Transcript Force 2

Forces and Motion
What words can you think of to
describe the motion of an
object?
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VELOCITY
The word scientists use to describe how
fast an object is moving and in what
direction it is moving
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Velocity is measured as:
a change in distance per
change in time
change in distance
change in time
What are some things you can think of that are
measured like this?
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What are some examples of
objects that move at high
velocities?
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What are some examples of
objects that move at low
velocities?
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What units would you use
to measure the velocity of
an object moving in this
classroom?
meters per second
m
s
Meters per second tells us how many meters
something can move in one second.
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What else can we know about an
object’s motion besides velocity?
•What if the object speeds up?
• What if the object slows down?
What if the object’s velocity changes?
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Scientists have a special way to
measure a change in velocity:
•Acceleration
Acceleration is a change in
velocity.
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The Way Acceleration Works:
• If you’re speeding up, you have positive
acceleration.
• If you’re slowing down, you have negative
acceleration.
• If you’re changing speed quickly, you have high
acceleration.
• If you’re changing speed slowly, you have low
acceleration.
• If you are moving at a constant speed, you have 0
acceleration.
• If you are changing direction, you are also
accelerating.
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Example:
You’re driving along in
your car and suddenly
have to slam on your
brakes for a red light.
Negative or positive
acceleration? High or
low acceleration?
High negative acceleration because you’re
slowing down quickly.
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Example:
You’re an astronaut waiting in
your shuttle for takeoff.
Suddenly the rockets fire
and you’re on your way to
Mars. High or low positive or
negative acceleration?
High positive acceleration because you’re
speeding up quickly.
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Example:
You’re driving down the
highway when your car
runs out of gas and you
slowly roll to a stop on the
side of the road.
Low negative acceleration because you’re
slowing down slowly.
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Challenge Question:
You throw a baseball straight
up in the air. It rises to a
certain height and then falls
back to earth, where you
catch it again in your glove.
First, high positive acceleration because it speeds up quickly as it leaves
your hand.
Then, as it rises, low negative acceleration, because it begins to slow down
slowly because of gravity.
As it begins to fall again, low positive acceleration, because it speeds up
slowly.
Finally, when you catch it, high negative acceleration because it slows
down very quickly in your hand.
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Graphing acceleration
• Velocity is plotted on the y-axis and time is on the x-axis
for a velocity vs. time graph. Its’ slope shows
acceleration.
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Graphing acceleration
•
•
When an object is moving with a constant velocity, the line on the graph is horizontal. When an
object is moving with a constant acceleration, the line on the graph is straight, but sloped. The
steeper the line, the greater the acceleration of the object. The blue line is steeper than the red
line because it represents an object with a greater acceleration.
Notice that a line sloping downwards - with a negative slope represents an object with a constant
deceleration - slowing down.
•
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Comparing speed and acceleration
graphs
• Click to see these graphs.
• Another cool simulation showing the
differences between graphs of position,
speed and acceleration. Moving Man
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Law of Inertia
• The Law of Inertia is also known as Newton’s
First Law of Motion. An object at rest stays at
rest and an object in motion stays in motion
unless it is acted upon by an unbalanced force.
An unbalanced force is also called a net force.
• Inertia
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Quick Review
• Velocity: speed of motion
• Acceleration: change in velocity
What causes an object’s
velocity to change?
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• If you’re riding your bike and want to
speed up, what do you do?
• If you’re driving a car and
want to slow down,
what do you do?
• If you’re playing soccer and
want to move the ball,
what do you do?
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Pushing pedals, pressing
brakes, and kicking soccer balls
are all examples of forces.
A force is anything that changes the velocity of
an object.
But what’s a change in velocity?
A change in velocity is acceleration, so . . .
A force is anything that gives an object
acceleration.
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Nothing will change speed
unless acted upon by a force.
If it’s sitting still, it will keep
sitting still. If it’s moving
fast, it will keep moving fast.
Only a force can change the
velocity of an object.
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Then why do things slow down?
• Flying baseballs slow down and eventually
stop because of the force of gravity.
• Cars and bikes eventually slow down and
stop because of the force of friction.
• Spacecraft in outer space with no friction
and no gravity will keep going at the same
speed forever without a force to slow them
down or speed them up.
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Final Review
• What is an object’s speed and
direction called?
• What is a change of an object’s
velocity?
• What is the only thing that can
change an object’s velocity and give it
acceleration?
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