Surviving Collisions
Car manufacturers spend enormous
amounts of money and time to
ensure that, in the event of a
collision, the occupants of the car
have the best chance possible of
survival.
Why?
The European New Car Assessment Programme (NCAP) tests
new cars and gives a safety rating for adult occupants,
children and pedestrians.
Results are published online.
NCAP also publishes information on the protection
for occupants offered by each new vehicle in the
event of a rear impact collision, which commonly
results in whiplash injuries.
Explain whiplash injuries in terms of Newton’s laws.
Car design engineers require a good understanding
of the physics of collisions to be able to design cars
that offer maximum protection to occupants and
pedestrians in the event of collision.
Which features of car design protect occupants and
pedestrians in the event of a collision?
Your research project is to design a
crumple zone for our remotecontrolled car and provide
evidence that your crumple zone
design is the most effective
protection in the event of a headon collision.
You will research the physics of car
safety, plan and carry out
appropriate investigative work and
prepare a scientific communication
presenting the aim, results and
conclusion of your work.
Through this research project you
will develop skills in:
researching
investigating
analysing
evaluating
communicating
Failure could have serious consequences…
Don’t Mess with Impulse
Why are crash barriers constructed on the central
reservation of motorways and some dual
carriageways?
Why are cars designed with crumple zones?
How are crumple zones designed?
What materials are used for effective crumple zones?
How can a comparison of the effectiveness of different
crumple zone materials be made?
The research
Within your group you will each research
individually the four key questions.
As you review information, make notes and
ensure you record full references.
To be successful in this work you must
demonstrate that you can evaluate materials on
the internet for reliability and can summarise
and explain information in your own words.
Acting on impulse?
When you jump, what’s the least
painful way to land?
Explain,
using your
research.
If you fall off something, what sort
of surface would you prefer to land
on? Explain!
The greater the force and the
longer the time for which it acts,
the greater its effect.
So the relationship F ×
important one.
t is an
Impulse = F × t
Impulse = Ft
What are the units of
impulse?
Impulse and momentum
In the clip, the mass of the ball is m.
During the collision, the average force
exerted is F or F .
Impulse and momentum
The velocity of the ball before the
collision is u (in this case u = 0 m s–1)
Immediately after the collision, the
velocity is v.
The force applied to the ball causes an
acceleration a.
Impulse and momentum
F  ma
v u
a
t
Impulse and momentum
so
v u 
F  m

 t 
Mulitply each side by t
Ft  m(v – u )
Ft  mv – mu
Impulse and momentum
mv – mu = change in momentum
(measured in kg m s–1)
impulse Ft = mv – mu = Δmv
Impulse and momentum
impulse Ft = mv – mu = Δmv
Impulse = change in momentum
caused by a force
Impulse and golf practical
diameter of ball =
mass of ball =
time to pass through light gate =
time of contact =
initial speed of ball =
final speed of ball =
Impulse and golf practical
Calculate the average force
exerted on the golf ball:
Ft = mv – mu
Impulse and golf practical
How could the force on the ball be
increased?
Or decreased?
What does this have to do with our
crumple zone investigation?
Look at each of the velocity–time
and acceleration–time graphs.
Describe the motion and decide
which graphs match the collision
without the crumple zone and
which match the collision with the
crumple zone.
What information are you looking
for to help you decide?
What does a greater acceleration
indicate about the unbalanced
force?
Without the crumple zone, the time taken for the vehicle to stop is 0.01 s.
The maximum acceleration is – 65.63 m s–2.
With crumple zone, the time taken to come to a stop is just under 0.03 s.
That is three times the time taken for it to stop without the crumple zone.
The maximum acceleration is – 42.77 m s–2.
Investigate and compare the
effectiveness of different crumple
zone materials.
With crumple zone, the time taken to come to a stop is just under 0.07 s.
Explain why the acceleration is much less than in the collision without the crash barrier.
