Transcript Chapter 6: Forces

Forces
Chapter 6
Pages: 116-147
Force
A force is a push or pull upon
an object resulting from the
object's interaction with
another object.
Contact
Forces
Long-Range Forces
Contact Forces
Contact forces are types of
forces in which the two
interacting objects are
physically in contact with each
other.
Friction
Tension
Air
Resistance
Long-Range Forces
Long-Range Forces are types of
forces in which the two interacting
objects are not in physical contact
with each other, but are able to
exert a push or pull despite the
physical separation.
Force
Force
F for use in equations.
Newton is the unit for Force.
N abbreviation for Newton.
Net Force can accelerate.
2
N = kg m/s
Force is a Vector Quantity
Magnitude
Direction
4000lb
Read Pages 118-119
Answer Question 1 in Notebook
Homework
Page: 124
Questions: 7-11
Free Body Diagrams
The purpose of a free-body force
diagram is to assist you in trying
to determine the net force
acting on a body.
Net Force
The purpose of a free-body force
diagram is to assist you in trying
to determine the net force
acting on a body.
Free Body Diagrams
The net force is the vector sum of
all the individual forces acting on a
system.
Fnet = F1 ± F2 ± F3 ± F4 …
Constructing
“free-body force diagram”
1.
Identify the object(s) you
will draw a diagram for.
Constructing
“free-body force diagram”
2. Identify all the forces acting
directly on the object and
the object exerting them.
 Gravity
 Table
Constructing
“free-body force diagram”
3.Draw a dot to represent the
object of interest.
Constructing
“free-body force diagram”
4. Draw a vector to represent
each force.
Table
Gravity
Constructing
“free-body force diagram”
5.
If the object is stationary
or is moving at a constant
velocity, the vectors should
graphically add up to zero.
Constructing
“free-body force diagram”
5.
If the object is accelerating,
the sum of the vectors should
produce a vector in the same
direction as the acceleration.
Constructing
“free-body force diagram”
Floor
Gravity
F
=F
floor
gravity
Standing on Floor
Constructing
“free-body force diagram”
Muscle
Gravity
Jumping
Fmuscle>Fgravity
Constructing
“free-body force diagram”
Gravity
In the Air
Fgravity
Types of Motion
No
Motion
Motor
FMotor=FBrakes
FRoad=FGravity
Road
Gravity
Brakes
No Net Force
Types of Motion
Constant
Motor
FMotor=FFriction
Velocity
FRoad=FGravity
Road
Gravity
Friction
No Net Force
Types of Motion
Speeding
Motor
FMotor>FFriction
Up
FRoad=FGravity
Road
Gravity
Friction
Net Force
Types of Motion
Slowing
Down
Motor
FMotor<FFriction
FRoad=FGravity
Road
Gravity
Friction
Net Force
Finding Net Force
FM=400N
Vertical
Fnet = FR - FG
FR=4000N
FF=400N
FG=4000N
Horizontal
Fnet = FF - FM
Fnet=4000N–4000N
Fnet=400N–400N
Fnet= 0N
Fnet= 0N
Finding Net Force
FM=400N
FR=4000N
FF=400N
FG=4000N
Not Moving
Or
Constant Velocity
Finding Net Force
FM=400N
Vertical
Fnet = FR - FG
FR=4000N
FF=100N
FG=4000N
Horizontal
Fnet = FF - FM
Fnet=4000N–4000N
Fnet=100N–400N
Fnet= 0N
Fnet= -300N
Finding Net Force
FM=400N
FR=4000N
FF=100N
FG=4000N
Accelerating
to the Left.
Newton’s Second Law of Motion
Law
of
Acceleration
Newton’s Second Law of Motion
The acceleration of an object as
produced by a net force is directly
proportional to the magnitude of
the net force, in the same direction
as the net force, and inversely
proportional to the mass of the
object.
Newton’s Second Law of Motion
Accelerates
Ff = 40N FNet
1N
P ==41N
Newton’s Second Law of Motion
a
m
FNet = ma
F
Newton’s First Law of Motion
Law
of
Inertia
Inertia
Inertia is the resistance an
object has to a change in its
state of motion.
Mass
Newton’s First Law of Motion
Seat Belt!!!!
Newton’s First Law of Motion
An object at rest tends to stay
at rest and an object in
motion tends to stay in
motion with the same speed
and in the same direction
unless acted upon by an
unbalanced force.
Mass vs. Weight
Mass is the amount of stuff you
are made up of. (kg or slugs)
Does not change!!!!
Mass vs. Weight
Weight depends on how much
gravity is acting on you at the
moment; you'd weigh less on the
moon than on Earth.
