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Brain injuries can be caused by strong linear acceleration of the head. Imaging measurements of a human head upon impact show that the magnitude of displacement of the brain relative to the skull is, at most, approximately 1 mm, independent of the magnitude of the acceleration. Studies indicate that the probability of mild traumatic brain injury (MTBI) is approximately 25% for linear accelerations of the head of 50g, where g is the acceleration due to gravity (g = 9.8 m s 2 ). The probability of MTBI is ≈100% for linear accelerations of 150g. For this problem, model the case of a bicycle accident as a human head falling straight down from rest at a height of 2 meters. Assume the ground is very hard and does not compress upon impact. Also assume constant deceleration of the brain upon impact.(a) For the case when the cyclist is not wearing a helmet, what is the deceleration that the cyclist’s head undergoes upon contact with the ground? Express your answer in g’s. What is the probability of suffering a MTBI?(b) Suppose you want to design a helmet that reduces the probability of MTBI to less than 25%. By how much must the helmet compress after impact to achieve this goal for our model assumptions? Express your answer in cm. Do typical bicycle helmets provide this amount of compression?(c) Discuss ways in which our model (a human head falling straight down from rest at a height of approximately two meters onto an incompressible surface) could overor under-estimate the probability of MTBI in a bicycle accident.

Respuesta :

Answer:

a) a = 2000 g ,  b)  y = 4 cm

Explanation:

a) We can solve this part using the free fall relations

           

Let's start by finding the speed with which it reaches the ground, just before it starts to decelerate

           v₁² = v₀² + 2 g y

as it comes out of rest the initial velocity is v₀ = 0

           v₁ = √ 2g y

           v₁ = √ (2 9.8 2)

           v₁ = 6.26 m / s

in the braking part the distance is y₂ = 1 mm = 0.001 m

          v² = v₁² - 2 a y₂

       when stopping its velocity is zero v = 0

        a = v₁² / 2y₂

        a = 6.26² / (2 0.001)

        a = 1.96 10⁴ m / s²

This is the braking acceleration.

Let's look for its relationship with the acceleration of gravity

          a / g = 1.96 10⁴ / 9.8

          a / g = 2000

          a = 2000 g

b) to reduce the injury to less than 25%, the maximum acceleration that must be a = 50g = 490 m / s²

Let's find the distance it must travel to have this acceleration

          v² = v₁² - 2 a y

          y = (0 + v₁²) / 2a

          y = 6.26² / (2 490)

          y = 0.04 m

          y = 4 cm

therefore we must have a helmet that deforms 4 cm before the head stops

Most bicycle helmets provide this protection since they are quite elongated

c) The main error in the calculations is not taking into account the  

resistance with the air, in this sense the speed would be less than the calculated one and our calculations would be over estimated.

The second factor is that in a fall there may be an initial velocity different from zero, so the velocity would be higher and the acceleration as well, in this case our calculations are underestimated.