THE PHYSICS
OF A CAR CRASH
Sam Ventresca
Carla Marano
Scenario
• A moving car rams into a stationary car. We
will analyze the crash in the following
chronological steps:
1. Moving car applies its brake.
2. Moving car’s velocity is too great and it hits
the still car, creating an inelastic collision.
3. Passenger of still car experiences whiplash
and then comes to rest.
1. Moving Car Applies its Brake
• The brake pedal in a car is connected to a
lever
– When you push down on the brake pedal (apply
force), the lever multiplies this force
– This multiplication is necessary since your foot/leg
would not be able to provide a great enough force
to slow or stop the car
– See illustration on next slide
Braking Lever
• The lever has a fulcrum that is placed at two-thirds the
distance of the lever towards the right side. This means
that the length of the lever to the right of the fulcrum is
half that of the left.
• When force is applied on either side, the multiplier is
either double or half of the applied force since it must
correspond to lever length and be equal on both sides
Brake Pedal and Force
• The physics concept that applies here is the
impulse equation: F*Δt = m*Δv.
– If a car with mass m changes its velocity, v, in a
certain span of time, t, then an applied force must
have been used to assist this change. So, the mass
multiplied by the change in the velocity will be
equal to the elapsed time multiplied by the force
2. Moving Car Strikes Stationary Car to
Create an Inelastic Collision
• An inelastic collision is a collision of two objects
in which energy is not fully conserved due to the
transfer of kinetic energy to other forms outside
of the two-car-system
• These forms can consist of things like heat and
sound
• In the scenario of the car crash, the sound of the
crash and the release of heat to the road or
surrounding air are examples of lost kinetic
energy
2. Moving Car Strikes Stationary Car to
Create an Inelastic Collision (Con’.t)
• In an ideal collision, all kinetic energy remains
within the system of the two cars
• This is referred to as an elastic collision, in which
all energy is conserved
• In real life, however, some of the kinetic energy
will be lost to things like the sound of the cars
striking
• Regardless, momentum is always conserved!
m1v1i + m2v2i = m1v1f + m2v2f
3. Passenger of Still Car Experiences
Whiplash and then Comes to Rest.
• The velocity of the moving car will be transferred
proportionally to the combined mass of the two
cars into a new, lesser final velocity
• The passenger’s body will act according to
Newton’s first law of motion, which states that
the velocity of a body remains constant unless
the body is acted upon by an external force
• Also known as inertia, this means that the
passenger will stay at rest while his car moves
forward
• Only the normal force of the seat will propel him
forward with his car
3. Passenger of Still Car Experiences
Whiplash and then Comes to Rest.
• In shock, the muscles in his neck will tighten and strain
• Without a headrest, his body would be pushed forward
while his head and neck stayed at rest
• Thus, his head would be bent far back, behind his seat,
injuring his neck muscles and head
• This is referred to as whiplash!
• After the car comes to a stop, this process will start
again, but in reverse
– Instead, his head will want to continue at this velocity
going backward, but the car will have stopped moving at
this velocity already
• Thus, his head would snap forward, injuring the neck
again
Insight
• This project allowed us to take a deeper look into the
physics behind certain aspects of car/car parts, as well
as what exactly goes on during a car crash
• The lever application of car brakes was particularly new
to us, as well as the scientific description of whiplash,
which can be FATAL!
• As a result, the workings of car brakes are more clear,
and we are able to see the necessity for headrests
• We also unintentionally dispelled the misconception
that headrests keep your head stationary during a
collision; they actually push your head forward so that
it stays at the same velocity as the car
DRIVE SAFELY!
Works Cited
• http://www.cliffprosser.com/Crash_test_3.JPG
• http://auto.howstuffworks.com/autoparts/brakes/brake-types/brake1.htm
• http://www.whatagreatsmile.com/images/whipla
sh1.jpg
• http://www.clubs.psu.edu/up/whiplash/Pictures/
Whiplash%20Photo%202009/rachel2.JPG
• http://nerdapproved.com/wpcontent/uploads/2010/04/iron-man-2-whiplashfigure.jpg?cb5e28
• http://csep10.phys.utk.edu/astr161/lect/history/
newton3laws.html