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Seatbelt Physics

Three laws of motion were identified by Isaac Newton in the mid to late 1600s, and all three apply to the physics and operation of a seatbelt.

  1. The law of inertia. It states that objects will maintain their current state of motion unless acted upon by an external force.
  2. Force equals mass times acceleration, or [A=NF/M].
  3. For every action, there is an equal or opposite reaction.

How do they apply?

  1. First law: A person sitting in the seat of a car during a crash will continue to move forward at the same speed the car was moving even if the car has been stopped or hits a solid object. The seatbelt is there to stop this inertia from proceeding.
seat belt physics
  1. Second law: if a driver or passenger weighs a substantial amount, the acceleration applied to that person will be less, due to the equation of Acceleration = Net Force / Mass. For instance, if a vehicle were to strike another vehicle (the force) with speed constant, a lighter person not wearing a seatbelt will fly out of the car further than a heavier person who is not wearing a seatbelt.
  2. Third law: Newton proved that for every action there is an equal or opposite reaction. For example, you are driving down a country road as the sun is setting. Your seatbelt is on and you are fully aware of your surroundings. Suddenly, a deer jumps in front of your vehicle and you jolt forward after slamming on your brakes. This causes you to be pushed back into your seat again. The force that is pushing you back is the equal and opposite reaction. 

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