Download 1 Newton`s Laws of Motion (Ch 5) Newton`s First Law

Newton’s Laws of Motion (Ch 5)
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Isaac Newton
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1642-1727
English physicist &
mathematician
By the age of 31, discovered:
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laws of motion
universal gravitation
calculus
Eccentric
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read Coming of Age in the
Milky Way, by Timothy
Ferris
Newton’s First Law of Motion
Newton’s first law is
often called the law of
inertia.
Force
A force is a push or pull. An
object at rest needs a force to
get it moving; a moving object
needs a force to change its
velocity.
The magnitude of a force can
be measured using a spring
scale.
Newton’s First Law
Every object continues in its state of rest, or of
uniform velocity in a straight line, as long as
no net force acts on it.
An object in motion
stays in motion and an
object at rest stays at
rest, unless acted upon
by a force.
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Newton’s First Law of Motion
Inertial reference frames:
An inertial reference frame is one in which
Newton’s first law is valid.
This excludes rotating and accelerating frames.
ConcepTest
A book is lying at rest on a table. The book will
remain there at rest because:
1) there is a net force but the book has too much inertia
2) there are no forces acting on it at all
3) it does move, but too slowly to be seen
4) there is no net force on the book
5) there is a net force, but the book is too heavy to
move
ConcepTest
A hockey puck slides on ice at constant
velocity. What is the net force acting on the
puck?
1) more than its weight
2) equal to its weight
ConcepTest
You put your book on the bus seat next to
you. When the bus stops suddenly, the book
slides forward off the seat. Why?
1) a net force acted on it
3) less than its weight but more than zero
2) no net force acted on it
4) depends on the speed of the puck
3) it remained at rest
5) zero
4) it did not move, but only seemed to
5) gravity briefly stopped acting on it
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ConcepTest
Mass
You kick a smooth flat stone out on a frozen pond.
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The stone slides, slows down and eventually stops.
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You conclude that:
1) the force pushing the stone forward finally stopped pushing on it
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2) no net force acted on the stone
3) a net force acted on it all along
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4) the stone simply “ran out of steam”
5) the stone has a natural tendency to be at rest
Newton’s Second Law of Motion
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Mass is the measure of inertia of an object.
In the SI system, mass is measured in
kilograms.
Mass is not weight:
Newton’s second law is
the relation between
acceleration and force.
Acceleration is
proportional to force
and inversely
proportional to mass.
Mass is a property of an object. Weight is the force
exerted on that object by gravity.
If you go to the moon, whose gravitational
acceleration is about 1/6 g, you will weigh much
less. Your mass, however, will be the same.
Newton’s Second Law
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Force is a vector, so
is true along
each coordinate axis.
The unit of force in the SI system is the
newton (N).
Note that the pound is a unit of force, not of
mass, and can therefore be equated to
newtons but not to kilograms.
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ConcepTest
From rest, we step on the gas of our Ferrari,
providing a force F for 4 secs, speeding it up to a
final speed v. If the applied force were only 1/2 F,
how long would it have to be applied to reach the
same final speed?
1) 16 s
2) 8 s
3) 4 s
ConcepTest
A force F acts on mass m 1 giving
acceleration a 1. The same force
acts on a different mass m 2 giving
acceleration a 2 = 2a1. If m 1 and
m2 are glued together and the
same force F acts on this
combination, what is the resulting
acceleration?
1) 3/4 a1
3) 1/2 a1
5) 1 s
4) 4/3 a1
ConcepTest
a1
m2
m2 m1
a2 = 2a1
a3
ConcepTest
What can you say about the
acceleration of gravity acting
force of gravity Fg acting on
a stone and a feather?
1) it is greater on the feather
1) Fg is greater on the feather
2) it is greater on the stone
2) Fg is greater on the stone
3) it is zero on both due to
vacuum
3) Fg is zero on both due to vacuum
4) it is equal on both always
m1
5) 2/3 a1
What can you say about the
on a stone and a feather?
F
F
2) 3/2 a1
4) 2 s
F
4) Fg is equal on both always
5) Fg is zero on both always
5) it is zero on both always
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Newton’s Third Law of Motion
More traditionally
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Any time a force is exerted on
an object, that force is caused
by another object.
For every action, there is an equal and
opposite reaction.
Newton’s third law:
Whenever one object exerts a
force on a second object, the
second exerts an equal force in
the opposite direction on the
first.
Newton’s Third Law of Motion
A key to the correct
application of the third
law is that the forces are
exerted on different
objects. Make sure you
don’t use them as if they
were acting on the same
object.
Newton’s Third Law of Motion
Rocket propulsion can
also be explained using
Newton’s third law: hot
gases from combustion
spew out of the tail of
the rocket at high
speeds. The reaction
force is what propels the
rocket.
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ConcepTest
A very large truck sits on a frozen lake. Assume
there is no friction between the tires and the ice. A fly
suddenly smashes against the front window. What
will happen to the truck?
1) it is too heavy, so it just sits there
2) it moves backward at const. speed
3) it accelerates backward
4) it moves forward at const. speed
5) it accelerates forward
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