Transcript Chapter 12. Friction and Newton Laws

Ch. 12
Motion & Forces
I. Newton’s Laws of Motion
“If I have seen far, it is because I have stood
on the shoulders of giants.”
- Sir Isaac Newton
(referring to Galileo)
Newton’s First Law
• Newton’s First Law of Motion
– An object at rest will remain at rest and an
object in motion will continue moving at a
constant velocity unless acted upon by a net
force.
Newton’s Second Law
• Newton’s Second Law of Motion
– The acceleration of an object is directly
proportional to the net force acting on it and
inversely proportional to its mass.
F = ma
Newton’s Third Law
• Newton’s Third Law of Motion
– When one object exerts a force on a second
object, the second object exerts an equal but
opposite force on the first.
III. Defining Force
Force
 Newton’s First Law
 Friction

A. Force
• Force
– a push or pull that one body exerts on
another
– What forces are being
exerted on the football?
Fkick
Fgrav
A. Force
• Balanced Forces
– forces acting on
an object that are
opposite in
direction and
equal in size
– no change in
velocity
A. Force
• Net Force
– unbalanced forces that are not opposite
and equal
– velocity changes (object accelerates)
Fnet
Ffriction
Fpull
N
W
N
Calculating Net Force from Images
• Net force is the sum of forces acting on an object
– If forces are acting in the same direction they
combine (add together)
– If forces are acting in opposite directions they reduce
(subtract)
• If net force is zero the motion of the object will
NOT change
• If net force is not zero the object will accelerate
(this may be positive or negative acceleration).
What is the net force on the objects below?
Opposite = subtract
8N – 3N = 5 N
2 N + 5N – 8N = 1 N
Same = addition
5N + 2N = 7 N
7N + 2N – 3N = 6N
B. Newton’s First Law
• Newton’s First Law of Motion
– “Law of Inertia”
• Inertia
–tendency of an object to resist any change
in its motion
–increases as mass increases
• Newton’s First Law of Motion
– An object at rest will remain at rest and an
object in motion will continue moving at a
constant velocity unless acted upon by a net
force.
ConcepTest 1
TRUE or FALSE?
The object shown in the diagram must be
at rest since there is no net force acting on
it.
FALSE! A net force does not
cause motion. A net force
causes a change in motion,
or acceleration.
ConcepTest 2
You are a passenger in a car and not wearing your seat belt.
Without increasing or decreasing its speed, the car makes a
sharp left turn, and you find yourself colliding with the righthand door.
Which is the correct analysis of the situation? ...
1. Before and after the collision, there is a
rightward force pushing you into the door.
2.
at the
timetime
of collision,
the door
2.Starting
Starting
at the
of collision,
the
exerts
leftward
force on you.
door aexerts
a leftward
force on you.
3. both of the above
C. Newton’s Second Law
• Newton’s Second Law of Motion
– The acceleration of an object is directly
proportional to the net force acting on
it and inversely proportional to its mass.
F = ma
Newton’s Second Law
F
m a
F = ma
F: force (N)
m: mass (kg)
a: accel (m/s2)
1 N = 1 kg ·m/s2
Calculations

What force would be required to
accelerate a 40 kg mass by 4 m/s2?
GIVEN:
WORK:
F=?
m = 40 kg
a = 4 m/s2
F = ma
F
m a
F = (40 kg)(4 m/s2)
F = 160 N
Calculations

A 4.0 kg shot-put is thrown with 30 N of
force. What is its acceleration?
GIVEN:
WORK:
m = 4.0 kg
F = 30 N
a=?
a=F÷m
F
m a
a = (30 N) ÷ (4.0 kg)
a = 7.5 m/s2
D. Gravity
• Gravity
– force of attraction between any two
objects in the universe
– increases as...
• mass increases
• distance decreases
B. Gravity
• Who experiences more gravity - the
astronaut or the politician?
 Which exerts more gravity the Earth or the moon?
less
distance
more
mass
D. Gravity
• Would you weigh more on Earth or
Jupiter?
 Jupiter because...
greater mass
greater gravity
greater weight
Free Fall
• When gravity is the
only force acting on
an object
• Represented by the
letter g
• Near Earth’s surface g
2
= 9.8 m/s
B. Gravity
• Weight
– the force of gravity on an object
W = mg
W: weight (N)
m: mass (kg)
g: acceleration due
to gravity (m/s2)
MASS
WEIGHT
always the same
(kg)
depends on gravity
(N)
D. Gravity
• Accel. due to gravity (g)
 In the absence of air resistance, all
falling objects have the same
acceleration!
 On Earth: g = 9.8 m/s2
W
g
m
elephant
g
W
m
feather
Calculations

