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Collisions often are described using “g” forces. What does this commonly used term really mean? Since the acceleration due to gravity is nearly constant on Earth’s surface, g is an intuitive unit. One g has a magnitude of 9.8 m/s2. What happens if a 1.4 kg adult brain experiences 20gs? According to Newton’s second law, it experiences a net force of (1.4 kg)(20×9.8 m/s2), or 270 N. That impulsive force is 20 times the brain’s weight. Units of g have the following advantage: The force that results in a one g acceleration is a force equal to your weight. Lie down with your back flat on the floor and raise your legs and arms, as if you were driving a car. Now imagine that you are gaining speed at a rate of 0 to 60 mph in just under 3 s. The floor is pushing your back about as hard as your car seat would. (An acceleration from 0 to 60 mph in 2.7 s is equal to about one g.) 
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Play a sport such as football, ice hockey, or lacrosse, and sooner or later your head will almost certainly experience an abrupt impulse. Recent studies suggest that the human brain has a 75% chance of being concussed when it experiences a 100g acceleration (which corresponds to a force of roughly 1,400 N!). In the hope of reducing concussion risk, new football helmets are being designed with flexible face masks and foam layers of varying densities. If the helmet’s cushioning can increase the amount of time the head is decelerated, then the forces imparted on the player’s brain are reduced. Padding likewise reduces the forces felt by other parts of a contestant’s body. 
The same reasoning lies behind many features of today’s automobiles. While it might seem odd to make a car crash last longer, increasing its duration is exactly what improves the odds of survival for the vehicle’s occupants. Consider a collision that abruptly stops your 35 mph (15.6 m/s) car. If your body mass were 60 kg, your initial momentum would have been roughly pi = 936 kg m/s. You might have transferred that momentum to the steering wheel in a dangerous 100 milliseconds (0.1 s) if it were not for several life-saving technologies such as the crumple zone. The corresponding force of 9,360 N is approximately the weight of your car, and it might be sufficient to kill you. The front end of the car, however, is designed to collapse in the event of a crash—rather than remain rigid—thereby extending the time of the collision. The force imparted on the passenger in the case of a crash is reduced. An easily crushed engine compartment is not a flaw but a well-designed, possibly life-saving feature. 
