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/s^{2}. 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/s^{2}), 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.)
