Download Friction and Gravity

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Think about pushing a book slowly across a long
table..If you stopped pushing it most likely it
would stop moving without you exerting force
upon it.
Now think about picking the book up slowly until
it is over your head and then letting go..will it
continue to move after you let it go? YES!!!
If both situations, you are exerting force upon an
object and then removing the force..but in one
situation there is continued movement and one
there is not..WHY?
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According to Newton’s first law of motion,
the book’s motion changes ONLY if an
unbalanced force acts on it…
When book one slides across the table, the
force of FRICTION causes the book to stop.
When book two falls, the force of GRAVITY
causes it to accelerate downward.
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When you pushed the book across the table,
the surface of the book rubs against the
surface of the table.
Even if an object looks smooth, it can still
have things that we cannot see that can rub
against other surfaces and cause the object
to decelerate.
Friction: the force that one surface exerts on
another when the two rub together.
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Friction acts in a direction opposite to the
object’s direction of motion.
Without friction, an object that was pushed or
pulled would continue at a constant speed
forever.
The strength of the force of friction depends
on two factors: the types of surfaces involved
and how hard the surfaces push together.
Rough surfaces produce a greater friction
than smooth surfaces.
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Rough surfaces
produce a greater
friction than smooth
surfaces.
If you tried to sled on
snow and sled on
sand…on which
surface would you go
faster??
THE SNOW because it
is a much smoother
surface.
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You are able to walk because of the friction
between the floor and the soles of your
shoes. Without friction you would only slide
across the floor and you would never move
forward.
An automobile moves because of friction
between its tires and the road.
Ballet dancers spread a powder on the soles
of their shoes so they don’t slip on the floor.
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There are three different types of friction:
sliding friction, rolling friction, and fluid
friction.
Sliding friction: when objects slide over a
surface. (ex.skiing)
Rolling friction: when an object rolls over
another object. (rolling pin)
Fluid Friction: an object moves through a
liquid or gas. (swimming)
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The force required to overcome ROLLING
friction is much less than the force needed to
overcome SLIDING friction.
The force need to overcome FLUID friction is
usually less than that needed to overcome
SLIDING friction.
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Friction explains why a book comes to a stop
after it is pushed…but why does the same
book fall to the ground if you lift it and let it
go??
Newton realized that a force acts to pull an
object straight down toward the Earth.
He called this force gravity!!
Gravity: the force that pulls objects toward
each other.
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When the only force acting upon an object is
gravity, it is said to be in FREE FALL.
An object in free fall accelerates as it falls.
In a free fall, the force of gravity is an
unbalanced force, and an unbalanced force
cause an object to accelerate.
Even though it seems hard to believe, ALL
objects in free fall accelerate at the same rate
regardless of mass!!!
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Have you ever played catch with someone?
If so, you have used PROJECTILE MOTION.
Will a projectile object land on the ground at
the same time as an object that is dropped
down??
An object that is dropped and a object that a
thrown from one side to another are BOTH in
free fall and will hit the ground at the same
time.
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Even though all objects are SUPPOSED to fall at
the same rate..sometimes that is not the case.
Objects falling through the air experience a
type of fluid friction called air resistance.
Air resistance is NOT the same for all objects.
The larger the mass of the object, the greater
the air resistance.
Example: a leaf falls more slowly than an acorn
because it is wider and has a greater mass.
The greatest velocity an object can reach is
known as TERMINAL VELOCITY.
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Weight: the force of gravity on a person or
object at the surface of a planet.
Formula for weight: Mass X Acceleration due
to gravity.
Acceleration due to gravity = 9.8 m/s ^ 2
Example: A 50kg person weighs…
50 kg X 9.8 m/s ^2 = 490 N
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The law of universal gravitation states that
the force of gravity acts between all objects in
the universe.
Any two objects in the universe attract each
other.
Earth and other objects around you all attract
you and each other.