Download Newton*s First Law

Newton’s First Law
Galileo’s View of Force & Motion
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Prior to the 1600s, early scientists
held simplistic views of motion
They noticed that moving objects
would spontaneously slow down for
no apparent reason (thought they
were “lazy”…..seriously)
Since they did not know about
friction, they concluded that a
constant net force was needed to
keep an object moving
They thought – based upon the
teachings of Aristotle – that larger
net forces made things move faster
and smaller net forces made things
move slower
Galileo’s View of Force & Motion
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Galileo questioned this view
He performed real experiments with a ball rolling down
and up sloped ramps
Galileo’s View of Force & Motion
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He reasoned that a ball speeds up as it rolls down a slope,
then moves with constant velocity along the horizontal
surface, and finally rolls up the far side to the same level it
started from
Galileo’s View of Force & Motion
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And if the ramp is made less steep, the ball still rises to
the same height
Galileo’s View of Force & Motion
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He further reasoned, logically, that if the slope of the
second plane was zero, the ball, once rolling, would
continue forever with no loss of speed, in an attempt to
reach its original height
Galileo’s View of Force & Motion
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Of course, when he performed the real experiment he
observed that the ball rolling along the horizontal surface
eventually stopped
However, he was able to assume correctly that the ball
slowed down to zero velocity because of friction
Inertia
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Galileo concluded that once an object starts moving, it
will continue moving with a constant velocity if there is
no friction present
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Galileo used the concept of inertia to help explain his
conclusion
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Inertia is the property of matter that causes it to resist
changes in motion
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the inertia of an object depends on the mass of the object
Examples of Inertia
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When a car stops suddenly, a passenger in the front seat
not wearing a seat belt continue to move forward (due to
the passenger’s inertia) and collides with the car’s
windshield
Examples of Inertia
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A magician pulls a smooth tablecloth quickly out from
under a place-setting of expensive china
Due to the inertia, the dishes remain at rest where they
were, and are not broken
Examples of Inertia
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It is more difficult to get a stalled car moving by pushing it
than it is to keep it moving
Also, it is more difficult to stop a stalled car that is
coasting by pushing backwards on it
The car’s inertia makes it difficult to move when stopped,
and difficult to stop, when moving
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Newton’s First Law of Motion: The Law of Inertia
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Although Galileo discovered the notion of ‘inertia’, it was
Newton who coined the term when he summarized
Galileo’s work
Newton’s First Law of Motion
- Also known as the law of inertia (the property of matter
that causes it to resist changes in motion; it is directly
proportional to the mass of the object)
- “ If the net external force on an object is zero, the object
will remain at rest or continue to move at a constant
velocity”
Implications of Newton’s First law
1. Objects at rest remain at rest unless acted upon by an
external unbalanced force. Eg. A ball on a horizontal
floor will remain at rest forever, unless someone gives it
a push
Implications
2.
Moving objects continue to move in a straight line at a
constant speed unless acted upon by an external
unbalanced force. Eg. A car moving into a flat icy curve
will tend to continue in a straight line, off the side of the
road
Implications
3. An external force is required to change the velocity of
an object. Internal forces have no effect on an object’s
motion. Eg, a passenger pushing on a dashboard of the
car does not cause the car’s velocity to change
Implications
4.
The external force must be unbalanced; that is, two
equal opposing forces acting on an object will not
change its velocity. For the object’s velocity to change,
the vector sum of the applied forces on the object must
be different than zero
Implications of Newton’s First Law
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Objects at rest tend to remain at rest
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Objects in motion tend to remain in motion
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If the velocity is constant, the net force acting on it must
be zero
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If the velocity is changing (in direction and/or magnitude)
the change must be caused by a net external force acting
on the object
Practice (together)
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Older cars did not have headrests, but all new cards do.
How do headrests help prevent injuries during a rearend collision? Use Newton’s First law to explain your
answer.
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During a read-end collision, the car will suddenly
accelerate forward and so will your body because the
seat exerts a force direction forward on your torso
In a vintage car with no headrest, there is no force
applied to the head
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As a result, your head will continue to remain at rest (law
of inertia)
Your head will eventually appear to snap backwards
relative to your body as your body accelerates forward
possibly resulting in a neck injury
So how does a seat belt work?
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A seat belt uses the sudden
decrease in velocity of the car to
activate a gear mechanism
In this design, the seat belt strap is
attached to a spool which in turn
is attached to a gear
Beneath this gear is a pendulum
that is free to swing back and
forth
So, how does a seatbelt work?
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When the car comes to a sudden
stop, the pendulum swings forward
due to inertia
This causes the pendulum to move
a metal stop into the teeth of the
gear, locking the seat belt in place
Interaction of Science & Technology
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The invention and use of airbags provides a typical
example of the interaction of science and technology in
our society
A problem in society, in this case injuries and deaths in
traffic mishaps, leads to research by scientists, which in
turn leads to technological development
Interaction of Science & Technology
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But then, more problems are identified that are a result of
the initial solution to the problem – the use of airbags can
cause injuries and deaths in certain instances
Thus, the process of scientific research starts again,
followed by more technological development
Interaction of Science & Technology
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Now, manufacturers are testing
airbags that reduce deaths and injuries
of pedestrians struck by cars
Similar to interior airbags, the
exterior ones are computercontrolled
As the car approaches a pedestrian, an
infrared detector senses the heat
radiating from the human body and if
necessary a front-end airbag deploys
If needed, a second airbag on the car’s
hood deploys, softening the
pedestrian’s landing
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Textbook: read pages 123 – 128
Answer #2,3,4,6, 10, 11, 12, 13, 14