, 1000 mph for Ecuador
• 850 mph for Los Angeles
• Yearly motion – Earth’s revolution once per __
– Distance: 1AU = ____
– Speed: average ~66,000 mph
– Direction “tilted” 23.5˚ relative to rotation.
• Sun orbits Milky Way center once per 200-230 Million yr
– 28,000 light-year radius, speed ~500,000 mph
– Studying this motion  dark matter discovered
• Galaxies moving relative to each other.
– Nearest galaxies get closer to us. Most move away.
Acceleration
• Acceleration – any change in velocity
– Changing speed (getting faster OR slower)
– AND/OR Changing direction
•
In everyday English, slowing down is called
“decelerating” but physicists use “accelerating” to
mean ANY form of acceleration/deceleration.
• Give some examples of things that are
“accelerating” in the physics sense.
Velocity vs. acceleration
• Identify the following as velocity (arm points
right) or acceleration (arm points left)
– Going 55 mph
– Using the gas pedal on your car
– Rolling in a straight line
– Using the brakes on your car
– Using the steering wheel on your car
– Running 10 meters per second
– Things falling due to gravity.
– Things thrown upwards on their way up to the top
Which does the human body
feel?
1. Velocity
2. Acceleration
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Debrief
• Talk to your neighbors:
– Did you get it right?
– Why or why not?
– Make sure you understand.
• ** For this question, the same is true for
ALL objects. They FEEL acceleration.
Accelerating – changing velocity
• When things accelerate, they change
velocity in a predictable way
– Cars go from zero to ____ in ____ seconds.
– That’s an acceleration of _______________
– Sprinters run at a top speed of
_________ meters/sec (or _____mph).
– How long does it take to speed up?
(Or slow down?)
– Calculate the accelerations.
Airplanes travel at a constant speed of 400
miles/hr. Is the airplane accelerating?
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1. Yes
2. No
3. Not enough information provided
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Debrief
• Talk to your neighbors:
– Did you get it right?
– Why or why not?
– Make sure you understand.
Two track demo
• See page 57, question 40.
• Clicker question coming:
– Which reaches end of track first?
Which ball reaches the end of
the track first?
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1. Same time
2. Ball on a straight path
3. Ball on the downhill/uphill path
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Debrief
• Talk to your neighbors:
– Did you get it right?
– Why or why not?
– Make sure you understand.
Acceleration changing an object’s speed
• Ramp demo
• Imagine something accelerates at a constant
rate of 2 miles/hr per second of motion. It starts
at rest. Work with your neighbor.
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Initially, its speed is _____________
After 1 second, its speed is ____________
After 2 second, its speed is ____________
After 3 second, its speed is ____________
After 10 secs, its speed is ____________
After 2000 secs, its speed is ____________
Write a rule for the speed after “T” seconds.
After you write your rule, notice the textbook gives an
equation on page 47.
A super-fast car and truck start at the same place. The car goes
100 m/s (220 miles/hr) always. The truck starts at rest and
accelerates 1 m/s per HOUR. Will the truck ever catch the car?
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1. Yes
2. No
3. Not enough information is provided
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Debrief
• Talk to your neighbors:
– Did you get it right?
– Why or why not?
– Make sure you understand.
When the truck catches the car, which
vehicle covered more distance?
1.
2.
3.
4.
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Car
Truck
Same
Not enough information is provided
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Debrief
• Talk to your neighbors:
– Did you get it right?
– Why or why not?
– Make sure you understand.
When the truck catches the car, which
vehicle is moving faster?
1.
2.
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Car
Truck
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Debrief
• Talk to your neighbors:
– Did you get it right?
– Why or why not?
– Make sure you understand.
Extra credit question – due Monday
• For 3 points, determine how much time it
takes the car to catch the truck. Do NOT use
any equations other than this one:
– Velocity = acceleration * time
– (You may not even need this equation)
• Your answer must be conceptual (i.e. words)
not mathematical.
• Reminder: The car goes 100 m/s always. The
truck starts at rest and accelerates 1 m/s per
HOUR.
• Due Monday
Calif. Science Standards for motion
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From California Science Standards, grade 2:
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Students know the way to change how
something is moving is by giving it a push
or a pull. The size of the change is related to
the strength, or the amount of force, of the
push or pull.
And from grade 8:
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Students know the velocity of an object must be
described by specifying both the direction and
the speed of the object.
Students know changes in velocity may be due
to changes in speed, direction, or both.
Gravity near ground
• Free fall
– Toss something up, what happens on way up?
– Drop something, what happens on way down?
• When things fall, measure same accel. for:
– speeding up
– slowing down.
– 10 meters/sec (=22 mph) per second of falling.
• Paper/pen experiment
• SAME GRAVITATIONAL ACCELERATION for ALL
objects, all masses, everywhere near ground.*
– *Unless there are other forces, such as _____________
– Then we’re dealing with NET force, not gravity’s force.
• Galileo observed this (in Pisa). Galileo’s observations
helped Newton formulate ideas.
From Apollo 15 mission
Men on the Moon
• Show the hammer-feather movie
• (Vakil’s computer  Videos  Planets &
Moons  hammer)
Gravitational acceleration
• g = 10 m/s per second or 22 mph per sec
• Drop something from rest. How fast will it be
moving after:
– 1 sec
– 2 sec
– 10 sec
– T sec
_______________
_______________
_______________
_______________*
• *After writing your rule, compare yours to the book’s
version on p. 48
– 10 m/s2 (on Earth’s surface) is abbreviated “g”**
• **More precisely it’s 9.8 m/s2.
Free fall questions
• What happens to an object if you drop it at
rest?
– How quickly?
• If you toss something straight upwards
– What happens on the upwards part of motion?
• How quickly?
– What happens on the downward part?
• How quickly?
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How fast is something moving at the top of
the motion after being thrown upwards? The
answer depends on:
1. Nothing – it is moving w/ speed=zero
2. how fast it was thrown
3. How long you wait
4. how fast thrown & how long you wait
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What is the acceleration at the top of the
motion after being thrown upwards?
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1. Zero
2. g
3. Not enough information provided
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You have two spheres of equal size and smoothness, and you can
ignore air resistance. One is heavy, the other much lighter. You hold one
in each hand at the same height above the ground. You release them at
the same time. What will happen?
1. The heavier one will hit the ground first.
2. They will hit the ground at the same time.
3. The lighter one will hit the ground first.
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Same objects. How does the acceleration of gravity
compare?
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The heavier one has a larger gravitational acceleration.
They have the same gravitational acceleration.
The lighter one has a larger gravitational acceleration.
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Same objects. How does the force of gravity compare? Be
careful!
0 1.
The heavier one has a stronger gravitational force.
They have the same gravitational force.
The lighter one has a stronger gravitational force.
0 2.
0 3.
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California Elementary School
Science Standards for gravity
•
From California Science Standards, hgih
school
a. Students know the relationship
between the universal law of
gravitation and the effect of gravity
on an object at the surface of Earth.
Summary
1.
2.
3.
4.
5.
Definitions: speed, velocity, acceleration
Differences between these three
Simple computations of speed & accel.
Gravity is an acceleration
All things freefall with same acceleration