Download Friction - Mayfield City Schools

Friction
•
•
Friction is a force
that resists the
motion of objects or
surfaces.
Many kinds of
friction exist.
Friction
•
•
Friction depends on both of the surfaces in
contact.
When the hockey puck slides on ice, a thin layer
of water between the rubber and the ice allows
the puck to slide easily.
Identifying friction forces
•
•
Friction is a force,
measured in newtons
just like any other
force.
Static friction keeps
an object at rest from
moving.
Identifying friction forces
•
Sliding friction is a
force that resists the
motion of an object
moving across a
surface.
A model for friction
•
The greater the force
squeezing two
surfaces together, the
greater the friction
force.
Reducing the force of friction
•
•
Unless a force is
constantly applied,
friction will slow all
motion to a stop
eventually.
It is impossible to
completely get rid of
friction, but it can be
reduced.
Reducing the force of friction
•
•
The friction between a
shaft (the long pole in
the picture) and an outer
part of a machine
produces a lot of heat.
Friction can be reduced
by placing ball bearings
between the shaft and
the outer part.
Useful friction
•
•
Friction is also important
to anyone driving a car.
Grooved tire treads
allow space for water to
be channeled away from
the road-tire contac
point, allowing for more
friction in wet conditions.
Useful friction
•
Shoes are designed
to increase the
friction between their
soles and the ground.
Why do you think these shoes increase friction?
•
Players wearing cleats can apply much
greater forces against the ground to help
them move and to keep from slipping.
FRICTIONAL
FRICTIO
FORCES
Free body diagram:
A book lying on a level table
Free body diagram:
A person floating in still water
Free body diagram:
A wrecking ball hanging vertically from a
cable
Free body diagram:
A helicopter hovering in place
3 Free Body Diagrams:
Move R, accel R
Move R, no accel
L
Move R, accel
Newton’s 3 Laws of Motion
Force changes motion
•
A force is a push or pull, or any action that
is able to change motion.
Law of inertia
•
•
•
Newton’s first law says that objects
continue the motion they already have
unless they are acted on by a net force.
If the net force is zero, an object at rest
will stay at rest.
If an object is acted upon by unbalanced
forces, its motion will change.
Force changes motion
•
Forces can be used to increase or
decrease the speed of an object, or to
change the direction an object is moving.
Law of inertia
•
•
Inertia is the
property of an object
that resists changes
in motion.
Objects with more
mass have more
inertia and are more
resistant to changes
in their motion.
Which ball has more
inertia?
The net force
•
•
Newton’s first law is
often written in terms of
the net force:
“An object at rest will
stay at rest and an
object in motion will
continue in motion at
constant velocity
UNLESS acted upon by
an unbalanced force.”
In other words :
Newton’s 1st Law
a.k.a. law of inertia
• Every object
continues in a state
of rest (or motion in
a straight line at a
constant speed)
unless it is
compelled to change
by forces exerted on
it.
Describe the magic card.
Would a coarse sandpaper work too?
• How about the table
cloth pull?
• (what had more
resistance to change
(inertia)…the full
beaker or empty
beaker?)
MASS IS A MEASURE OF INERTIA
MASS RESISTS ACCELERATION
Crash-test
action figure
•How do seat belts
work?
•Why are infant seats
rear facing?
• What happens?
• Do air bags deploy
when hit from the
front/back or
both?
MASS RESISTS ACCELERATION (+ OR -)
• How does
inertia account
for removing
the dirt from
your shoes
before coming
into the house?
Inertia
Ketch-up from
a bottle?
Flying forward
when hit a
curb?
Newton’s Second Law
links force, acceleration and mass
Fxm
6.00
acceleration
(m/s2)
acceleration
(m/s2)
F+m
4.00
2.00
0.00
0.00
2.00
4.00
3.00
2.00
1.00
0.00
0.00
2.00
Force + m ass (N + kg)
Force x Mass (N * kg)
4.00
Newton’s Second Law
links force, acceleration and mass
• The acceleration produced by a net force on an
object
– is directly proportional to the net force,
– is in the same direction as the net force, and
– is inversely proportional to the mass of the
object.
F = ma
acceleration = Force / mass
F
A
M
Newtons are the SI units of Force
Gravity
and weight
•
•
A 10-kilogram rock has
a mass of 10 kilograms
no matter where it is in
the universe.
A 10-kilogram rock’s
weight however, can
vary greatly depending
on where it is.
Newton’s Second Law
links force, acceleration and mass
• In the picture –
cars already in
motion…
• So will continue
in Motion.
• Force is NOT
needed to keep
an object in
motion.
• Force Causes
Acceleration
Imagine NO friction – How far will
these cars travel?
2nd Law explains why…
• Objects in free fall have equal
acceleration
• i.e. plastic and steel ball
experiment
• Less with air drag
•When air drag cancels out
•weight, speed becomes constant
(acceleration = 0)
•(i.e. terminal speed)
•Directional (downward) motion
with constant speed = terminal
velocity
Forces always come in matched
pairs
•
Newton’s Third Law
(action-reaction)
applies when a force is
placed on any object,
such as a basketball.
The Third Law: Action/Reaction
•
Newton’s Third Law
states that every action
force creates a reaction
force that is equal in
strength and opposite in
direction.
•
There can never be a
single force, alone, without
its action-reaction partner.
The Third Law: Action/Reaction
•
•
Here, one force acts
on the ball, and the
other force acts on
the hand.
It doesn’t matter which
force you call the action
and which the reaction.
The forces do not
cancel because we can
only cancel forces
acting on the same
object.
Action and reaction
•
When sorting out
action and reaction
forces it is helpful to
examine or draw
diagrams.
Here, one force acts on the ________________,
and the other force acts on the _______________.
Action and reaction forces
Below are some guidelines to help you
sort out action and reaction forces:
1. Both are always present whenever any force
appears.
2. They always have the exact same strength.
3. They always act in opposite directions.
4. They always act on different objects.
5. Both are real forces and can cause changes
in motion.
Action
/
Reaction
Gas pushes on shuttle
Shuttle pushes on gas
Woman pulls on spring
Tire pushes on road
Earth pulls on ball
Spring pulls on woman
Road pushes on tire
Ball pulls on Earth
Collisions
•
•
Newton’s third law tells us that any time two objects hit each
other, they exert equal and opposite forces on each other.
However, the effect of the force is not always the same.
THE LAW OF MOMENTUM GOVERNS THESE COLLISIONS
Collisions
• When a large truck
hits a small car, the
forces are equal.
• However, the small
car experiences a
much greater
change in velocity
much more rapidly
than the big truck. Which vehicle ends up
with more damage?
Momentum – mass in motion
• The mass of an object multiplied by
its velocity
• p=mv p - momentum
m - mass
v- velocity
What has momentum?
Slow moving bus?
Bullet?
Big Elephant eating from a tree?
Combining the 2nd and 3rd
Laws rd
2nd Law F=ma
3 Law F1 = - F2
Collision – both objects experience forces
=/opposite
Moving club head
strikes a golf ball (at
rest).
Is the force =/opposite?
m1v1 = m2v2