Friction I
11/8/13
Bellwork
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What is the description of friction force?
The force exerted by a surface as an object
moves across it or makes an effort to move
across it.
Agenda
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Go over H.W.
Sports figure video – “That Mu You Do”
Notes
Friction I
11/8/13
Physics Questions
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Answer True or False, in your notes, for the
following questions
14 questions total
You will be answering them by a show of
hands
Question #1
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Ben travels to a distant planet. In outer
space, with essentially no gravity, he can
throw a baseball (mass = 0.5 kg) and a shot
put (mass = 4 kg) horizontally at the same
speed with the same force.
Question #2
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Gravity in the International Space Station is
zero.
Question #3
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The force of gravity on an object is the same
as the weight of the object.
Question #4
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Mass and weight have essentially the same
meaning. To increase the weight of an object
requires an increase in the mass of the
object.
Question #5
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Friction is caused by surface roughness.
Question #6
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An object will slow down if there is no net
force.
Question #7
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Passive forces exist (tables can exert force).
Question #8
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Surface forces can't exceed the weight
(active force) on an object.
Question #9
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An object with a constant net force will have
a constant acceleration.
Question #10
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Forces can be negative.
Question #11
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Larger (smaller) velocity means larger
(smaller) acceleration.
Question #12
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Zero velocity means zero acceleration.
Question #13
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Spacecraft need thrusters to keep them
going.
Question #14
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Objects fall at the same rate because they
have the same force of gravity acting on
them.
Question #1
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Ben travels to a distant planet. In outer
space, with essentially no gravity, he can
throw a baseball (mass = 0.5 kg) and a shot
put (mass = 4 kg) horizontally at the same
speed with no discernable difference in
effort.
FALSE!
It still will take more force to accelerate a
more massive object.
Question #2
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Gravity in the space station is zero.
FALSE
The reason that the astronauts act
weightless is that they're inside a container
which is FALLING!
Question #3
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The force of gravity on an object is the same
as the weight of the object.
True
Force of gravity is the same as the weight.
Question #4
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Mass and weight have essentially the same
meaning. To increase the weight of an object
requires an increase in the mass of the
object.
FALSE, Weight is the gravitational
attraction force on an object from a nearby
planet. Mass in chemistry is the amount of
matter present. In physics it’s inertia, or in
other words the resistance to changes in
motion.
Question #5
Friction is caused by surface roughness.
 True, Friction is mostly caused by chemical
bonding between the moving surfaces; it is
caused by stickiness.
(When thinking about friction, don't think
about grains of sand on sandpaper. Instead
think about sticky adhesive tape being
dragged along a surface.)
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Question #6
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An object will slow down if there is no net force.
False
This is the Aristotelian idea that an object's
natural state is rest. Where objects only move
when a net force is exerted upon them. This stems
from common everyday observations, e.g. students
seeing that objects which have been pushed across
the floor come to a stop (and not seeing friction as
a dissipative force acting on the object).
Question #7
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Passive forces exist (tables can exert force).
True
Some students believe that inert objects
cannot exert a force. They can alter an
object's motion, but they don't exert a force.
This is untrue, the table DOES exert a force.
Question #8
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Surface forces can't exceed the weight
(active force) on an object.
False
In acceleration situations surface forces are
greater then the force of gravity, elevator
starting upward, roller coaster at the
bottom of a loop, an object hitting the
ground.
Question #9
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An object with a constant net force will have
a constant acceleration.
True
Constant net force will accelerate an objects
at a constant rate
Question #10
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Forces can be negative.
True
The negative means direction (down, left)
Question #11
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Larger (smaller) velocity means larger
(smaller) acceleration.
False
A small velocity that stops quickly has a
high acceleration
A large velocity that takes a long time to
stop has a low acceleration
Question #12
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Zero velocity means zero acceleration.
False
A ball thrown straight up at the top of it
path has zero velocity, but is accelerating do
to gravity (-9.81 m/s2)
Question #13
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Spacecraft need thrusters to keep them
going.
False
Newton’s 1st Law – An object in motion will
stay in motion
Question #14
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Objects fall at the same rate because they
have the same force of gravity acting on
them.
False!
So what is the reason? (Will be answered
shortly.)
Mass vs. Weight
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Mass (scalar): Is the amount of matter an
object has Unit is the kilogram (kg)
Weight (vector): The force of gravity that
acts on an object
Unit is the Newton (kg · m/s2)
Mass and weight are two different
quantities!
Coefficient of Friction
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Coefficient of Friction (scalar): The ratio
between the force necessary to move one
surface horizontally over another. It is
reduced once the motion has started.
Unit is a ratio (unitless)
Types of Friction
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Static Friction – is the friction force
resisting the relative motion of two surfaces
that are not moving
Kinetic Friction – is the friction force of two
surfaces that are sliding past each other
Inertia
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Inertia (scalar): The resistance of a body or
object to change its motion.
Mass is the measurement of inertia (kg)
Force Quantities
Quantity
Type
Unit
Acceleration
Vector
m/s
Acceleration of
gravity
Vector
m/s
2
2
2
Force of gravity
Vector
N (kg m / s )
Mass
Scalar
kg
Coefficient of
Friction
Scalar
(ratio)
Mythbusters – Phonebook Friction
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http://www.youtube.com/watch?v=AX_lCOjL
CTo
http://www.youtube.com/watch?v=QMW_uY
WwHWQ&feature=related
http://www.youtube.com/watch?v=hG7lGZqW
FpM