Transcript Car Safety Features - SASTA-cars

CARS
and Safety Features
Inertia
To be able to:
All
Most
Some
Describe what inertia is Explain how inertia Describe how car
works
manufactures have
tried to over come
(MYP 2/3)
inertia
(MYP 3/4)
Criteria
A,C
(MYP 6/6)
Newton’s First Law of Motion
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".
Newton’s Second Law of
Motion
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).
Newton’s Second Law of
Motion
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
Rearrangements of formula
5
F = ma
mass
m=F
a
acceleration
a=F
m
F = ma
F
m
a
m=F
a
a=F
m
7
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 =?
Which equation?
F=ma
= 270 x 4
= 1080 N
6
Problem Solving
1.
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?
7
ANSWERS
1. The furniture van would require more
force to stop because its mass is greater
7
3. What would be the acceleration of 85kg wagon
with a force of 382.5N?
Which equation?
a=F
m
= 382.5
85
= 4.5 m/s/s
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=ma
= 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 collisions
In the “Forces” module we looked at how to calculate an object’s
kinetic energy:
Kinetic energy = ½ x mass x velocity squared
in J
in kg
in m/s
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?
Energy loss in collisions
Before
Speed = 50m/s
Speed = 20m/s
Mass = 1000kg
Mass = 800kg
After
Speed = 20m/s
Mass = 1000kg
Speed = 30m/s
Mass = 800kg
Energy loss in collisions
Consider a head-on collision where the cars stick together. How much kinetic
energy is lost in this example? Where does all the energy go?
Before
Speed = 50m/s
Speed = 30m/s
After
Speed = 10m/s
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.
Car safety Features
To be able to:
All
Most
Some
Describe what features
have to improve
safety
Explain how each
feature works
(MYP 2/3)
(MYP 3/4)
Describe how car
manufactures have
improved safety
features over time
(MYP 6/6)
Criteria
A,C
NEW
vs
OLD
Car Safety Features
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 Features
Seatbelts
Air bags
Head restraints
Crumple Zones
Collapsible Steering Column
Padded dashboards
Collapsible bumper bar
Car Features
• ABS
• Crumple zones
• Seatbelt
• 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
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
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.
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.
Hydraulics
Hydraulic systems use the principle that
pressure is transmitted throughout a liquid.
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.
Hydraulics
Force
applied
here
Force
transferred
here
Pressure inside all parts of the
hydraulic system is the same
Hydraulic Brake
All hydraulic brake systems (e.g. in a car) use a small
master piston and a bigger slave piston.
hydraulic
fluid
slave
pistons
foot
pedal
master
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 – Pressure
Equations
The pressure exerted by the master piston on the
hydraulic fluid can be calculated using this equation:
force applied
pressure =
area of master piston
Hydraulic Brake – Pressure
Equations
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 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.
Hydraulic Brake – calculations
The master piston of a car has an area of 5 cm2.
1. If a force of 10 N is
applied to the master
piston, calculate the
pressure created in
the brake pipes.
brake pedal
wheel
2. If the slave piston (the
brake pads) has an
area of 50 cm2,
calculate the force
exerted on the brake
disc.
master piston
axle
slave piston brake disc
Hydraulic brake – Answers
Calculations:
1. At the master piston,
p = 10 N
5 cm2
Brake pads
= 2 N/cm2
2. At the slave piston,
f=p×a
= 2 N/cm2 × 50 cm2
= 100 N
So, the force exerted on the
brake disc is ten times greater
than the original force applied to
the master piston.
Brake
discs
Hydraulic car brake – labelling the parts
ABS- Antilock Brakes
Movement 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 Brakes
Anti 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 Brakes