Download Forces - Mandan Public School District

Chapter 3
Newton’s
nd
2
law
 2nd – an object
accelerates in the
direction of the net force
acting upon it
 A= net force/Mass
 A force on an object is
equal to the change in
momentum of the object
 Force = mass * accel.
 F=m*a
 a= Fnet/m
Practice
 Your push your
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
Microweldsdeformations that hold
cause static friction
Ever notice how it takes
more force to get an
object moving
Air resistance
 When drag is equal to
weight, there is no net
external force on the
object.
 The object then falls at a
constant velocity as
described by Newton's
first law of motion.
 The constant velocity is
called the terminal
velocity.
 This is due to the air
resistance – Friction
force caused when
objects fall through air
 Dependant upon the
speed, shape, mass, and
size of the object, and
the density of the air.
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_dOEyAf
k
 http://nssdc.gsfc.nasa.go
v/planetary/image/feath
erdrop_sound.mov
surface of the earth
accelerate at a rate of
9.8m/s/s
 Objects of different
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 car makes a
turn, the force of
friction acting
upon the turned
wheels of the car
provide the
centripetal force
required for
circular motion.
As a bucket of
water is tied to a
string and spun
in a circle, the
force of tension
acting upon the
bucket provides
the centripetal
force required
for circular
motion
As the moon orbits
the Earth, the
force of gravity
acting upon the
moon provides the
centripetal force
required for
circular motion –
p.s. the moon is a
banana
Weight
 Weight – The
measure of the force
of gravity on a body.
 On earth your weight
in Newtons (N) is
equal to your mass in
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
8.7 m/s/s. About 10% less
than the surface
Projectile motion
 Projectile motion:
Anything that has
horizontal motion is a
projectile
 Horizontal and vertical
motion act
independently of one
another-
 Projectiles follow a path
called a trajectory
So a bullet dropped and a bullet fired from the same
height will hit the ground at the same time.
Newton’s
rd
3
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
objects when they strike
each other
 F=mvf – mvi/t
combining the 2nd and
3rd laws
 A baseball strikes your
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