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» A 75kg man is standing on a scale in
an elevator. The elevator is at rest.
What is the reading on the scale (in
Newtons)?
Scale
» A 75kg man is standing on a scale in
an elevator. The elevator is at rest.
What is the reading on the scale (in
Newtons)?
FN scale on man
Scale
FW earth on man
FN = F W
» A 75kg man is standing on a scale in
an elevator. The elevator
accelerating upwards. Does the
reading on the scale (apparent
weight) change? If so, how?
FN scale on man
a
FW earth on man
Scale
» A 75kg man is standing on a scale in
an elevator. The elevator
accelerating downwards. Does the
reading on the scale (apparent
weight) change? If so, how?
FN scale on man
a
FW earth on man
Scale
» A stack of books whose true weight is
165N is placed on a scale in an
elevator. The scale reads 165N. From
this information alone, can you tell
whether the elevator is moving with a
constant velocity of 2m/s upward, is
moving with a constant velocity of
2m/s downward, or is at rest?
Scale
» A 10kg suitcase is placed on a scale
that is in an elevator. In which
direction is the elevator
accelerating …
» when the scale reads 75N?
» When the scale reads 120N?
Scale
» DOES NEWTON’S THIRD LAW PREVENT THE
HORSE FROM MOVING THE WAGON?
» Noah Formula notes that Newton's Third Law
says that if I pull on the wagon, the wagon
exerts an equal and opposite force on me. He
believes that if these two forces are equal and
opposite, they will cancel, so that the net force
is zero. If the net force is zero, then Newton's
Second Law (and Newton's First Law, too) says
that the acceleration of the wagon must be
zero, so the wagon can never move.
» An object accelerates (or not) because of
the forces that push or pull on it.
(Newton's 2nd Law)
» Only the forces that act on an object can
cancel. Forces that act on different
objects don't cancel because they affect
the motion of different objects!
» What are the Newton's Third Law Force Pairs?
» 1. The two forces colored yellow in the
diagram are a Newton's Third Law force pair "horse pulls wagon" and "wagon pulls horse".
They are equal in magnitude and opposite in
direction.
» 2. The two forces colored blue in the diagram
are a Newton's Third Law force pair - "horse
pushes ground" and "ground pushes horse".
They are also equal in magnitude and opposite
in direction.
» Friction Force = a contact force between two
surfaces. It results from the electromagnetic
force due to interactions between molecules of
the two surfaces.
» Frictional forces are parallel to the surfaces, and
in a direction opposite to the motion of the
objects.
» Friction depends upon how hard the surfaces are
pressing against each other (the normal force)
and the rough/smoothness of the surfaces in
contact.
» Static Friction – The
magnitude, fs, of the
static frictional force
can have any value
from zero up to a
maximum value of
fsMAX, depending on
the applied force.
» fsMAX = μsFN
» μs is the coefficient of
static friction
(describes the
roughness of the two
surfaces in contact)
» FN is the magnitude of
the normal force (tells
how hard the surfaces
are pressing together)
» Static friction is
essential – for
example rock
climbing, walking, a
car moving on the
road...
» Once two surfaces
begin sliding over one
another, the static
frictional force is no
longer of any concern.
» Kinetic friction comes
into play.
» Kinetic frictional force
opposes the relative
sliding motion.
» fk = μkFN
» μk is the coefficient of
kinetic friction
» fk is independent of the
area of contact between
the surfaces
» is independent of the speed
(if speed is small).
» Is proportional to the
Normal force
» μk is usually less than μs
» Kinetic friction is typically
less that static friction
» It takes less force to keep
an object sliding across a
floor than it takes to get it
going in the first place.
Kinetic Friction
When two surfaces are moving with respect to one another, the frictional resistance is almost
Index
constant over a wide range of low speeds, and in the standard model of friction the frictional force
is described by the relationship below. The coefficient is typically less than the coefficient of static Friction
friction, reflecting the common experience that it is easier to keep something in motion across a
concepts
horizontal surface than to start it in motion from rest.
» A skier is standing motionless on a
horizontal patch of snow. She is holding
onto a horizontal tow rope which will soon
pull her forward. Her mass is 59 kg and
the coefficient of static friction between
her skis and the existing snow is 0.14.
What is the magnitude of the maximum
force that the tow rope can apply to the
skier without causing her to move?
» 80.95 Newtons
» A horse is pulling a plow along a field. The
coefficient of kinetic friction between the
plow and the dirt is .45 and the mass of
the plow is 75 kg.
» a) Find the frictional force exerted on the
plow.
» b) Find the frictional force exerted on the
dirt.
» a) 330 N
» b) -330 N (the opposite direction)
» A sled and its rider are moving at a speed
of 4.0 m/s on a horizontal stretch of snow.
The sled is moving to the right. The snow
exerts a kinetic frictional force on the sled
runners so the sled slows down and
eventually stops. The coefficient of kinetic
friction is 0.050.
» a) Draw the free body diagram.
» b) What is the displacement of the sled?
Use kinematics!
» A sled and its rider are moving at a speed
of 4.0 m/s on a horizontal stretch of snow.
The sled is moving to the right. The snow
exerts a kinetic frictional force on the sled
runners so the sled slows down and
eventually stops. The coefficient of kinetic
friction is 0.050.
» a) 16.3m