CARSand Safety Features

Inertia Inertia

To be able to:

All Most Some

Describe what inertia is

(MYP 2/3)

Explain how inertia works

(MYP 3/4)

Describe how car manufactures have tried to over come inertia

(MYP 6/6)

Criteria A,C

Newton’s First Law of Newton’s First Law of MotionMotion

An object at rest will remain at rest unless acted on by an unbalanced force. An object in

motion continues in motion with the same speed and in the same direction unless acted

upon by an unbalanced force.

This law is often called "the law of inertia". "the law of inertia".

Newton’s Second Law of Newton’s Second Law of MotionMotion

Acceleration is produced when a force acts on a mass. The greater the mass (of the object being

accelerated) the greater the amount of force needed (to accelerate the object).

Force (N) = mass (kg) x acceleration (m/s/s)

The bigger the force, the …higher/lower…. the acceleration

The bigger the mass, the …higher/lower…. the acceleration

Newton’s Second Law of Newton’s Second Law of MotionMotion

Rearrangements of formulaRearrangements of formula

mass

acceleration

m a

F

a = F m

m = F a

F = mam = F

aa = F m

F = ma

5

How much force is needed to accelerate a 70kg

rider and her 200kg motorcycle at 4 m/s/s?

m = (200 + 70) kg

a = 4 m/s/s

F = ?

F = m a

= 270 x 4

= 1080 N

Which equation?

7

Problem SolvingProblem Solving1. A sports car and a furniture van are both travelling at a speed

of 60km/h. Which vehicle would require more force to stop it? Explain.

2. What would be the acceleration of 85kg wagon with a force of 382.5N?

3. a) What force is needed to accelerate an empty 1000kg car at 3 m/s/s.

b) The same force is then applied to the same car with a student inside. If the resultant acceleration is 2.8m/s/s, what is the mass of the student?

6

1. The furniture van would require more force to stop because its mass is greater

7 ANSWERSANSWERS

3. What would be the acceleration of 85kg wagon with a force of 382.5N?

a = F m

= 382.585

= 4.5 m/s/s

Which equation?

7

4. What force is needed to accelerate an empty 1000 kg car at 3 m/s/s. The same force is then applied to the same car with a student inside. If the resultant acceleration is 2.8m/s/s, what is the mass of the student?

F = m a = 1000 x 3 = 3000 N

m = F a = 3000 2.8 = 1071.4 kg

student’s mass = (1071.4 – 1000) = 71.4kg

Energy loss in collisionsEnergy loss in collisions

We’ve also said that in a collision momentum is conserved (unless an external force acts). The same cannot usually be said for

kinetic energy…

For example, consider the following collision. How much kinetic energy is lost?

Kinetic energy = ½ x mass x velocity squared

in J in kg in m/s

In the “Forces” module we looked at how to calculate an object’s kinetic energy:

Energy loss in collisionsEnergy loss in collisions

Mass = 1000kgMass = 1000kg Mass = 800kgMass = 800kg

Speed = 50m/sSpeed = 50m/s Speed = 20m/sSpeed = 20m/s

Mass = 1000kgMass = 1000kg Mass = 800kgMass = 800kg

Speed = 20m/sSpeed = 20m/s Speed = 30m/sSpeed = 30m/s

Before

After

Energy loss in collisionsEnergy loss in collisionsConsider a head-on collision where the cars stick together. How much kinetic

energy is lost in this example? Where does all the energy go?

In this example more kinetic energy was lost. We say it was a “less elastic collision”. An “elastic collision” is one where the kinetic energy is conserved.

Speed = 50m/s Speed = 30m/s

Speed = 10m/s

Before

After

Car safety Features Car safety Features

To be able to:

All Most Some

Describe what features have to improve safety

(MYP 2/3)

Explain how each feature works

(MYP 3/4)

Describe how car manufactures have improved safety features over time

(MYP 6/6)

Criteria A,C

NEW

vs

OLD

Car Safety Car Safety FeaturesFeatures

The greater the speed, the longer it will take to decelerate in a collision, leading to a greater force and greater damage and injury.

Car Safety FeaturesCar Safety FeaturesSeatbeltsAir bags

Head restraintsCrumple Zones

Collapsible Steering ColumnPadded dashboards

Collapsible bumper bar

Car FeaturesCar Features• ABS• Crumple zones• Seatbelt• Air bags

Air Bags Air Bags Airbags are a flexible envelope that is designed to

inflate rapidly in the advent of an accident.

They are triggered automatically if the car suffers a sudden impact, and once inflated they provide

cushioning for occupants.

This is meant to limit and even prevent the kinds of major trauma occupants of cars can receive in a

crash.

