Background information

Moving objects have momentum . Newton's First Law of Motion says that unless an outside force acts on an object, the object will continue to move at its present speed and direction.[1]
Automobiles consist of several objects, including the vehicle itself, the passengers inside and any other loose objects in the vehicle.[2]
Unless the objects inside the car are restrained they will continue moving at whatever speed the car is traveling even if the car is stopped by a crash.
Changing or stopping an object's momentum requires a force acting over a period of time. If momentum changes instantly, as in a car crash, the force is very, very great! If the momentum can be changed over a period of time, even a fraction of a second, much less force needs to be applied with less damage or injury.[3]
In a head-on collision, if a passenger flies into the dashboard of a car, their momentum is instantly stopped, and serious injury is often the result.[4]
If the passenger is restrained by a seat-belt, their momentum is reduced more gradually by the constant and smaller force of the belt acting over a longer period of time. Seat-belts can reduce the impact of a passenger to one-fifth of the impact suffered by the body of the car.

Seat Belts Affect on Force.....

Did you know that over 40,000 people die each year in car accidents?

Seat belts can prevent death in about half of these accidents. If you know this and are still not wearing a safety belt,

you may need to ask yourself why not. Imagine running as

fast as you can into a wall. You would most likely expect to get pretty well
injured. Do you think you could stop yourself if the wall suddenly softened up[5]
when you were two feet away from it? This is exactly the situation you face
when the front of your car hits something at about twenty miles an hour. The
car stops in the first tenth of a second, but you keep accelerating at the same
rate you were going in the car until something stops you, this could be the steering wheel, dashboard or the windshield, if you are not wearing [6]
your seat belt at
Fifteen miles an hour you would seriously hurt yourself and anyone in the car,
but a thirty miles an hour when you crash it would be four times as bad as if you would at fifteen miles an hour. Or
imagine the impact you would endure from falling three stories.

There are a lot of different factors that come into effect during a car crash.

There are tons of things to take into consideration when looking into a car Seat belts by dorrianmace

Like it? Create your own at It's free and fun!

crash, but we’ll be focusing on momentum, force, inertia, and how the seat belt
affects the passengers of a car crash. All of these factors are considered in
Newton’s laws of motion. Newton’s first law states that an object in motion
stay in motion (everything in the car not that’s not attached to the vehicle.)
unless an equal and opposite force acts on it (This being the seat belt,
slowing the driver down.). A driver of a vehicle has many things against
him/her with this being said they need much more to make sure that they are
safe during a crash and/or an accident. One of the safety measures taken is the
use of the seat belt itself. The task of the seat belt is to stop you with the
car so that your stopping distance is greater than if you had no seat belt.

A crash which stops the car and driver must take away all its kinetic energy.

this shows that a longer stopping distance decreases the impact force. A largecarcr6.gif
amount of stretch in the seat belt will reduce the average impact force. The[7]
best help a driver has is his or her seat belt. Another important thing about
the safety of a seat belt is that it helps restrain the person in the seat.
Seat belts can reduce the impact of a passenger to about a fifth of the impact
suffered by the body of the car. If the seat belt does what it is made to do
then it would slow you down extremely as soon as the crash occurs. The more the
seat belt holds you back the less momentum you have after making contact in a
crash. With no seat belt to stop the driver with the car, the driver would fly
free until stopped suddenly by the impact on the steering column, windshield,

The stopping distance is about a fifth of that with a seat belt,
causing the average impact force to be about five times as great. The work done to stop the driver
is equal to the average impact force on the driver multiplied by the distance
traveled in stopping during the crash. A crash which stops the car and driver
must take away all the kinetic energy, and the work-energy principle The

change in the kinetic energy of an object is equal to the net work done on the object.[8]
For a straight-line collision, the net work done is equal to the average force
of impact times the distance traveled during the impact. Average impact force multiplied

by the distance traveled is equal to the change in kinetic energy then shows

that a shorter stopping distance increases the impact force. If the car was
going forty-five miles per hour everything inside the car is also going forty-five
miles per hour. This is due to the law of inertia or Newton's First law. Newton's

Law states that unless an outside force acts on the object, the object will
remain with its constant speed and direction.

In this case the object or objects in motion
are the car and the driver. The car is stopped by another car, its
breaks, a tree, etc. For the driver and/or passenger his/her stopping force
would be the force of the seat belt stopping the passenger from hitting the
windshield. Newton's
Three laws of Motion are all physical laws focusing on the
relationships between force,
velocity, mass and acceleration. Newton's Second law can be applied to
understand the effectiveness of wearing seat belts to prevent injury and death
during collisions. Newton's Second Law of Motion is the most powerful and
influential of Newton's three laws because of the equation provided and
quantitative calculations it encompasses, rather than laws that are just a
statement or idea. Newton's Second Law focuses on the relationship between
mass, acceleration and force of an object. This relationship is expressed by
the equation of force is equal to mass multiplied by acceleration.

