Managing Managing AccelerationAcceleration

Wikipedia

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What is acceleration?What is acceleration? Why we need to manage acceleration Why we need to manage acceleration

in cars and other high speed in cars and other high speed conveyances.conveyances. Crash Tests with and without restraint Crash Tests with and without restraint

systemssystems How to minimize dangerous How to minimize dangerous

accelerations.accelerations. Air bagsAir bags SeatbeltsSeatbelts Anti-lock and Hydraulic brakes (avoiding Anti-lock and Hydraulic brakes (avoiding

accidents)accidents) Softer/deformable interior materials.Softer/deformable interior materials. The Crumple ZoneThe Crumple Zone

What is velocity?What is velocity?

speed with direction includedspeed with direction included 40 mph due West40 mph due West is an example of is an example of

velocityvelocity 40 mph40 mph is speed (because no is speed (because no

direction is given)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/timeVelocity = distance/time(in a certain (in a certain

direction)direction)

examples of units:examples of units: Meters/secondMeters/second Miles per hourMiles per hour Feet per secondFeet per second Kilometers per hourKilometers per hour

AccelerationAcceleration

Change in speedChange in speedChange in directionChange in direction

Slowing down is negative accelerationSlowing down is negative acceleration

Making a sharp right turn is accelerationMaking a sharp right turn is acceleration

Flooring the accelerator is positive Flooring the accelerator is positive accelerationacceleration

Driving over a speed bump is Driving over a speed bump is accelerationacceleration

Riding on a Merry-go-RoundRiding on a Merry-go-Round

Examples of zero Examples of zero accelerationacceleration

A plane flying steadily and in a straight A plane flying steadily and in a straight line at 700 mphline at 700 mph

An elevator that is moving at a steady An elevator that is moving at a steady pacepace

Your house when there isn’t an Your house when there isn’t an earthquakeearthquake

Equations for finding Equations for finding accelerationacceleration

a = (Va = (Vff – V – Vii)/ t)/ t

a = -½ Va = -½ Vii22 / d / d

a – average accelerationa – average acceleration

VVii – initial speed/velocity – initial speed/velocity

VVff – final speed – final speed

t – time it takes to stopt – time it takes to stop

d – distance taken to stopd – distance taken to stop

Galileo Galilei (1564-1642)Galileo Galilei (1564-1642)

Galileo showed that all objects fall at Galileo showed that all objects fall at the same rate at the earth’s surfacethe same rate at the earth’s surface

Galileo used an Galileo used an inclined planeinclined plane to to lessen the earth’s pull on objectslessen the earth’s pull on objects

This allowed Galileo to do his This allowed Galileo to do his experiments without extremely experiments without extremely accurate time pieces.accurate time pieces.

Prior to Galileo’s experiments with Prior to Galileo’s experiments with falling objects, most scientists falling objects, most scientists believed that heavier objects fell believed that heavier objects fell faster than lighter objects.faster than lighter objects.

Galileo’s ApparatusGalileo’s Apparatus

Galileo found Galileo found that objects fall that objects fall at 9.8 m/sat 9.8 m/s22

Every second Every second the velocity of a the velocity of a falling object falling object increases by increases by 9.8 m/s9.8 m/s

http://ircamera.as.arizona.edu

Gravitational AccelerationGravitational Acceleration All objects fall to the earth with the All objects fall to the earth with the

same acceleration. (ex. Feather and same acceleration. (ex. Feather and penny in tube)penny in tube)

This acceleration = 9.8 m/sThis acceleration = 9.8 m/s22

A falling object will increase its speed by 9.8 A falling object will increase its speed by 9.8 m/s every second it falls. (m/s every second it falls. (assuming no air assuming no air resistanceresistance))

1 g = 9.8 m/s1 g = 9.8 m/s22

You are experiencing 1 g “pull” towards earth You are experiencing 1 g “pull” towards earth as you sit in the classroomas you sit in the classroom

Isaac Newton (1642-1727)Isaac Newton (1642-1727)

Continued Galileo’s Continued Galileo’s study of motionstudy of motion

Studied motion while Studied motion while alone on his farm during alone on his farm during the plague while the plague while Cambridge was closed.Cambridge was closed.

http://content.answers.com

Isaac Newton discovered the Isaac Newton discovered the laws of motion and laws of motion and

expressed them mathematicallyexpressed them mathematically

Newton’s First Law of Motion states:Newton’s First Law of Motion states: Objects at rest tend to stay at rest unless Objects at rest tend to stay at rest unless

a net force is applied to them.a net force is applied to them.