The airbag is connected to a range of sensors in the car which measure things like acceleration,

impact, wheel speed etc.

When an impact is sensed by these, the airbag is inflated by a gas propellant that inflates a nylon

bag.

The actual process occurs in a fraction of a second so that a cushion appears in front of or beside an

occupant pretty much instantaneously.

Crumple Zones Crumple Zones The crumple zone of the automobile is the thing

that keeps the passengers safe inside the passenger compartment of the automobile,

because around your car, the fenders and certain portions of the car have been designed by

automobile manufactures to crumple, to collapse, to absorb the crash forces during the collision.

The car won't make out so well, but you'll survive the crash.

Crumple Zones Crumple Zones

Seat Belts Seat Belts The job of the seatbelt is to hold the passenger in place so the passenger is almost part of the car

which prevents the passenger from flying forward as the car stops abruptly in the case of a collision.

When a car stops suddenly due to a collision with another object the car's acceleration decreases

very quickly in a short period of time.

This is called deceleration. deceleration.

As the car collides with another object, the other object provides the force which changes

the speed and direction.

The car stops going in the direction it was going in, and in some cases bounces back depending how hard of a force hits it or how

much momentum the car had.

Also, the speed decelerates quickly due to the impact.

When all this happens the passenger is not being acted upon by a force to slow them

down.

As the person continues in their same direction and speed ( forward and the same speed that the car was going) the seatbelt catches them, holding them back from flying through the air.

The alternative is to not wear a seatbelt, but a force will still have to act on the person in order

to slow them down.

This force will come from the dashboard or windshield as the person crashes into it causing a lot of damage to themselves.

Hydraulic systems use the principle that pressure is transmitted throughout a liquid.

HydraulicsHydraulics

They are used to transfer movement from one part of a machine to another without linking the

parts mechanically.

All hydraulic systems use two pistons linked via a pipe which carries special oil called hydraulic

fluid.

HydraulicsHydraulics

Pressure inside all parts of the hydraulic system is the same

Force applied

here

Force transferred

here

All hydraulic brake systems (e.g. in a car) use a small master piston and a bigger slave piston.

The master piston is used to apply a force. This puts the liquid under pressure. The pressure is transmitted to the pistons on all

four wheels of the car.

Hydraulic Brake Hydraulic Brake

foot pedal

master piston

hydraulic fluid

slave pistons

The pressure exerted by the master piston on the hydraulic fluid can be calculated using this equation:

pressure = force applied

area of master piston

Hydraulic Brake – Pressure Hydraulic Brake – Pressure Equations Equations

Hydraulic Brake – Pressure Hydraulic Brake – Pressure Equations Equations

The slave piston has a larger area than the master piston, so the force exerted by the slave pistons is greater than the

force exerted by the driver on the brake pedal.

The pressure is transmitted to the slave pistons, so the force exerted by the slave piston can be calculated using:

pressure = force exerted

area of slave piston

force exerted = pressure × area of slave piston

The master piston of a car has an area of 5 cm2.

Hydraulic Brake – Hydraulic Brake – calculationscalculations1. If a force of 10 N is

applied to the master piston, calculate the pressure created in the brake pipes.

2. If the slave piston (the brake pads) has an area of 50 cm2, calculate the force exerted on the brake disc.

brake pedal

axle

master piston

brake discslave piston

wheel

Hydraulic brake – Hydraulic brake – AnswersAnswers

2. At the slave piston, f = p × a

= 2 N/cm2 × 50 cm2 = 100100 NN

So, the force exerted on the brake disc is ten times greater than the original force applied to

the master piston.

Calculations:

1.At the master piston, p = 10 N

= 22 N/cmN/cm22

Brake pads

Brake discs

5 cm2

Hydraulic car brake – labelling the Hydraulic car brake – labelling the partsparts

ABS- Antilock BrakesABS- Antilock BrakesMovement of a vehicle when the wheels are

locked or stationary is known as skidding and this is what anti lock brakes is supposed to prevent.

ABS monitors the rotation of each wheel during braking and any wheel that locks or is about to

lock the system reduces the braking force and re-applies it at a rate of several times per

second.

ABS- Antilock BrakesABS- Antilock BrakesAnti lock brakes allows you to steer around obstacles if

you can't stop in time as a skidding vehicle can't be properly steered.

ABS also allows you to have more control of your vehicle in adverse conditions such as ice, mud and gravel. It also prevents excessive tyre damage as skidding damages your tires and if you apply the

brakes hard constantly and skid a lot it can severely reduce the lifespan of your tyres.

ABS- Antilock BrakesABS- Antilock Brakes