(newtons or pounds)
Mass vs. Weight
Weight
Mass vs. Weight
Mass
Weight
To calculate weight use the
acceleration due to gravity
(9.8m/s2). This will be called g.
F=ma
Fg=mg
Weight Problems
Mr. Clune has a mass of 110kg.
How much does he weight?
Given: m=110kg
Find: Fg=?
g=9.8m/s2
Equation: Fg=mg
=(110kg)(9.8m/s2)
F =1078N
F=ma Problems
A boy pulls a sled that has a mass of
5kg across the snow. The sled
accelerates at a rate of 0.5m/s2. What
is the net force of on the sled?
2
a=0.5m/s
Fnet
Given: m=5kg
a=0.5m/s2
Find: Fnet=?
Equation: Fnet=mg
=(5kg)(0.5m/s2)
Fnet=2.5N
F=ma Problems
A rock with a mass of 10kg fell off a
cliff. At a specific time during its’ fall
it had an acceleration of 3m/s2, due
to air resistance. What is the force
of air on this rock at this time?
+
Fnet= Fair + Fg
Fair
Fnet
a
Fnet= ma
Fg= mg
Fg
Fair= Fnet - Fg
Fair= ma - mg
Fair= m(a – g)
2
2
10kg{(-3m/s )–(-9.8m/s )}
Fair=
2
2
Fair= 10kg{(-3m/s )+(9.8m/s )}
Fair= 10kg(6.8m/s2)
Fair= 68N
Homework
Page: 147
Questions: 22, 27,29
Due: 10/25/06
Factors that determine Friction
FNg
Ff
FT
Friction Forces
FP
Friction Forces
FN
– Normal Force: This force
which will affect frictional
resistance is the component of
applied force which acts
perpendicular or "normal" to the
surfaces which are in contact and
is typically referred to as the
normal force.
Friction Forces
FT
– Surface Force: This force
opposite the normal force
which is equal to this force.
Friction Forces
FP
– Push or Pull Force: This
force is pushing or pulling the
object.
Friction Forces
Ff
– Friction Force: Frictional
resistance to the relative motion of
two solid objects.
Friction Forces
– Static Friction Force:
Static frictional forces are nonmoving forces between two
Ffs
surfaces. It will increase to
prevent any relative motion up
until some limit where motion
occurs.
Friction Forces
– Kinetic Friction Force:
The force between two surfaces
that are moving with respect
to one another, the frictional
resistance is almost constant
over a wide range of low
speeds.
Ffk
Friction Forces
μ
– Coefficient of Friction:
The ratio of the force of
friction (Ff) between two
bodies and the force pressing
them together (FN).
FN
Ff
Coefficient of
Friction
Ffs
μs=
FN
FN
Ff
Coefficient of
Friction
Ffk
μk=
FN
Friction Problem
A refrigerator of total weight 400N is
pushed at a constant speed across
a room by pushing horizontally on
one side with a force of 160N. What
is the coefficient of kinetic friction?
Ffk
FN
Ffk = 160N
FN = 400N
μk = ?
Ffk
μk=
FN
140N
μk=
400N
μk= 0.35
If the coefficient of static friction between
the floor and the refrigerator was 0.6, how
much force would be needed to start the
refrigerator moving?
μs = 0.6
FN = 400N
Ffs = ?
Ffs
μs=
FN
Ffs = μk FN
Ffs = μk FN
Ffs = (0.6)(400N)
Ffs = 240N
Homework
Page: 133
Questions: 14,15
Page: 145
Questions: 33-35
Due: 11/2/06
Newton’s Third Law
"For every action, there
is an equal and
opposite reaction."
While driving, Anna Litical observed a
bug striking the windshield of her car.
Obviously, a case of Newton's third
law of motion. The bug hit the
windshield and the windshield hit the
bug. Which of the two forces is
greater: the force on the bug or the
force on the windshield?
Rockets are unable to accelerate in space
because ...
There is no air in space for the
rockets to push off of.
There is no gravity is in space.
There is no air resistance in space.
... nonsense! Rockets do accelerate in
space.
A gun recoils when it is fired. The recoil is the
result of action-reaction force pairs. As the
gases from the gunpowder explosion expand,
the gun pushes the bullet forwards and the
bullet pushes the gun backwards. The
acceleration of the recoiling gun is ...
a.greater than the acceleration of the bullet.
b.smaller than the acceleration of the bullet.
c.the same size as the acceleration of the bullet.
In the top picture, a physics student is
pulling upon a rope which is attached
to a wall. In the bottom picture, the
physics student is pulling upon a rope
which is held by the Strongman. In
each case, the force scale reads 500
Newtons. The physics student is
pulling…
with more force when the rope is
attached to the wall.
with more force when the rope is
attached to the Strongman.
the same force in each case.