Download CH 3 Forces

CH 3 Forces
Sec 1 Newton’s
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nd
2
Law
Net force acting on an object
causes the object to accelerate
in the direction of the net force
Amount of “a” depends on the
mass of the object and the size
of the net force
F=mxa
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Force is measured in Newtons
(N), m=kg, a=m/s2
So…. 1 N = 1 kg x 1 m/s2
Let’s Practice!
How much force is
needed to accelerate an
80 kg rider and her 320
kg motorcycle at 6 m/s2?
 It takes a force of 3000 N
to accelerate an empty
1000 kg car at 3m/s2. If
a 160 kg wrestler is
inside the car, how much
force will be needed to
produce the same “a”?
 A 75 kg skater pushes off
from a wall with a force
of 300 N. What is the
skater’s “a”?
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Types of Friction
Friction (f) is the force that
opposes motion between 2
surfaces that are touching each
other
 Amount depends on surface
types and amount of force
pressing surfaces together
 3 different types:
 Static-prevents 2 surfaces from
sliding past each other
 Sliding-opposes the motion of 2
surfaces sliding past each other
 Rolling-frictional force between
a rolling object and the surface
it is rolling on
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Air Resistance
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Force that opposes
gravity on a falling object
Depends on 4 things:
Speed
Shape
Density
Size
Air resistance is why a
feather falls slower than
an object of the same
mass but has a different
shape
Terminal Velocity
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Highest velocity that will
be reached by a falling
object
Air resistance balances
the pull of gravity
According to the Law of
Inertia, when the forces
acting on an object are
balanced, the motion of
the object will not change
and acceleration will stop
Sec 2 Gravity
Gravity (g)– force that
every object in the
universe exerts on
every other object in
the universe
 Amount depends on
the 2 objects masses
and the distance
between them
 That is why a pencil
falls to the floor
(Earth) and not
toward you when you
drop it
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Falling objects  Ignoring all forces
except gravity on falling
objects, the object is
said to be in “free fall”
(more later…)
 Then all objects would
fall with the same
acceleration and hit the
ground at the same time
when dropped from
same height (Let’s try
it!)
 That acceleration is 9.8
m/s2
Weight
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Even if you are standing still, Earth exerts a gravitational
force on you
This is your weight (W) in Newtons
W=mxg
m= your mass in kg
g= acceleration of gravity (m/s2) which is 9.8 m/s2
W = m x 9.8 m/s2
Try converting your weight in kgs (divide lbs by 2.2) to N
Weight and Mass
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They are NOT the same
measurement
Weight is a force and mass
is the amount of matter an
object contains
Compare your weight in N
on Earth to the other
planets (pg 78)
Which one would you weigh
the most? The least?
Astronauts in space only
appear to be weightless—
they seem to be floating
because they are inside the
shuttle and they are all
falling with the same
acceleration
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Projectile Motion
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Anything thrown or shot
through the air is called a
projectile
They follow a curved path
due to Earth’s
gravitational pull and its
own inertia
When the quarterback
throws the ball it has
horizontal motion
(parallel to the Earth’s
surface) due to inertia
Gravity pulls the ball to
Earth, creating an
increasing vertical motion
These 2 motions are
independent of each
other
Circular Motion
Acceleration toward the
center of a curved or
circular path is called
centripetal acceleration
 Force that causes a
moving object to move
in a curved or circular
path is centripetal force
 Gravity, the ultimate
centripetal force:
Earth’s gravity exerts
this force on the Moon
which causes it to
move in a circular orbit
around the Earth
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Sec 3 The
rd
3
Law of Motion
Forces always act in pairs—
action/reaction pairs
 Newton’s 3rd Law—to every action
there is an equal and opposite
reaction
 Action/reaction forces always act
on different objects, so forces
might be equal, but they are NOT
balanced
 Action—rockets burn fuel and
produce hot gases that push
against the inside of the rocket and
escape out the back
 Reaction—causes the rocket to
move in the opposite direction
 Let’s try it: Balloon Racers!
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Momentum
Property a moving object has
due to its mass and velocity
 Designated by “p”
 p=mxv
 kg·m/s = kg x m/s
Practice!
 A horse with a mass of 450.0
kg finished a race with a
velocity of 20.0 m/s. What
was his “p”?
 What was the mass of another
horse that was running at a
velocity of 18.0 m/s if his “p”
was 7500 kg·m/s?
 Law of Conservation of
Momentum– the total amount
of p of a group of objects does
not change unless outside
forces act on the objects.
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Law of Conservation of Momentum
The total amount of
p of a group of
objects does not
change unless
outside forces act on
the objects
 BUT…p can be
transferred from one
object to another
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