Objects in motion tend to stay in motion Objects in motion tend to stay in motion and continue in a straight line unless a and continue in a straight line unless a force is applied to themforce is applied to them

Constant speed in a straight line is called Constant speed in a straight line is called Uniform MotionUniform Motion

Example of the first lawExample of the first law

Objects lying around don’t “walk away” on Objects lying around don’t “walk away” on their own. (i.e. blame your room-mates)their own. (i.e. blame your room-mates)

Without seatbelts, a person can get ejected Without seatbelts, a person can get ejected from a car that suddenly stops.from a car that suddenly stops.

http://youtube.com/watch?v=giYQE1Hskjc&mhttp://youtube.com/watch?v=giYQE1Hskjc&mode=related&searchode=related&search==

http://youtube.com/watch?v=xU2jrQ4uunU&fhttp://youtube.com/watch?v=xU2jrQ4uunU&feature=relatedeature=related

http://www.nhtsa.dot.gov

What is Inertia?What is Inertia?

InertiaInertia is the resistance an object has is the resistance an object has to a change in its state of motion to a change in its state of motion

InertiaInertia is the tendency of an object to is the tendency of an object to stay at rest or stay in motionstay at rest or stay in motion

Any motion that IS NOT Uniform Motion, is Any motion that IS NOT Uniform Motion, is called called AccelerationAcceleration

Prior to Newton, scientists thought that Prior to Newton, scientists thought that objects that moved in circles did not have objects that moved in circles did not have forces acting on them.forces acting on them.

Heavenly bodies (moon, planets) never Heavenly bodies (moon, planets) never stopped orbiting so people assumed the stopped orbiting so people assumed the steady state of the heavens implied that no steady state of the heavens implied that no forces acted on orbiting bodies seen in spaceforces acted on orbiting bodies seen in space

Newton and the Newton and the AppleApple

Newton did tend an apple orchardNewton did tend an apple orchard Claims he did have a breakthrough Claims he did have a breakthrough

moment while daydreaming theremoment while daydreaming there Newton saw the apple and moon togetherNewton saw the apple and moon together Newton makes the connection that the Newton makes the connection that the

moon is like a VERY large apple only moon is like a VERY large apple only farther awayfarther away

Realizes that the moon IS falling towards Realizes that the moon IS falling towards the earth but that its distance from the the earth but that its distance from the earth never changes due to the curvature earth never changes due to the curvature of the earthof the earth

Satellite MotionSatellite Motion

http://www.edumedia-sciences.chttp://www.edumedia-sciences.com/a271_l2-satellite-motion.htmom/a271_l2-satellite-motion.htmll

Newton’s Laws of Motion are Newton’s Laws of Motion are UniversalUniversal

Newton realizes that laws of Newton realizes that laws of motion that describe motion on motion that describe motion on earth should be universal and earth should be universal and apply to motion of bodies in the apply to motion of bodies in the universe.universe.

Newton’s 2Newton’s 2ndnd Law of Motion states: Law of Motion states:

Force on an object is the object’sForce on an object is the object’s

Mass times Acceleration.Mass times Acceleration.

F = maF = ma

Examples of the 2Examples of the 2ndnd Law Law

More massive objects weigh moreMore massive objects weigh more

F = mgF = mg It is harder to throw a bowling ball It is harder to throw a bowling ball

than a baseballthan a baseball I’d rather be hit by an acorn than a I’d rather be hit by an acorn than a

big green pine cone going the same big green pine cone going the same speedspeed

F = maF = ma

If there is a force on you right If there is a force on you right now, then why are you not now, then why are you not

accelerating?accelerating?

A force does not always produce A force does not always produce acceleration, but if all the forces acceleration, but if all the forces added together do not equal ZERO, added together do not equal ZERO, then the net force will produce then the net force will produce accelerationacceleration

This leads us to the Newton’s 3This leads us to the Newton’s 3rdrd Law Law

Newton’s Third Law of Motion states:Newton’s Third Law of Motion states:

For Every force there is an equal but For Every force there is an equal but opposite force.opposite force.

