Transcript Managing Acceleration Wikipedia

Managing
Acceleration
http://www.local6.com/news
http://www.gregscoasterphotos.com
www.dowwallpaper.com
Wikipedia


What is acceleration?
Why we need to manage acceleration in
cars and other high speed conveyances.


Crash Tests with and without restraint
systems
How to minimize dangerous accelerations.





Air bags
Seatbelts
Anti-lock and Hydraulic brakes (avoiding
accidents)
Softer/deformable interior materials.
The Crumple Zone
What is velocity?



speed with direction included
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)

examples of units:




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



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
a – average acceleration
Vi – initial speed/velocity
Vf – final speed
t – time it takes to stop
d – distance taken to stop
Galileo Galilei (1564-1642)




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


Galileo found that
objects fall at 9.8
m/s2
Every second the
velocity of a
falling object
increases by
9.8 m/s
http://ircamera.as.arizona.edu
Gravitational Acceleration


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)

1 g = 9.8 m/s2
You are experiencing 1 g “pull” towards earth as
you sit in the classroom
Isaac Newton (1642-1727)


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 discovered the
laws of motion and
expressed them mathematically
Newton’s First Law of Motion states:
 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
Constant speed in a straight line is called
Uniform Motion
Example of the first law


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
What is Inertia?
Inertia is the resistance an object has to a
change in its state of motion
Inertia is the tendency of an object to stay
at rest or stay in motion



Any motion that IS NOT Uniform Motion, is
called Acceleration
Prior to Newton, scientists thought that objects
that moved in circles did not have forces acting
on them.
Heavenly bodies (moon, planets) never stopped
orbiting so people assumed the steady state of
the heavens implied that no forces acted on
orbiting bodies seen in space
Newton and the Apple





Newton did tend an apple orchard
Claims he did have a breakthrough moment
while daydreaming there
Newton saw the apple and moon together
Newton makes the connection that the moon is
like a VERY large apple only farther away
Realizes that the moon IS falling towards the
earth 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
Examples of the 2nd Law



More massive objects weigh more
F = mg
It is harder to throw a bowling ball than
a baseball
I’d rather be hit by an acorn than a big
green pine cone going the same speed
F = ma
If there is a force on you right now,
then why are you not accelerating?


A force does not always produce
acceleration, but if all the forces 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
Where’s the equal
but opposite force?
The earth is also
accelerating
towards the
skydiver but the
acceleration is
very, very small.
www.blackfive.net
Forces always come in pairs
Examples:

Sitting on a chair

Swimming

A car hitting a metal guard rail

Two cars colliding

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



Newton’s Law of Universal Gravitation
Force = G x m1 x m2 / d2
G is a universal constant
M1
d
M2
Cavendish devises an experiment that allows him to find G
What is a potentially
dangerous acceleration?




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. Funk & Wagnall's Encyclopedia.
Crash victims sustain greater g forces but
experience them for 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 Acceleration (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
Technologies used to minimize
dangerous accelerations






Seatbelts and the webbing
Air bags
The Crumple zone
Deformable dashboard and steering wheel
Hydraulic brakes
Anti-lock brakes
Seatbelts do three things:



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
Pawl
Ratchet Gear
Belt Triggered Retractor – page 75
Clutch
Toothed
Plate
Seatbelts typically lock up around ½ g



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



Triggered by yanking on the belt or by
changes to acceleration?
Locate the pawl, ratchet, toothed plate,
and any other important component.
Describe in two-three sentences what
happens when the car undergoes high
negative acceleration.
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?



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 100 mph.
How effective are air bags?


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%




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 the seat is too 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




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


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
Anti-lock Brakes (ABS)




Safer only when used properly.
Accident data does not show fewer fatalities as a
result of having ABS
ABS functions only when brakes are not
pumped. ABS creates pedal chatter which
means ABS is working.
ABS doesn’t mean you will stop quicker.
However control of the car is maintained,
because skidding is avoided.
How does an
Anti-lock braking systems work?


ABS keeps the tires from skidding as long
as the driver keeps the brake pedal down.
When slamming on the brakes, tires can
stop spinning much faster than a car can
stop. This is called “lock up”. When this
happens the car must now skid because
the wheels are not turning.
Static (rolling) friction
vs. Kinetic (sliding) friction


Kinetic (sliding) friction is much less than
static (rolling) friction.
Once a car skids, the tires lose grip with
the road and the driver loses control.
Tire tread, road, and driving conditions
determine the coefficient of friction.
A bald tire on a dry roads has a much
higher friction coefficient than on a wet
road.
Static friction will
counteract any applied
force up to a certain
threshold at which friction
is overcome and the
object begins to slide.
Static Friction
Once in motion, kinetic
friction resists motion.
Kinetic Friction
FStatic > FKinetic
 is the friction coefficient
 is between 0 and 1
FStatic > FKinetic
For this reason, we DO NOT want to skid if we
want to maintain traction and control of the car.
When stopping, we want to the tire to “stick” to
the road so that we can use the contact forces
to stop the car.
Static friction is higher than sliding friction,
therefore for quick stopping we do not want to
skid.
The ABS controller (computer)



Ensures that the tire slows down at the same
rate as the car, but keeps the tires forces very
near the point where sliding would occur –
just under Fstatic
This maximizes the braking power.
Friction is at it’s highest right before lock-up
Demo of conventional brakes vs. ABS:
http://www.youtube.com/watch?v=uq4DDMMoomU
How is this done?
The system uses a computer to monitor
the speed of each wheel. When it detects
that one or more wheels are turning
slower than the remaining wheels, the
computer sends a signal to momentarily
remove and reapply the pressure to the
affected wheels to allow them to continue
turning. This "pumping" of the brakes
occurs at ten or more times a second.
http://www.familycar.com/brakes.htm
The system consists of an electronic
control unit, a hydraulic actuator*, and
wheel speed sensors at each wheel.
Information from sensors is fed
to the controller.
http://www.aa1car.com/library/abs1.htm
* An actuator is a device in a car that uses electricity to do some sort of
mechanical movement- Such as power door locks or a remote trunk opener.
In this case the actuator releases pressure in the brakes to pulse the
brakes.
If the controller determines that one tire is
decelerating more than the other tires, the
controller sends a signal to the wheel’s actuator to
lower the pressure to prevent skidding.
Solenoid valve closes,
preventing more fluid
from getting to the brake.
It then is reopened, thus
pumping the brake.
www.lake-link.com
Sources
http://auto.howstuffworks.com/seatbelt.htm
http://auto.howstuffworks.com/airbag.htm