Transcript 12.1 Forces - TeacherWeb

Name: _______________________________ Class: ____
12.1
Forces
Chapter 12
Forces and Motion
What are we going to learn?
• _____________________________________
_____________________________________
Misconceptions
• A soccer ball sits _______________. You come along
and ____________, sending it _________ across the
field. It eventually ________ to a _______. You
_____________ a _________ to start it ___________,
and then it ____________ all on its own, right?
• No!
• _________ cause all __________ in _________. Just
as you __________ a __________ to the ball to
__________ it up from ________, the ____________
applied a ________ to _________ it down to a
_______. If the ground _________ apply a force to the
ball, it would ________________ forever __________
slowing down or ______________.
Forced to Change
• Misconception: ______________________________
____________________________________________
• Fact: ______________________________________
____________________________________________
How are Forces Described?
• Force – a _________________________________
• When one __________ pushes or ________ another
________, the first object ________ a _________ on
the ___________ object
• Ex: You exert a __________ on a computer ______
when you ___________
• Ex: You exert a ________ on a ________ when you
pull it ________________________
• Force is _____________ by its _____________ and
by the _____________ in which it acts
• ___________ to the _______ is a _______________
from ___________ to the __________
• The _____________ and _____________ can be
represented by an ___________
• The arrow _________ in the ____________ of the
________
• The arrow _________ tells you the ___________ (ex:
the ___________________, the
_______________________)
• The ___________ of the force is ______________ in
the SI unit called a _____________, after the
scientist Sir ___________________
• _____________ is the force that ___________ a
_______________ mass to _______________ at a
rate of 1 meter per second each second __________
• A _________ can cause a ___________ object to
________, or it can _____________ a
_________________ by ______________ the
object’s ___________________________
How do Forces Affect Motion?
• Often ____________________ force acts on an
_________ at the __________________
• The _________________ of all the __________ on
an object is called the ______________
• The net force _______________ if and _______ an
___________ will _________________
• When the ____________ on an object are
_____________, the net force is ________ and
there is ______________ in the object’s _________
• When an _________________________ acts on an
object, the _____________________________
• You can ________ the ________________ on an
object by ____________ together the
______________ of _____ the individual
__________ acting on the object
• Look at the figure below
• The big dog pushes on the box with a force of 16 N
to the right
• The small dog pushes on the box with a force of 10
N to the right
• The net force on the box is the sum of these forces
• In the situation, there was a nonzero net force
• A nonzero net force causes a change in the object’s
motion
What if the Forces on an Object are not acting
in the same Direction?
• The big dog pushes with a force of 20 N
• The small dog pushes with a force of 10 N
• The dogs are pushing against each other
• When the forces on an object act in opposite
directions, the strength of the net force is found by
subtracting the strength of the smaller force (think
negative) from the strength of the larger force (think
positive)
• When forces act in opposite directions, the net force is
in the same direction as the larger force
• In the example, the box will accelerate to the
right
Let’s try a few examples...
• Use what you know about net force to describe the
motion of the box in the figure. Assume the box starts
at rest.
• 1. You pull on your dog’s leash to the right with a 12 N
force. Your dog pulls to the left with a 6 N force.
Sketch this situation, including labeled force arrows.
• 2. What is the net force on the leash? Calculate it.
Draw and label it.
• What's the definition of balanced forces?
____________________________________________
____________________________________________
• What's the definition of unbalanced forces?
____________________________________________
____________________________________________
• When forces are unbalanced and in opposite
directions, what is the combined force equal to?
What direction is it?
____________________________________________
____________________________________________
____________________________________________
•
Use your answers above to explain what happens
in a tug-of-war stand-off.
____________________________________________
____________________________________________
• Use your answers above to explain what happens
in a arm-wrestling stand-off.
____________________________________________
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• How is a tug-of-war stand-off similar to an armwrestling stand-off?
____________________________________________
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•
How are the two stand-offs different?
_______________________________________
_______________________________________
_______________________________________
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Friction
• The _________ that _____________ exert on each
other when they ________________ each _________
• Ex: If you _________ a ________ across a table, the
surface of the ____________________ the surface of
the _________
Factors that Affect Friction
• 2 ____________:
______________________________ involved
how ________ the surfaces are
_______________________________
• Ex: The ______________________ is ___________
on a blocking _______
• If his coach wanted to ______________________ to
move the sled, the coach could
____________________________ of the sled.
________________ the ____________ of the sled
________________ would ____________________
and make the sled __________________________.
• ___________ surfaces produce ________________
than __________________________
• Ex: if you ___________________________ together
______________, there would be _______________
than if you ___________ your
__________________________________
• Friction _____________ when
___________________________ against each other
• Friction ________ in a _________________________
to the direction of the __________________ (opposes
the motion of objects that touch as they move past
each other)
• _______________ friction, a ____________ object
__________________________ until it
_________________________________________
• How could you make it easier moving heavy objects?
_____________________________________________
• What would happen to the football player if he
switched to a much heavier sled?
_____________________________________________
_____________________________________________
Types of Friction
• There are ____ main types of _______________:
____________ friction
_____________ friction
_____________ friction
____________ friction
Static Friction
• The friction force that _________________________
____________________________________________
Sliding Friction
• A force that _____________ an object’s
______________ of motion as it
__________________________
Rolling Friction
• The friction force that _________________________
Fluid Friction
• This force _____________ an object’s
________________________________________
• Ex: ______________________ that acts on an object
moving through the air
• Why is it more difficult to walk on ice than on the floor?