When applying Newton's Second Law of Motion...

to car accidents and the use of seat belts,
the net external force on apassenger's body
is the mass of the vehicle multiplied by the cars
acceleration. The proper use of a seat belt reduces
the external force of an accident and distributes the
initial inertia of the collision across the body. This is
good to have happen during the crash, because this
reduces the force felt by the driver when he or she is
in a high impact car crash and/or finder bender.

importance or seat belts:

Seat Belts are the best protection in a car accident.4-point-seat-belt.gif
Failure to wear a seat belt contributes to more fatalities than any other single traffic safety-related behavior. 63 percent of people killed are not wearing seat belts. Wearing a seat belt use is still the single most effective thing we can do to save lives and reduce injuries on America's roadways.
Data suggests that education alone is not doing the job with young people, especially males age of 16 to 25 ­ the age group least likely to buckle up. They simply do not believe they will be injured or killed. Yet they are the nation's highest-risk drivers, with more drunk driving, more speeding, and more crashes. Seat belts are the most effective safety devices in vehicles today, estimated to save 9,500 lives each year. Yet only 68 percent of the motor vehicle occupants are buckled. In 1996, more than 60 percent of the occupants killed in fatal crashes were unrestrained.
If 90 percent of Americans buckle up, we will prevent more than 5,500 deaths and 132,000 injuries annually.
The cost of unbuckled drivers and passengers goes beyond those killed and the loss to their families.

The Lab:

During my lab experiment, I will be testing the
difference between car crashes with and without my safety belt and/or seat
belt. In doing so I will be able to tell how much safer you are with you seat
belt and how dangerous in can be without one. My hypothesis is that with the
seat belt I will slow down much quicker so that I feel a less intense crash or
displacement. Without I will most likely stop at the same time as the car and
suffer a high injury risk. With this being said, In the lab I will be lowering
the risk of injury by taking my car to a secluded street with my lab
assistants, camera, and lab notebook to record data, a stopwatch and ruler,
yard stick, or tape measure to measure distance needed to acquire the desired
speed to get accurate results and also to know how far I traveled before I
simulated a car crash or finder bender. To simulate a finder bender I will
travel at a speed of 35-45miles per hour, (this will be converted to Meters per
second for my calculations to be more accurate.) I will travel at this speed for a distance
of 50-60 yards, (this will become Meters.) ounce I have reached the end I will
slam on the breaks as fast as possible to represent what takes place during a
smaller scale crash.

During the lab I will have one of my lab
assistants record from both inside and outside. I will run the lab a desired
amount of times to get the best and most accurate results possible. In doing
this assures me that I will be getting the best and precise calculation to
assure the success of my lab and prove my thesis statement to be correct and/or
my hypothesis. In showing that newtons laws of motion are correctly used in
this experiment will prove to me to me that I have gotten closer to understanding
the things around me threw physics.

Planned Procedure
  1. select car to do lab in.EABD_FbeltLOCK.jpg
  2. take car to a empty street were you can drive without disturbance.
  3. step into car and secure your seat belt.
  4. turn your car on.
  5. accelerate the car up to 25 miles per hour for 10 seconds.
  6. step onto the break peddle as hard as possible to simulate a small car crash.
  7. record the time it takes your body to stop.
  8. do five more trials to make sure you have accurate information or data.
  9. do lab again, but do not wear seat belt, (remember to wear your helmet.) to see the difference in how long it takes your body to stop.
  10. in both situations remember to calculate the force the sat belt has on you.


  1. selected car for lab.
  2. selected street to run my test on.
  3. had me and another passenger sit in the vehicle.
  4. We then secured our seat belts.
  5. I drove the car at 25miles/hour.
  6. I drove at this speed for...
  7. Then to simulate a small car crash(finder bender) I slammed on the breaks.
  8. recorded data to see how long it took me to slow down.
  9. then we ran the same test again, this time we did not wear a seat belt.
  10. we recorded our data and looked at the difference in time it took for us to come to a stop or slow down.


  • With the seat belt i found that i felt like i stopped slower and increased my time i had to increase my impact time so that the force i felt was reduced.
  • without i nearly face planted into the windshield.

Pros and Cons


CONS: without seat belts

  • In a head on collision, if a passenger flies into the windshield of a car, their momentum is instantly stopped , and the passenger will suffer from severe injuries. the reason for this is because the force exerted is stop over a very short period of time.


Cars are designed with crumple zones so they may slow down over a longer period of time, which keeps the force smaller However, this safety feature alone will usually not prevent serious injury or death to the occupants of a car during an accident. The crumple zone only slows the car more gradually. The only way it slows the occupants more gradually is if they are attached to the car. Otherwise, the car may come to rest more slowly but the people come to rest immediately upon striking the already stopped interior of the car. Stopping in a small amount of time means the force must be very large. This video clip shows some very dramatic scenes of car crash tests with test dummies who are not wearing seat belts. Specifically look for cars crumpling and people stopping in very small amounts of time

PROS: with seat belt

  • if the passenger is restrained by a seat belt, their momentum is reduced more gradually by the constant and smaller force of the belt acting over a longer period of time. seat belts can reduce the impact of a passenger to 1/5 of the impact suffered by the body of the car.


Seat belts use two main ideas to protect passengers during a car accident. First, they slow the passenger down more slowly than the passenger running into steering wheel or dashboard. This keeps the force required to stop them smaller. It also prevents the person from contacting any of the glass windows in the car or continuing on to be stopped abruptly by the road, tree, or another automobile. The video clip above shows the role of the seat belt during an accident.

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