Download Managing Acceleration

Demos
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Ball
Tennis ball on string
Two same size cookie boxes
Styrofoam, piece of plywood, hammer
Master Cylinder Brake
Box with sandpaper on bottom
(datastudio)
Feather and penny in tube
Managing
Acceleration
http://www.local6.com/news
http://www.gregscoasterphotos.com
www.dowwallpaper.com
Wikipedia
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What is acceleration?
Why we need to manage acceleration in
cars and other high speed conveyances.
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Crash Tests with and without restraint
systems
How to minimize dangerous accelerations.
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Air bags
Seatbelts
Anti-lock and Hydraulic brakes (avoiding
accidents)
Softer/deformable interior materials.
The Crumple Zone
What is velocity?
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Motion
speed and direction
40 mph due West is an example of
velocity
40 mph is speed (because no direction is
given)
When any part of velocity changes (speed and/or direction),
we say there is acceleration.
Acceleration is change in velocity!
Velocity = distance/time
(in a certain direction)
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examples of velocity units:
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Meters/second
Miles per hour
Feet per second
Kilometers per hour
Acceleration
Change in speed
Change in direction
Slowing down is negative acceleration
Making a sharp right turn is acceleration
Flooring the accelerator is positive acceleration
Driving over a speed bump is acceleration
Riding on a Merry-go-Round
Examples of zero acceleration
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No change in motion
A plane flying steadily and in a straight line
at 700 mph
An elevator that is moving at a steady pace
Your house when there isn’t an earthquake
Equations for finding acceleration
a = (Vf – Vi)/ t
a = -½ Vi2 / d
(Vf = 0)
a – average acceleration
Vi – initial speed/velocity
Vf – final speed
t – time
d – distance taken to stop
Aristotle (384 BC – 322 BC), the
great Greek philosopher believed….
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It is natural for things to be at rest.
Objects at rest require no explanation.
Objects in motion require an explanation.
Aristotle believed a force is required to
keep an object in motion.
What do you think?
Galileo Galilei (1564-1642)
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Questioned this 1900
year-old Aristotelian
view
Thought that maybe
objects in constant
motion may not
need a force to
remain in motion.
http://www.practicalphysics.org
Galileo showed…
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Motion in a straight line at constant speed
does not require a force.
Objects at rest or in motion (moving in a
straight line at a constant speed) are
natural states.
Forces cause a change in motion
Aristotle was incorrect to believed that
forces cause motion.
A change in motion is a change in direction
and/or speed.
Acceleration is the rate of change of motion.
Acceleration depends on the force applied and
the mass of the object.
F a acceleration
Galileo’s other studies of motion
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Galileo showed that all objects fall at the
same rate at the earth’s surface.
Galileo used an inclined plane to lessen
the earth’s pull on objects.
This allowed Galileo to do his experiments
without extremely accurate time pieces.
Prior to Galileo’s experiments with falling
objects, most scientists believed that
heavier objects fell faster than lighter
objects.
Galileo’s Apparatus
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Galileo found that
objects fall at 9.8 m/s2
Every second the
velocity of a falling
object increases by
9.8 m/s
Experiment reduced
the effects of friction
and air resitance.
http://ircamera.as.arizona.edu
Gravitational Acceleration
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All objects fall to the earth with the same
acceleration. (ex. Feather and penny in
tube)
This acceleration = 9.8 m/s2
A falling object will increase its speed by 9.8 m/s
every second it falls. (assuming no air resistance)
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1 g = 9.8 m/s2
You are experiencing 1 g “pull” towards earth as
you sit in the classroom
Isaac Newton (1642-1727)
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Continued Galileo’s study of
motion
Studied motion while alone
on his farm during the
plague while Cambridge was
closed.
http://content.answers.com
Isaac Newton expressed the laws of motion as
3 laws and explained why they are UNIVERSAL laws
(they apply everywhere, not just on earth)
Newton’s First Law of Motion states:
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Objects at rest tend to stay at rest unless a net
force is applied to them.
Objects in motion tend to stay in motion and
continue in a straight line unless a force is
applied to them
Rest or Constant speed in a straight line is called
Uniform Motion.