_____________________________________________
_____________________________________________
• Which type of friction keeps your shoes from slipping
on the floor?
_____________________________________________
• Which type of friction keeps you skating on an icy
pond?
_____________________________________________
_____________________________________________
• Which type of friction keeps you skating on an icy
pond?
_____________________________________________
_____________________________________________
Question...
• How is friction between people like friction between
objects?
_____________________________________________
_____________________________________________
_____________________________________________
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• Name 4 types of friction and give an example of each.
_____________________________________________
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_____________________________________________
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• What types of friction occur between your bike tires
and the ground when you ride over cement
(__________), ride through a puddle (____________),
and apply your brakes (_________________)?
• What is friction affected by?
_____________________________________________
_____________________________________________
Gravity
• A ____________ that acts
_________________________________________
• It is an _____________________________
• Earth’s _____________ acts ______________ toward
the _______________ of Earth
• What forces are acting on the skydiver?
____________________________________________
• Which force is greater?
____________________________________________
• How can you tell?
____________________________________________
____________________________________________
• How will the skydiver land safely?
____________________________________________
____________________________________________
• Sir Isaac ___________ concluded that a
________________ objects ___________________
toward the __________ of Earth
• We are so used to _____________________ that we
may not have thought about ___________________
• Newton published his __________________ in 1687.
• What observations might you make today that would
lead you to the same conclusions about gravity?
____________________________________________
____________________________________________
Universal Gravitation
• Newton realized that __________ acts
______________ in the ____________, not just on
Earth
• Ex: makes _______________________ to the ground
keeps the ___________________________ Earth
keeps all the ___________ in our solar system
_______________________ the sun
Law of Universal Gravitation
• The force of __________ acts ___________________
__________ in the universe that _________________
• This means that _________________ in the universe
that _______________________________________
• You are _______________ not only ______________
but also to the ________, the _________________ in
the solar system, and _________________________
around you
• __________ and the ___________ around you are
_________________________ as well
• You do _______________ the ____________ among
_________ objects because these __________ are
_______________________ compared to the
_________ of __________________________
• The ________________ force __________________
objects is ____________________ the force
________________________
• You ____________ only the ___________ of the
________________ gravitational ___________
• Ex: You ___________ your ______________ toward
the ______ the way you ____________ toward Earth
Factors Affecting Gravity
• ____ factors: __________ & ______________
• Mass is a _____________ of the ________________
in an object (SI unit is the _______________)
• The ______________ an object has, the
____________ the ___________________________
between it and other objects
• Ex: __________ gravitational force on nearby objects
is __________ because the ___________________
_________. The ___________________ planets
interact with a ______________ gravitational
__________________ the _____________________
planets
• Gravitational _________ also _______________ the
______________ between the objects’ ___________
• Ex: As ______________________, gravitational
_____________________. That’s why ________ can
_______ a visible gravitational __________________
in your _______ and _____ on a pencil
________________.
• Ex: ______________________ would be able to
perform _______ that are ______________________
• Gravitational _________ are _________ so golf
______ and baseballs can float or
___________________
• Give an example of another sport which would be
affected by reduced gravitational force
_____________________________________________
What’s that Again?
• The ____________ an object has, the ___________
its __________________________
• The __________________________ is __________
one _________ and another, the _____________ the
__________________________ between the objects
• Between which 2 objects is the gravitational force
stronger, a robin and a worm or a bear and a cub?
Why?
____________________________________________
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• Between which 2 objects is the gravitational force
stronger, 2 apples in a fruit bowl or 2 apples on
different branches of an apple tree? Why?
_____________________________________________
_____________________________________________
• In the solar system, which body exerts the most
gravitational force? Why?
_____________________________________________
_____________________________________________
• 1. Circle the object in the outermost orbit which experiences the greatest
gravitational pull from the sun.
• 2. Planet B’s force arrow from the gravitational pull of the sun should be
(longer/shorter) than planet A’s arrow.
• 3. Draw what a planet would look like it is was the same distance from the
sun as Planet C but experienced a smaller gravitational pull from the sun.
Mass and Weight
• ________ is sometimes ____________ with
__________
• Mass is the ___________ of the
________________________ in an object
• __________ is a _____________ of the
______________________ on an object
• Ex: Your ________________________ displays the
______________________ Earth is
____________________
• What happens to the gravitational force between 2
objects when their masses are increased?
_____________________________________________
• What happens when the distance between the
objects increases?
____________________________________________
• If the mass of Earth increased, what would happen to
your weight?
____________________________________________
• What about your mass?
____________________________________________
• At any time, your _______ is the ________________
as it would be _______________________________
• But your _________________ with the ___________
of the __________________________
• The _______ has a _________________________ at
different places in the ________________________
• On the _________, he would weigh about _______ of
what he ___________________
• On _______, he would weigh just over ______ of what
he ____________________
The Ups and Downs of It...
• __________ causes __________ to
________________________________
• ________________________ acts in the
_____________________________ to the
________________ motion of ___________ and
________________________________________
Terminal Velocity
• The ____________________ of a falling object when
the _______________________________________
the _____________________
Projectile Motion
• The ________________ of an
____________________________________ and the
__________________________________________
causes the ball to ____________________________
• _______________________ is a falling object’s
___________________ it is given a
_______________________________