Download Newton`s Second Law I

Newton’s Second Law
But first…
Review: Newton’s First Law
If an object experiences NO net
force….
Resting objects remain at rest.
 Moving objects move at a constant
velocity.
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Newton’s First Law
Also known as the
Law of Inertia
Inertia: An object’s tendency to keep
on doing what it’s already doing.
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Newton’s First Law
Inertia: An object’s tendency to keep
on doing what it’s already doing.
An object at rest will remain at rest…
Law of Inertia:
An object in motion will
continue in motion…
Inertia
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Inertia is a term used to measure the ability of an
object to resist a change in its state of motion.
An object with a lot of inertia takes a lot of force to
start or stop; an object with a small amount of inertia
requires a small amount of force to start or stop.
The word “inertia” comes from the Latin word inertus,
which can be translated to mean “lazy.”
Inertia
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Inertia is a term used to measure the ability of an
object to resist a change in its state of motion.
Mass is a measure of inertia. The higher the mass of
an object is, the more it resists changes to its motion:
Law of inertia:
(click here)
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The law of inertia and YOU!
Equilibrium
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The condition of zero acceleration is
called equilibrium.
In equilibrium, all forces cancel out
leaving zero net force.
Objects that are standing still are in
equilibrium because their acceleration is
zero.
Objects that are moving at constant
speed and direction are also in
equilibrium.
A static problem usually means there is
no motion.
Calculate force
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A woman is holding
two dogs on a leash.
If each dog pulls with
a force of 80 newtons,
how much force does
the woman have to
exert to keep the dogs
from moving?
Newton’s Second Law
The acceleration of an object is:

Directly proportional to the net
force acting on it, and…
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Inversely proportional to its mass
What does that mean?
The acceleration of an object is:

Directly proportional to the net external force
acting on it
(The stronger the force applied to an object, the
greater the acceleration will be.)

Inversely proportional to its mass
(The heavier the object, the less it will accelerate
for a given force.)
Newton's Second Law

If you apply
more force to
an object, it
accelerates at
a higher rate.
Newton's Second Law
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If an object
has more
mass, it
accelerates at
a lower rate
because it
has more
inertia.
Practice: Calculating acceleration
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A cart rolls down a
ramp.
The cart has a mass of
500 grams (0.5 kg).
Using a spring scale,
you measure a net force
of 2 newtons pulling the
car down.
Calculate the
acceleration of the cart.
Calculating acceleration
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Three people are pulling on a wagon applying forces
of 100 N,150 N, and 200 N.
The wagon has a mass of 25 kilograms.
Determine the acceleration and the direction the
wagon moves.
Calculating force
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An airplane needs to
accelerate at 5 m/sec2 to
reach take-off speed before
reaching the end of the
runway.
The mass of the airplane is
5,000 kilograms.
How much force is needed
from the engine?
More on weight….
Weight (as you know) is a force.
 Weight is the force exerted on an
object by gravity.
 The magnitude of weight is found by
Newton’s Second Law:
F=mxa
Weight = mass x (9.8 m/s2)
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