Transcript Force and Newtons Laws

Class Starter
1. If a car travels west 75 kilometers takes a uturn and travels back east 25 kilometers
what is the car’s final displacement? 50 km west
2. If a car at rest, traveled north 5.5 s and
reached a final velocity of 22.0 m/s, what
was the car’s acceleration? 4.0 m/s2
3. If a force propels a car to move 300km in
4hrs what is the average speed (do not need
to convert)? 75 km/h
Ch 12
http://www1.teachertube.com/viewVideo.php?video_id=540&title=ForceMan_
WHAT IS A FORCE?
– a push or a pull
– one body exerts a force
on another
– SI units are Newton
2
(N), 1N=1kgm/s
m
=
(
m
)(
a
)
–
F
a
Effects of forces on objects
– Balanced forces – forces are
equal in size & opposite in
direction
–Net force – force that changes
the velocity of the object
•Object accelerates in the
direction of the greater force
• A group of animals are playing tug-of-war.
The elephants pull the rope with a force of
100N to the left. The donkeys pull the rope
with a force of 105N to the right. Which
way will the rope accelerate?
• To the RIGHT
Friction
– A force that opposes motion
between 2 surfaces that are
touching each other
Friction
–Amount of friction depends
on 2 factors
1. the kinds of surfaces
(texture)
2. the force pressing the
surfaces together
(pressure)
Four types of Friction
1. Static-friction force acts on
objects that are NOT moving,
acts in direction OPPOSITE to
the applied force
2. Sliding-force opposes the
direction of motion of object as
it slides over a surface (less
than static)
Four types of Friction
3. Rolling-force acts on rolling
objects (less than static or
sliding)
4. Fluid-opposes motion of
an object through
a liquid/gas
Gravitational Force
• Gravity – the attracting force
exerted by every object on
every other object
• Amount of gravitational
force between objects
depends on 2 things...
Gravitational Force
1) mass
a) more mass=more g force
b) less mass=less g force
2) distance
a) greater distance=less g force
b) lesser distance=more g force
ACCELERATION DUE TO GRAVITY
Falling objects accelerate
2
at 9.8 m/s
*same for ALL
(in a vacuum w/
no air resistance)
This means that
for each s the
body is falling, its
velocity increases
by 9.8 m per s
Air Resistance
• force air exerts on
a moving object
• Opposes gravity
• Amount depends
on:
1. Speed 2. Size
3. Shape 4.Density
Terminal Velocity
• The highest velocity that will be
reached by a falling object
• Downward Fg equals the
upward Fair resistance
Projectiles
• Anything shot/thrown through the air
• A thrown ball has constant horizontal
velocity
• Best angle to get farthest distance = 45O
• Gravity pulls the ball downward
• Vertical acceleration increases
changing the direction of the projectile
to forward and downward, making a
curved path
Projectiles
Circles
Centripetal
Acceleration
towards the
center of a
curved or
circular path
Circles
Centripetal Force
exerted toward
the center of a
curved path,
w/out this force
object will shoot
off in a straight
line in the
direction it was
traveling
STAR Questions
• List all 4 things that affect the amount of
air resistance on an object.
• Speed, size, shape and density
• List the 2 things that affect the amount of
friction btw 2 objects.
• Texture and pressure
• List the 2 things that affect the amt of
gravitational force btw 2 objects.
• Mass and distance
Newton’s 1st Law
• States an object moving at a
constant velocity keeps moving at
that velocity unless a net force acts
on it.
• If an object is at rest, it stays in
rest unless a net force acts on it.
• “If I push it, it goes”
• Also called the law
inertia
of
Inertia
• Tendency of an object to resist any
change in its motion
• If an object is moving, it keeps
moving at that speed & direction
unless a force acts on it
Inertia
• The more mass an object has, the
greater its inertia
• More force would be needed to change
the velocity of the object w/ greater
inertia (which also means greater mass)
Newton’s 2nd Law
• The acceleration of an object is equal
to the net force acting on it divided by
the object’s mass
• a=F/m or simply F=ma
• amt of acceleration is affected by
1. size of the force
F
2. mass of the object
m
a
Thanks Cambridge Physics Outlet for amazing graphics!
Examples:
• What is the force required to push a
25kg girl in a 100kg wagon with an
acceleration of 3m/s2?
• Find the acceleration of an object
with a mass of 20kg and a Force
of 50N being applied to it?
Mass
• amount of matter in an object
• constant independent of location
• measured in g & Kg
Weight
• measure of force of gravity
on an object Fg=W=mg
• measured in Newtons (N)
*g = 9.8m/s2
*Mass has to be in
kg to calculate Weight
Fg
m
g
• What is the weight of a person who
has a mass 50,000 g?
• If a person has a mass of 50kg on
Earth, would mass be less than 50kg
on moon?
• Would they weigh the same on the
moon?
Newton’s third law of motion
• Whenever one object exerts a force on a
second object, the second object exerts
an equal & opposite force on the first
object (action / reaction)
• They do NOT cancel out because they
act on different objects
Thanks Cambridge Physics Outlet for amazing graphics!
Momentum
• More mass =
More momentum
• More velocity =
More momentum
• Momentum is
transferred
Law of conservation of Momentum
• In a closed system, the
loss of momentum of
one objects equals the
gain in momentum of
another object –
momentum is
CONSERVED
STAR Questions
• What is the rate of acceleration due to
gravity for a car?
• 9.8 m/s2
• Describe Newton’s 1st law.
• If I push it = it goes, if no push = no go
• Describe Newton’s 3rd law.
• Action = reaction