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Where’s the equal Where’s the equal but opposite but opposite force?force?

The earth is also The earth is also accelerating accelerating towards the towards the skydiver but the skydiver but the acceleration is acceleration is very, very small.very, very small.

www.blackfive.net

Forces always come in pairsForces always come in pairs

Examples:Examples: Sitting on a chairSitting on a chair SwimmingSwimming A car hitting a metal guard railA car hitting a metal guard rail Two cars collidingTwo cars colliding If you want to determine the If you want to determine the

force force of a crash, you can either of a crash, you can either inspect inspect damage to the car or damage to the car or inspect inspect damage to what the car hit.damage to what the car hit.

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

called called gravitygravity

Newton’s Law of Newton’s Law of Universal Universal GravitationGravitation

Force = G x mForce = G x m11 x m x m22 / d / d22

G is a universal constantG is a universal constant

M1M2

d

Cavendish devises an experiment that allows him to find G

What is a potentially What is a potentially dangerous acceleration?dangerous acceleration?

Slamming on the breaksSlamming on the breaks Hitting a brick wallHitting a brick wall If a force of 4 to 6 g is sustained for If a force of 4 to 6 g is sustained for

more than a few seconds, the more than a few seconds, the resulting symptoms range from visual resulting symptoms range from visual impairment to total blackout. impairment to total blackout.

Crash victims sustain greater g forces Crash victims sustain greater g forces but experience them for less than 150 but experience them for less than 150 msms

Funk & Wagnall's Encyclopedia.

““The acceleration during the crash The acceleration during the crash that killed Diana, Princess of Wales, in that killed Diana, Princess of Wales, in 1997 was estimated to have been on 1997 was estimated to have been on the order of 70 to 100 g, which was the order of 70 to 100 g, which was intense enough to tear the pulmonary intense enough to tear the pulmonary artery from her heart -- an injury that artery from her heart -- an injury that is nearly impossible to survive. Had is nearly impossible to survive. Had she been wearing a seat belt, the she been wearing a seat belt, the acceleration would have been acceleration would have been something more like 30 or 35 g - something more like 30 or 35 g - enough to break a rib or two, but not enough to break a rib or two, but not nearly enough to kill most people.”nearly enough to kill most people.”

http://hypertextbook.com/physics/http://hypertextbook.com/physics/

Automotive Acceleration (g)

event typical carsports 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 Technologies used to minimize dangerous accelerationsdangerous accelerations

Seatbelts and the webbingSeatbelts and the webbing Air bagsAir bags The Crumple zoneThe Crumple zone Deformable dashboard and steering Deformable dashboard and steering

wheelwheel Hydraulic brakesHydraulic brakes Anti-lock brakesAnti-lock brakes

Seatbelts do three things:Seatbelts do three things:

Apply forces to the parts of the body that Apply forces to the parts of the body that are tough (rib cage and pelvis)are tough (rib cage and pelvis)

Prevent the human from impacting rigid Prevent the human from impacting rigid objects.objects.

Reduce the acceleration on the body by Reduce the acceleration on the body by restraining the body continuously restraining the body continuously throughout the crash.throughout the crash.

Energy = F x distanceEnergy = F x distance

By increasing the stopping distance of the By increasing the stopping distance of the human the forces are lowered.human the forces are lowered.

Inertia triggered retractorsInertia triggered retractors

Pawl Ratchet Gear

Belt Triggered Retractor – page Belt Triggered Retractor – page 7575

Seatbelts typically lock up around ½ g

Clutch

Toothed Plate

Clutch – a mechanism for transmitting Clutch – a mechanism for transmitting rotation, which can be engaged or rotation, which can be engaged or disengaged disengaged (Wikipedia) (Wikipedia)

Ratchet – a device that (when engaged) Ratchet – a device that (when engaged) allows for linear or rotational motion in allows for linear or rotational motion in one direction onlyone direction only

Pawl – a piece with a pointed end that Pawl – a piece with a pointed end that engages with the ratchet and locks itengages with the ratchet and locks it

Determine type of seatbeltDetermine type of seatbeltgiven in classgiven in class

Triggered by yanking on the belt or by Triggered by yanking on the belt or by changes to acceleration?changes to acceleration?