Impulse and momentum
impulse Ft = mv – mu = Δmv
impulse = change in momentum
caused by a force
What happens when the ball hits the racquet?
To the racquet strings?
To the ball?
The force–time graph for a tennis
racquet hitting a ball might look
like this:
F (N)
0
t (s)
What information can be obtained
from the graph?
F (N)
0
t (s)
Investigating force–time graphs
Start data-logging.
Slightly push it
from rest so it moves
down the runway and
collides on the
force sensor.
Fix a small
spring on the
force sensor.
© PASCO
Use the velocity–time graph to determine
the velocity of the trolley before and after the
collision. Calculate the change in
momentum of the trolley.
Use the force–time graph to determine the
area under the graph.
Compare the area under the graph with the
calculated change in momentum of the
trolley.
Velocity
v changes when the trolley
collides on the force sensor
Force (N)
Time (s)
max. F
F
F
very short time interval
1.60 s
1.65 s
Why does the force vary during impact?
When a tennis ball is being hit by
a racket, the ball is deformed.
The force of impact increases to
maximum when the ball is deformed
most.
As the ball regains
its shape, the
force decreases.
For the same change in momentum,
what happens to the average force as
the time of impact decreases?
F (N)
The change in
momentum
remains the
same.
0
t (s)
What does this
mean for the
area under the
graph?
F (N)
On the axes,
sketch the graph
for a shorter
time of impact.
0
t (s)
What happens
to the average
force?
Which ball would you prefer to be
dropped on your foot and why?
Use the words force, time, change in
momentum and impulse in your
explanation!
A trolley hits and rebounds from a
force sensor and the F–t graph is
obtained.
Force (N)
The area under graph is 0.46 N s.
8
7
6
5
4
3
2
1
1.75
1.80
1.85
1.90
Time (s)
1.95
2.00
What is the maximum force
acting on the sensor during
Force (N)
impact?
Area under the
graph = 0.46 N s
8
7
6
5
4
3
2
1
1.75
1.80
1.85
1.90
Time (s)
1.95
2.00
Force (N)
Maximum force = 7.8 N
8
7
6
5
4
3
2
1
Area under the
graph = 0.46 N s
7.8 N
1.75
1.80
1.85
1.90
Time (s)
1.95
2.00
Force (N)
Determine the change in momentum of the trolley.
Area under the
graph = 0.46 N s
8
7
6
5
4
3
2
1
1.75
1.80
1.85
1.90
Time (s)
Area under the graph = 0.46 N s
1.95
Obtained by
2.00
data-logging
program
Force (N)
Determine the change in momentum of the trolley.
Area under the
graph = 0.46 N s
8
7
6
5
4
3
2
1
1.75
1.80
1.85
1.90
Time (s)
1.95
obtained by
2.00
data-logging
program
Area under the graph = 0.46 N s
Change in momentum = 0.46 kg m s–1
Force (N)
Calculate the average force acting during impact.
8
7
6
5
4
3
2
1
Time of impact = 2.05 – 1.75 = 0.3 s
1.75
1.80
1.85
1.90
Time (s)
Average
 in momentum
=
force
time
1.95
0.46
=
0.3
2.00
= 1.53 N
324 m
A careless
school pupil
drops
a 1p coin
from his
pocket at
the top of
the Eiffel
Tower.
324 m
Calculate
the velocity
of the coin
on impact
on the
ground.
324 m
Calculate the
change in
momentum of
the coin,
making an
estimate of
the coin’s
mass and the
rebound
velocity.
324 m
Calculate the
average force
exerted by the
coin on
impact,
estimating the
time of
impact.
In this section you should have developed…
• an understanding of the concept of impulse as
a change in momentum, as it applies in sports
and vehicle safety
• skills in researching information on the
internet, evaluating its quality and referencing
appropriately
• independent investigative skills
• a greater understanding of graphs,
interpreting to understand and describe
motion.