Newton's laws, Force and Car Seat Belts

in #stemng5 years ago (edited)

Perhaps more important than the invention of motor vehicles is the invention of seat belts. Seat belts can save lives, so it is an important safety feature in a car; though it's use and awareness depends on individuals and environment. Modern vehicles have a lot more safety features than vehicles of yesteryears, but by just wearing a car seat belt one may turn a fatal car crash into a car crash without fatalities.

The best way folks can protect themselves in their cars is by wearing a seat belt," said U.S. Transportation Secretary Anthony Foxx. "Whether you're a driver or passenger, in the front seat or back, the simple act of wearing a seat belt significantly reduces the risk of fatality and major injury in a crash." Source

2010 Porsche 911 GT3 RS Seat Belt
[Car Spy: (CC BY 2.0] : Flickr

A study conducted by National Highway Safety Traffic Administration (NHTSA) of the USA in 2016 showed that awareness and use of car seat belts in the United States of America have grown to 90% and the estimated lives saved since 1975 at about 345,000. But, this can not be said for every environment, for example in Nigeria people rarely use seat belts and the majority use it to avoid paying fines not as a safety feature to save a life.
Before discussing more on a seat belt, I will pause and take a look at the principles behind a car Seat Belt.

Newton's First and Second Laws.

Sir Isaac Newton's three laws are known as Newton's Laws of Motion. For this topic, I will discuss the only Newton's first and second laws.

That is, if a body experiences a change of direction in motion, deceleration or acceleration, there must be another outside force acting on it. This means that a car would keep moving until it is acted upon by force. Hence, a force is required to restrain the vehicle, and it's occupants if not they would continue as stated above. Force is defined as a pull or a push working on a body as a result of its interaction with another body. It has has a direction since it is a vector quantity.

If we consider a body at rest to have a zero speed, and an unbalanced force causes it to move or change direction.

Unbalanced and Balanced Forces

Balanced Force: This is when all the forces acting on a body balance each other and the object is said to be on equilibrium. Let's illustrate with a simple example of what happens when you lie on your bed.
Two forces are acting upon you when you are lying on the bed. One is the Earth's gravitational force (Pull) that exerts a downward force on you, and the other is a push of the bed on you, This pushes an upward force you Since the two forces acting on you are equal in magnitude and opposite direction, they are balanced.
Unbalanced force: They are not equal and opposite and always cause a change of direction. If unbalanced forces act in different directions, the combined forces are equal to the difference between the two forces and are exerted in the course of the larger force.

Newton's second law is then represented mathematically as;
F = ma

  • F is the unbalanced force
  • m is the mass of the body
  • a is the acceleration caused by the body

The above laws are experienced when in moving cars and trucks. The unfortunate injuries that occur in car crashes happen a result of the tendencies of cars to continue its motion.

Lets us consider the unfortunate collision of a car and another body (wall). Upon contact with the body, a force (an Unbalanced force F) acts on the vehicle. The occupants of the car share the same motion with the car. Hence the unbalanced force abruptly decelerates the car to rest; the passengers are decelerated too by another force. This could be the car dashboard, windscreen or steering, side doors or the object the car collided with etc. ; However, if the passengers are buckled up, the seat belt would the object to slow you down. This action is known as Newton's first law of motion or law of inertia. Seat belts absorb your inertia better than other objects (dashboard, steering etc.) because of the flexibility. The seat belt design has a 'little give in' so it does not allow abrupt stop of the passengers. Also, seat belt straps spread over a large area of the body, hence distributing the impact.


The seat belt was invented in the 19th century by English Engineer George Cayley. However, the three-point seat belts in use in modern cars of today were patented in the USA by Roger Griswold and Hugh Dehaven. The state of Victoria in Australia in 1970, the world saw the first legislation for compulsory use of seat belts in cars for drivers and front seat passengers. The legislation came to be after it was found that the use of seat belts lowered incidence of officer's injuries and deaths.

Over the years, the world had seen various types of seat belts are in use in vehicles depending on when the car was manufactured.

  • Two Point/ Lap: This is the type of seat belt wraps around the waist, and it is still being used in aeroplanes. It has an adjustment that allows it to loosen or tighten. It is first the first type of seat belt.
  • Sash: This design was seen in the 1960s and has a design flaw that allows that occupant slide off the belt in certain accident conditions. The belt is fastened at the waist and worn above the occupant's shoulders.
  • Three-point: This has a similar design like the sash but spreads the energy of the body when a collision occurs, and it has a Y-shaped arrangement.
  • Belt-in-Seat: This type is a three-point seat belt with a shoulder attachment to the seat itself rather than to the car frame. It was used in cars like Range Rover Classic.
  • 4, 5, and 6 Points: Five-Points harness is favourite for child safety seats, car race and other high-performance cars.


Safety first! Having the culture of safety first is very important in life. Safety devices are crucial in our daily lives and should be taking very seriously. Accidents happen every day, and the use of safety belts have saved many lives. It is imperative to recognise the work of seat belt to reduce injuries and save lives.
In many countries, not wearing seat belts could attract fines or a more severe penalty from the law enforcement; and not wearing seat belts has the potential to hurt your insurance claims should one be unfortunate to be involved in an accident.
Your Safety First!

Thank you for reading.

References: 1, 2, 3, 4

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Great research and posting, keep it up !

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This Porche seatbelt resembled the type you see in an aircraft. I guess since it's a fast car they use more points to ensure safety. Good work.

You are correct! Porche 911 GT3 is a fast sports car; It is a racing machine, hence needs a multi-points seatbelt for safety

Safety is of utmost importance. Thank you for reemphasizing this fact for some of us that are yet to give 100% compliance to this safety device the seat belt.

Safety is very important in every aspect of human life. Thanks for reading my post