Locate the pawl, ratchet, toothed plate, Locate the pawl, ratchet, toothed plate, and any other important component.and any other important component.

Describe in two-three sentences what Describe in two-three sentences what happens when the car undergoes high happens when the car undergoes high negative acceleration.negative acceleration.

In the event of a crash, a pretensioner will tighten In the event of a crash, a pretensioner will tighten the belt almost instantaneously. Like the belt almost instantaneously. Like airbagsairbags, , pretensioners are triggered by sensors in the pretensioners are triggered by sensors in the car's body, and most pretensioners use car's body, and most pretensioners use explosively expanding gas to drive a piston that explosively expanding gas to drive a piston that retracts the belt. retracts the belt. WikipediaWikipedia

Gas is ignited here

www.abetterwindshield.com

Airbags Airbags inflate and inflate and apply forces apply forces evenlyevenly to the to the windshield, windshield, dash, and dash, and occupant occupant over a time over a time period of period of about 100 msabout 100 ms

What makes an airbag What makes an airbag inflate?inflate?

The accelerometer is builtinto a microchip. DuringLarge decelerations, the massof the accelerometer shifts.This closes an electrical contacttriggering the bag to inflate.

Sodium Azide (NaN3) reacts with Potassium Nitrate (KNO3) to produce nitrogen gas The bag inflates in 40 msThe bag has tiny holes in it allowing the gas to escape so that the bag absorbs energy.

Is an air bag dangerous?Is an air bag dangerous?

Inflating in 40 ms implies that the Inflating in 40 ms implies that the bag is actually exploding.bag is actually exploding.

Occupants should be >10 inches Occupants should be >10 inches away from the steering wheelaway from the steering wheel

If the occupant hits the air bag If the occupant hits the air bag before it is fully inflated injury can before it is fully inflated injury can occur. When the bag inflates it does occur. When the bag inflates it does so at 100 mph.so at 100 mph.

How effective are air bags?How effective are air bags?

Seatbelts are 42% effective at Seatbelts are 42% effective at preventing driver fatalities.preventing driver fatalities.

Seatbelts with air bags are 49% Seatbelts with air bags are 49% effective at preventing driver effective at preventing driver fatalities.fatalities.

Airbags reduce the risk of death by only Airbags reduce the risk of death by only 7%7%

246 people died from air bags from 246 people died from air bags from 1986-2001 (75% were women)1986-2001 (75% were women)

7,000 people were saved from air bags 7,000 people were saved from air bags during the same 15 year periodduring the same 15 year period

11,000 people are saved 11,000 people are saved annuallyannually due due to seat beltsto seat belts

People less than 5’3” tall are more People less than 5’3” tall are more harmed with an air bag than without it harmed with an air bag than without it during a crash because the seat is too during a crash because the seat is too close to the steering wheel.close to the steering wheel.

http://www.youtube.com/watch?v=-lHI5BwFl_w&NR=http://www.youtube.com/watch?v=-lHI5BwFl_w&NR=11

Mercury Sensorhttp://www.autoshop101.com

SensorsSensors

The role of deformable The role of deformable materialsmaterials

Reduce accelerations on a human’s Reduce accelerations on a human’s bodybody

Increase the stopping time and distanceIncrease the stopping time and distance E = Force * distance (increase distance E = Force * distance (increase distance

so that the energy is absorbed at a so that the energy is absorbed at a lower force)lower force)

Less bouncing – multiple hard hits are Less bouncing – multiple hard hits are more damaging that one force applied more damaging that one force applied more continuouslymore continuously

Crumple Zone absorbs Crumple Zone absorbs energyenergy

KE = ½ mvKE = ½ mv22

Kinetic Energy needs to be absorbed Kinetic Energy needs to be absorbed quickly in an area outside of the quickly in an area outside of the driving compartmentdriving compartment

Crumple ZoneCrumple Zone

http://www.aip.org/dbis/stories/2004/14124.html

Anti-lock Brakes (ABS)Anti-lock Brakes (ABS)

Safer only when used properly.Safer only when used properly. Accident data does not show fewer Accident data does not show fewer

fatalities as a result of having ABSfatalities as a result of having ABS ABS functions only when brakes are not ABS functions only when brakes are not

pumped. ABS creates pedal chatter which pumped. ABS creates pedal chatter which means ABS means ABS is workingis working..

