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.