Managing Acceleration - PowerPoint PPT Presentation

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Managing Acceleration

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Air bags Seatbelts Anti-lock and Hydraulic brakes (avoiding accidents) ... a hydraulic actuator*, and wheel speed sensors at each wheel. – PowerPoint PPT presentation

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Title: Managing Acceleration


1
Managing Acceleration
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Wikipedia
2
  • 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

3
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!
4
  • Velocity distance/time
  • (in a certain direction)
  • examples of units
  • Meters/second
  • Miles per hour
  • Feet per second
  • Kilometers per hour

5
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

6
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

7
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

8
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.

9
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
10
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

11
Isaac Newton (1642-1727)
  • Continued Galileos study of motion
  • Studied motion while alone on his farm during the
    plague while Cambridge was closed.

http//content.answers.com
12
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

13
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
14
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

15
  • 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

16
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

17
Satellite Motion
  • http//www.edumedia-sciences.com/a271_l2-satellit
    e-motion.html

18
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.

19
  • Newtons 2nd Law of Motion states
  • Force on an object is the objects
  • Mass times Acceleration.
  • F ma

20
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

21
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

22
  • 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
23
  • Wheres the equal but opposite force?
  • The earth is also accelerating towards the
    skydiver but the acceleration is very, very small.

www.blackfive.net
24
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.

25
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

26
Cavendish devises an experiment that allows him
to find G
27
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.
28
  • 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/

29
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
30
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

31
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.

32
Inertia triggered retractors
Pawl
Ratchet Gear
33
Belt Triggered Retractor page 75
Clutch
Toothed Plate
Seatbelts typically lock up around ½ g
34
(No Transcript)
35
  • 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

36
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.

37
  • 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
38
Airbags inflate and apply forces evenly to the
windshield, dash, and occupant over a time period
of about 100 ms
www.abetterwindshield.com
39
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.
40
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.

41
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

42
  • 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

43
Sensors
Mercury Sensor http//www.autoshop101.com
44
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

45
Crumple Zone absorbs energy
  • KE ½ mv2
  • Kinetic Energy needs to be absorbed quickly in an
    area outside of the driving compartment

46
Crumple Zone
http//www.aip.org/dbis/stories/2004/14124.html
47
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.

48
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.

49
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.

50
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
51
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

52
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.

53
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

54
Demo of conventional brakes vs. ABS
http//www.youtube.com/watch?vuq4DDMMoomU
55
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
56
  • 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.
57
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
58
Sources
  • http//auto.howstuffworks.com/seatbelt.htm
  • http//auto.howstuffworks.com/airbag.htm
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