ABS doesn’t mean you will stop quicker. ABS doesn’t mean you will stop quicker. However control of the car is maintained, However control of the car is maintained, because skidding is avoided.because skidding is avoided.

How does an How does an Anti-lock braking systems work?Anti-lock braking systems work?

ABS keeps the tires from skidding as ABS keeps the tires from skidding as long as the driver keeps the brake pedal long as the driver keeps the brake pedal down.down.

When slamming on the brakes, tires can When slamming on the brakes, tires can stop spinning much faster than a car stop spinning much faster than a car can stop. This is called “lock up”. When can stop. This is called “lock up”. When this happens the car must now skid this happens the car must now skid because the wheels are not turning.because the wheels are not turning.

Static (rolling) friction Static (rolling) friction vs. Kinetic (sliding) frictionvs. Kinetic (sliding) friction

Kinetic (sliding) friction is much less Kinetic (sliding) friction is much less than static (rolling) friction.than static (rolling) friction.

Once a car skids, the tires lose grip Once a car skids, the tires lose grip with the road and the driver loses with the road and the driver loses control.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.

http://hyperphysics.phy-astr.gsu.edu

Kinetic FrictionStatic Friction

FStatic > FKinetic

is the friction coefficient is between 0 and 1

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.

FStatic > FKinetic

For this reason, we DO NOT want to skid if For this reason, we DO NOT want to skid if we want to maintain traction and control we want to maintain traction and control of the car.of the car.

When stopping, we want to the tire to When stopping, we want to the tire to “stick” to the road so that we can use the “stick” to the road so that we can use the contact forces to stop the car. contact forces to stop the car.

Static friction is higher than sliding friction, Static friction is higher than sliding friction, therefore for quick stopping we do not therefore for quick stopping we do not want to skid.want to skid.

The ABS controller The ABS controller (computer)(computer)

Ensures that the tire slows down at the Ensures that the tire slows down at the same rate as the car, but keeps the tires same rate as the car, but keeps the tires forces very near the point where sliding forces very near the point where sliding would occur – just under Fwould occur – just under Fstaticstatic

This maximizes the braking power.This maximizes the braking power. Friction is at it’s highest right Friction is at it’s highest right beforebefore lock- lock-

upup

http://www.youtube.com/watch?v=uq4DDMMoomU

Demo of conventional brakes vs. ABS:

How is this done?How is this done?

The system uses a computer to monitor The system uses a computer to monitor the speed of each wheel. When it the speed of each wheel. When it detects that one or more wheels are detects that one or more wheels are turning slower than the remaining turning slower than the remaining wheels, the computer sends a signal to wheels, the computer sends a signal to momentarily remove and reapply the momentarily remove and reapply the pressure to the affected wheels to allow pressure to the affected wheels to allow them to continue turning. This them to continue turning. This "pumping" of the brakes occurs at ten or "pumping" of the brakes occurs at ten or more times a second.  more times a second. 

http://www.familycar.com/brakes.htm

The system consists of an electronic The system consists of an electronic control unit, a hydraulic actuatorcontrol unit, a hydraulic actuator**,  ,  and wheel speed sensors at each and wheel speed sensors at each wheel.  wheel. 

* 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.

http://www.aa1car.com/library/abs1.htm

Information from sensors is fed to the controller.

If the controller determines that one tire is If the controller determines that one tire is decelerating more than the other tires, the decelerating more than the other tires, the

controller sends a signal to the wheel’s controller sends a signal to the wheel’s actuator to lower the pressure to prevent actuator to lower the pressure to prevent

skidding.skidding.

www.lake-link.com

Solenoid valve closes,preventing more fluidfrom getting to the brake.It then is reopened, thuspumping the brake.

SourcesSources

http://auto.howstuffworks.com/seatbelt.http://auto.howstuffworks.com/seatbelt.htmhtm

http://auto.howstuffworks.com/airbag.hthttp://auto.howstuffworks.com/airbag.htmm