Example of the first law
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Objects lying around don’t “walk away” on their
own. (i.e. blame your room-mates)
Without seatbelts, a person can get ejected from
a car that suddenly stops.
http://youtube.com/watch?v=giYQE1Hskjc&mode
=related&search=
http://youtube.com/watch?v=xU2jrQ4uunU&featu
re=related
http://www.nhtsa.dot.gov
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Any motion that IS NOT Uniform Motion, is
acceleration and requires a force.
Prior to Newton, scientists thought that celestial
bodies (moons, planets) that moved in circles
did not have forces acting on them.
Heavenly bodies never stopped orbiting so
people assumed the “steady state” of the
heavens implied that no forces acted on orbiting
bodies seen in space.
Newton understands that circular motion
must require a force…. So what does this
mean for the earth’s moon?
Newton and the Apple
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Newton did tend an apple orchard
Claims he did have a breakthrough moment
while daydreaming there
Newton saw the apple and moon together in the
sky
Newton makes the connection that the moon is
like a VERY large apple only farther away
An apple thrown fast enough could be put into
orbit just like the moon.
Realizes that the moon IS falling towards the
earth due to gravity but that its distance from
the earth never changes due to the curvature of
the earth
Satellite Motion
http://www.edumediasciences.com/a271_l2-satellitemotion.html
Newton’s Laws of Motion are
Universal
Newton realizes that laws of motion
that describe motion on earth should
be universal and apply to motion of
bodies in the universe.
Newton’s 2nd Law of Motion states:
Force on an object is the object’s
Mass times Acceleration.
F = ma
F – Force
m – mass (how much matter or
atoms make up the object)
a – acceleration (rate at which
speed or direction changes)
Examples of the 2nd Law
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More massive objects weigh more
F = mg
It is harder to throw a bowling ball than a
tennis ball
I’d rather be hit by an acorn than a big
green pine cone going the same speed
F = ma
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It takes more force to accelerate a big
mass compared to a small mass. This is
one reason why heavy and light objects in
a vacuum fall to earth at the same rate.
If there is a force on you right now?
Why are you not accelerating?
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A force does not always produce
acceleration, but if all the forces on a
body/object added together do not equal
ZERO, then the net force will produce
acceleration
This leads us to the Newton’s 3rd Law
Newton’s Third Law of Motion states:
For Every force there is an equal but
opposite force.
www.the-fitness-motivator.com
www.primidi.com
http://www.answers.com
3rd Law: Forces come in pairs
Where’s the equal but
opposite force?
The earth is also
accelerating towards
the skydiver but the
acceleration is very,
very small … or
cancelled out by
someone skydiving on
the other side of the
earth.
www.blackfive.net
Forces always come in pairs
Examples:
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Sitting on a chair
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Squishing a styrofoam ball
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A car hitting a metal guard rail
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Two cars colliding
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If you want to determine the force
of a crash, you can either inspect
damage to the car or inspect
damage to what the car hit.
All objects with mass are attracted to each
other – this attractive force is called gravity
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Newton’s Law of Universal Gravitation
Force = G x m1 x m2 / d2
G is a universal constant
From this equation the earth’s mass can
be calculated.
M1
d
M2
Henry Cavendish devises an experiment that allows him to find Big G
The force between two 150 lb.
people 3’ apart is about the
weight of a flea.
Gravity is a VERY weak force but the
earth is so massive that the force is
exerts on us is very noticeable.
What is a potentially
dangerous acceleration?
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Slamming on the breaks
Hitting a brick wall
If a force of 4 to 6 g is sustained for more
than a few seconds, the resulting
symptoms range from visual impairment
to total blackout.
