# Forces

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```					Chapter 3
Newton’s            2nd   law
 2nd – an object               A force on an object is
accelerates in the             equal to the change in
direction of the net force     momentum of the object
acting upon it
 A= net force/Mass             Force = mass * accel.
 F=m*a
 a= Fnet/m
Practice
friends trike with a
force of 150N, his
mass is 35kg the
trikes is 15kg, what is
the acceleration?
 150N/50kg=
 3m/s/s
Friction
 Friction is the force
that opposes motion
between two
opposing surfaces
 Friction always acts
against motion
 Causes of friction –
unsmooth surface,
deformations,
molecular attraction
2 types of friction
 Static friction – force
of a resting object
 Sliding or Kinetic
friction - force on a
moving object
Microwelds-
deformations that hold
cause static friction
Ever notice how it takes
more force to get an
object moving
Air resistance
 When drag is equal to         This is due to the air
weight, there is no net        resistance – Friction
external force on the          force caused when
object.                        objects fall through air
 The object then falls at a
 Dependant upon the
constant velocity as
described by Newton's          speed, shape, mass, and
first law of motion.           size of the object, and
 The constant velocity is       the density of the air.
called the terminal
velocity.
More massive objects fall faster than less massive
objects because they are acted upon by a larger force
of gravity; for this reason, they accelerate to higher
speeds until the air resistance force equals the gravity
force.
Stuff falls
 http://www.youtube.co        All objects near the
m/watch?v=5C5_dOEyA           surface of the earth
fk                            accelerate at a rate of
 http://nssdc.gsfc.nasa.go     9.8m/s/s
v/planetary/image/feath      Objects of different
erdrop_sound.mov              masses will fall at the
same rate regardless of
their mass
Where is he falling fastest?
Where is accelerating the most?
Where is he not accelerating?
Gravitational accelertion
 More force acts upon a
more massive object, but
its larger mass requires
more force to accelerate.

 There is a myth about
Galileo dropping canon
balls off the leaning
tower of Pisa to prove
this.
Gravitational acceleration
 Velocity of a freefalling object can be found with the
formula v= g*t
 Where gravities acceleration is 9.8m/s/s

 How can you find falling distance
 D= ½ g *t2
Centripetal
 Centripetal
acceleration:
acceleration toward
the center of a curved
path
 Centripetal means
center seeking- it is
merely a net force
Centripetal ex.

As a bucket of
As a car makes a     water is tied to a   As the moon orbits
turn, the force of   string and spun      the Earth, the
friction acting      in a circle, the     force of gravity
upon the turned      force of tension     acting upon the
wheels of the car    acting upon the      moon provides the
provide the          bucket provides      centripetal force
centripetal force    the centripetal      required for
required for         force required       circular motion –
circular motion.     for circular         p.s. the moon is a
motion               banana
Weight
 Weight – The
measure of the force
of gravity on a body.
in Newtons (N) is
kg times gravity’s
acceleration
(9.8m/s2)
 W= m * g
Weightlessness
 Weightlessness is sensation experienced when
there are no external objects touching one's
body and exerting a push or pull
 Astronauts on the orbiting space shuttle are
weightless because..
 a. there is no gravity in space and they do not weigh
anything.
 b. space is a vacuum and there is no gravity in a
vacuum.
 c. space is a vacuum and there is no air resistance
in a vacuum.
 d. the astronauts are far from earth's surface at a
location where gravitation has a minimal effect.
Freefallin’
 Orbit is a state of
constant freefall – the
Earth’s surface is
falling away at the
same rate of orbit
 400 km above the earth's
surface, the the value of g
will have been reduced
from 9.8 m/s/s (at earth's
surface) to approximately
than the surface
Projectile motion
 Projectile motion:         Projectiles follow a path
Anything that has           called a trajectory
horizontal motion is a
projectile
 Horizontal and vertical
motion act
independently of one
another-
So a bullet dropped and a bullet fired from the same
height will hit the ground at the same time.
Newton’s   3rd   law
 For every action
(force) in nature
there is an equal and
opposite reaction.
 Force pairs: Action
and reaction forces
do not act on the
same object.
 What would happen
if they did?
Many reaction forces are not noticed due to other
forces or smaller accelerations.
Introduction to Rocket Performance - Level 3 - Jump
Animation
Momentum
 Momentum: property
of a moving object
based on its velocity
and mass.
 p = mv
 kg x m/s – label
Momentum is conserved
 Law of Conservation
of Momentum:
Momentum may be
exchanged but the
total amount of
momentum remains
the same.
In a game of pool where is the
momentum lost
Practice
 A scooter and a big
red truck, both
moving at 20m/s.
Which has more
momentum.
 Mass of red truck
3000kg
 Mass of scooter 8kg
 Momentum of truck:
 of Scooter:
Force and momentum
 Objects with momentum       This is a formula
can apply force to other     combining the 2nd and
objects when they strike     3rd laws
each other                  A baseball strikes your
 F=mvf – mvi/t                glove with a velocity of
49m/s, its mass is .145kg.
The ball comes to a stop
in .05s. What force is
applied?
Artificial Satellite vs. Natural Satellite
 Sputnik: 1957 Russian
(October 4th) “Fellow
Traveler or Satellite”

 What do we use satellites
for?
 Weather           TV
Communications
Science and Research
Satellites
 How many satellites
currently orbit the earth?
 Geosynchronous
Satellites: Orbital speed
matches the earth’s
rotation
 Where?       35,790 km
above the equator.
 International Space
Station: 390 km

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