Mrs. J. weighs 557 N. What is her
mass?
GIVEN:
WORK:
W = 557 N
m=?
g = 9.8 m/s2
m=W÷g
m = (557 N) ÷ (9.8 m/s2)
m = 56.8 kg
W
m g
ConcepTest
• Is the following statement true or false?
– An astronaut has less mass on the moon
since the moon exerts a weaker
gravitational force.
 False! Mass does not depend on
gravity, weight does. The astronaut has
less weight on the moon.
E. Newton’s
• For every action
force, there is an
equal and
opposite reaction
force.
rd
3
Law
rd
3
law: Action / Reaction
• The action force and the
reaction force occur to
different objects so the
force is not balanced.
• Not all action and
reaction forces produce
motion.
• Unbalanced forces
equal changes in
motion
Newton’s third law
• Kicking a soccer ball
– Action force = your foot hits the ball
– Reaction force = the ball pushes against your foot.
• Ball moves b/c action force is larger than reaction force
• Leaning against a wall
– Action force = you pushing against a wall
– Reaction force = wall pushing against you
• Nothing moves b/c the action force equals the reaction
force.
F. Friction
• Friction is a force that opposes the
motion of objects that touch as they
move past each other.
• Friction acts at the surface where
object are in contact
• Four main types of friction: static
friction, sliding friction, rolling friction
and fluid friction.
C. Friction
• Friction
– is the force that opposes motion
between 2 surfaces
– depends on the:
• types of surfaces
• force between the surfaces
C. Friction
• Friction is greater...
– between rough surfaces
– when there’s a greater
force between the
surfaces
(e.g. more weight)
• Pros and Cons?
Static Friction
• Static friction is the friction
force that acts on objects
that are not moving.
• Static friction always acts in
the direction opposite to
that of the applied force
• Prevents objects from
sliding.
• Example: pushing a dresser
that does NOT move
Sliding Friction
• Sliding Friction is a force
that opposes the direction
of motion of an object as it
slides over a surface.
• Occurs when there is
enough force to overcome
the static friction
• There will be a net force in
the direction of motion
• Pushing a desk that slides
against the floor.
Rolling Friction
• Rolling friction is the
force that acts on
rolling objects.
• Rolling friction is about
100 to 1000 times less
than the force of static
or sliding friction
• Skate boarding down a
hill
Fluid Friction
• Fluid friction opposes
the motion of an object
through a fluid.
• Fluid examples: air,
water, quick sand, and
cake batter
• Air Resistance is the fluid
friction action on an
object moving through
the air.
What are the forces
acting on a falling
leaf?
Air Resistance
• Terminal Velocity
– maximum velocity reached
by a falling object
– reached when…
Fair
Fgrav = Fair
 no net force
 no acceleration
 constant velocity
 still falling
Fgrav
Projectile Motion
• Projectile motion the motion of a
falling object after it is given an
initial forward velocity.
• this is a curved path
• Objects with different mass fall at
the same rate.
• The combination of an initial
forward velocity and the
downward vertical force of gravity
causes the ball to follow a curved
path.
Momentum
• Momentum is the product of an object’s mass
and its velocity, an object with large
momentum is hard to stop
• momentum for any object at rest is zero
• Law of conservation of momentum if not net
forces acts on a system, then the total
momentum of the system does not change
• In a closed system the loss of momentum of one
object equals the gain in momentum of another
object--- momentum is conserved