Managing Acceleration

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Managing Acceleration
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Wikipedia
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• 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

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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!
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• Velocity distance/time
• (in a certain direction)
• examples of units
• Meters/second
• Miles per hour
• Feet per second
• Kilometers per hour

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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

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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 isnt an earthquake

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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

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Galileo Galilei (1564-1642)

• Galileo showed that all objects fall at the same
  rate at the earths surface
• Galileo used an inclined plane to lessen the
  earths pull on objects
• This allowed Galileo to do his experiments
  without extremely accurate time pieces.
• Prior to Galileos experiments with falling
  objects, most scientists believed that heavier
  objects fell faster than lighter objects.

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Galileos 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
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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

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Isaac Newton (1642-1727)

• Continued Galileos study of motion
• Studied motion while alone on his farm during the
  plague while Cambridge was closed.

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Isaac Newton discovered the laws of motion and
expressed them mathematically

• Newtons 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

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Example of the first law

• Objects lying around dont 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?vgiYQE1Hskjcmoderelat
  edsearch
• http//youtube.com/watch?vxU2jrQ4uunUfeaturere
  lated

http//www.nhtsa.dot.gov
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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

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• 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

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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

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Satellite Motion

• http//www.edumedia-sciences.com/a271_l2-satellit
  e-motion.html

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Newtons 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.

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• Newtons 2nd Law of Motion states
• Force on an object is the objects
• Mass times Acceleration.
• F ma

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Examples of the 2nd Law

• More massive objects weigh more
• F mg
• It is harder to throw a bowling ball than a
  baseball
• Id rather be hit by an acorn than a big green
  pine cone going the same speed
• F ma

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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 Newtons 3rd Law

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• Newtons Third Law of Motion states
• For Every force there is an equal but opposite
  force.

www.the-fitness-motivator.com
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http//www.answers.com
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• Wheres the equal but opposite force?
• The earth is also accelerating towards the
  skydiver but the acceleration is very, very small.

www.blackfive.net
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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.

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All objects with mass are attracted to each other
this attractive force is called gravity

• Newtons Law of Universal Gravitation
• Force G x m1 x m2 / d2
• G is a universal constant

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Cavendish devises an experiment that allows him
to find G
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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.
• Crash victims sustain greater g forces but
  experience them for less than 150 ms

Funk Wagnall's Encyclopedia.
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• 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/

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Automotive Acceleration (g) Automotive Acceleration (g) Automotive Acceleration (g) Automotive Acceleration (g) Automotive Acceleration (g)
event typical car sports car race car large truck
starting 0.3 - 0.5 gt 0.9 1.7 lt 0.2
braking 0.8 - 1.0 gt 1.3 2  0.6
cornering 0.6 - 1.0 gt 2.5 3 ??
 
http//hypertextbook.com
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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

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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.

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Inertia triggered retractors
Pawl
Ratchet Gear
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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

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Determine type of seatbeltgiven 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.

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• 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
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Airbags inflate and apply forces evenly to the
windshield, dash, and occupant over a time period
of about 100 ms
www.abetterwindshield.com
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What makes an airbag 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.
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.
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Is an air bag dangerous?

• Inflating in 40 ms implies that the bag is
  actually exploding.
• Occupants should be gt10 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.

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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

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• 246 people died from air bags from 1986-2001 (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 53 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_wNR1

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Sensors
Mercury Sensor http//www.autoshop101.com
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The role of deformable materials

• Reduce accelerations on a humans 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

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Crumple Zone absorbs energy

• KE ½ mv2
• Kinetic Energy needs to be absorbed quickly in an
  area outside of the driving compartment

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Crumple Zone
http//www.aip.org/dbis/stories/2004/14124.html
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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 doesnt mean you will stop quicker. However
  control of the car is maintained, because
  skidding is avoided.

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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.

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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.

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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.
http//hyperphysics.phy-astr.gsu.edu
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Static friction will counteract any applied force
up to a certain threshold at which friction is
overcome and the object begins to slide.
Once in motion, kinetic friction resists motion.
Kinetic Friction
Static Friction

• FStatic gt FKinetic
• is the friction coefficient
• is between 0 and 1

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FStatic gt 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.

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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 its highest right before lock-up

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Demo of conventional brakes vs. ABS
http//www.youtube.com/watch?vuq4DDMMoomU
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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
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• 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.
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If the controller determines that one tire is
decelerating more than the other tires, the
controller sends a signal to the wheels 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
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Sources

• http//auto.howstuffworks.com/seatbelt.htm
• http//auto.howstuffworks.com/airbag.htm