Newton's Law and Momentum Application

Common practice problems we do when dealing with momentum involve car collisions, but not only does this involve physics theoretically, but also explains some safety features on cars. Newton's first law states that an object in motion will stay in motion, and an object at rest will stay at rest unless acted on by an unbalanced force. For cars, this is true as cars will continue moving unless the brake is applied, or it collides with an object. Newton's third law states that for every action, there is an opposite and equal reaction. For example, if a car and a truck collide, they will experience the same magnitude of force, but the driver of the car will be worse off because of Newton's second law, which means that the car experiences a larger acceleration due to the truck having a greater mass (F=ma).

Since we know that momentum is conserved, this means that it can only be transferred from one object to another. The law of conservation of momentum states that in a closed system, the total momentum stays the same. In a car collision, this means that if one car loses x amount of momentum, the other car will gain that same amount, as it is being transferred.

This explains the reason for seat belts in vehicles. Going back to Newton's First Law, and object in motion will stay in motion. This means that when a car hits an object, the car will be acted on by an unbalanced force and stop, however the driver will continue forward. We wear seat belts to hold the driver or passengers back from flying forward. If we don't wear seat belts, a force will act on the person to slow them down, however this force will come from the windshield causing much more damage, and possibly throwing the person out of the car. The seat belt provides a softer, stopping force to minimize the damage in car accidents.

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Airbags are placed in cars to reduce the speed of the passenger in a car during a collision. The passenger of the car has momentum because of the passenger's mass and velocity. In order to stop this momentum, another force must act on the passenger. Instead of the passenger hitting the dashboard or windshield of the car, airbags are used to soften the blow and minimize the injuries of the person. The airbag provides a change in momentum, also known as impulse. The more time a force has to act on an object, the less damage to the passenger. 

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In addition to having airbags and seat belts, car also have crumple zones to reduce the impact of collisions. The front and back ends of cars have what are called crumple zones. These increase the amount of time that the force acts on the car during a collision. With an increased amount of time, the force acts over a longer period of time, which reduces the affects of the force when it reaches the middle of the car, where the passengers are. Crumple zones help minimize the injuries sustained in car collisions. 

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Lastly, another safety feature of a car are the headrests. Not only do headrests provide comfort, but they also prevent neck injuries during collisions. In some collisions, when a car hits another object, or hit the brakes, the car stops moving, but from what we know from Newton's first law, the driver's body will continue moving. In this case, the driver's head could be thrown either forwards or backwards, and once the head is thrown one way, it will naturally be thrown the other way due to the way our neck muscles react. Headrests help prevent neck injuries by providing a softer way to stop the head and neck of a driver. 

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