Crash victims sustain greater g forces but
experience them for approx. less than 150
ms
“The acceleration during the crash that
killed Diana, Princess of Wales, in 1997
was estimated to have been on the order
of 70 to 100 g, which was intense enough
to tear the pulmonary artery from her
heart -- an injury that is nearly impossible
to survive. Had she been wearing a seat
belt, the acceleration would have been
something more like 30 or 35 g - enough
to break a rib or two, but not nearly
enough to kill most people.”
http://hypertextbook.com/physics/
Automotive Accelerations (g)
event
typical car
sports
car
race
car
large truck
starting
0.3 - 0.5
> 0.9
1.7
< 0.2
braking
0.8 - 1.0
> 1.3
2
~ 0.6
cornering
0.6 - 1.0
> 2.5
3
??
http://hypertextbook.com
Fighter Pilots may experience
up to 9g
A pilot weighing 180 lbs. will feel as if s/he
weighs 180 X 9 or 1620 lbs.
Technologies used to minimize
dangerous accelerations in cars
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Seatbelts and the webbing
Air bags
The Crumple zone
Deformable dashboard and steering wheel
Hydraulic brakes
Anti-lock brakes
Seatbelts do three things:
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Apply forces to the parts of the body that are
tough (rib cage and pelvis)
Prevent the human from impacting rigid objects.
Reduce the acceleration on the body by
restraining the body continuously throughout the
crash.
Energy = F x distance
By increasing the stopping distance of the
human the forces are lowered.
Inertia triggered retractors
(Inertia is determined by mass and it is the
resistance to changes in motion)
Pawl
Ratchet Gear
Belt Triggered Retractor – page 75
Clutch
Toothed
Plate
Seatbelts typically lock up around ½ g
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Clutch – a mechanism for transmitting
rotation, which can be engaged or
disengaged (Wikipedia)
Ratchet – a device that (when engaged)
allows for linear or rotational motion in
one direction only
Pawl – a piece with a pointed end that
engages with the ratchet and locks it
Determine type of seatbelt
given in class
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Triggered by yanking on the belt or by
changes to acceleration?
Locate the pawl, ratchet, toothed plate,
and any other important component.
In the event of a crash, a pretensioner will tighten the
belt almost instantaneously. Like airbags, pretensioners
are triggered by sensors in the car's body, and most
pretensioners use explosively expanding gas to drive a
piston that retracts the belt. Wikipedia
Gas is ignited here
Airbags inflate
and apply
forces evenly to
the windshield,
dash, and
occupant over a
time period of
about 100 ms
www.abetterwindshield.com
What makes an airbag inflate?
The accelerometer is built
into a microchip. During
Large decelerations, the mass
of the accelerometer shifts.
This closes an electrical contact
triggering the bag to inflate.
Sodium Azide (NaN3) reacts
with Potassium Nitrate (KNO3)
to produce nitrogen gas
The bag inflates in 40 ms
The bag has tiny holes in it
allowing the gas to escape so
that the bag absorbs energy.
Is an air bag dangerous?
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Inflating in 40 ms implies that the bag is
actually exploding.
Occupants should be >10 inches away from
the steering wheel
If the occupant hits the air bag before it is
fully inflated injury can occur. When the
bag inflates it does so at approx. 100 mph.
How effective are air bags?
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Seatbelts are 42% effective at preventing
driver fatalities.
Seatbelts with air bags are 49% effective
at preventing driver fatalities.
Airbags reduce the risk of death by only 7%
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246 people died from air bags from 19862001 (75% were women)
7,000 people were saved from air bags
during the same 15 year period
11,000 people are saved annually due to
seat belts
People less than 5’3” tall are more harmed
with an air bag than without it during a crash
because shorter drivers sit close to the
steering wheel.
http://www.youtube.com/watch?v=-lHI5BwFl_w&NR=1
Sensors
Mercury Sensor
http://www.autoshop101.com
The role of deformable materials
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Reduce accelerations on a human’s body
Increase the stopping time and distance
E = Force * distance (increase distance
so that the energy is absorbed at a lower
force)
Less bouncing – multiple hard hits are
more damaging that one force applied
more continuously
Crumple Zone absorbs energy
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KE = ½ mv2
Kinetic Energy needs to be absorbed
quickly in an area outside of the driving
compartment
Crumple Zone
http://www.aip.org/dbis/stories/2004/14124.html
Sources
http://auto.howstuffworks.com/seatbelt.htm
http://auto.howstuffworks.com/airbag.htm