a car crashes into a tree, coming to rest from an initial velocity of 11 m/s whi
ID: 1279966 • Letter: A
Question
a car crashes into a tree, coming to rest from an initial velocity of 11 m/s while the hood is crumpled by 0.7m Assume the acceleration is constant how much time does the crash take? B. a police inspector notices that the skidd marks leading to the crash in part A are 122 m long. He already knows that the mass of the car is 1828 kg, the final velocity of the car(whenthe crash began) was 11 m/s, and the coefficient of kinetic friction between car tires and asphalt is 0.55. With this info, he deduces the initial speed of the car before the driver slammed on the brakes. What was the car's initial velocity?
Explanation / Answer
v^2 - u^2 = 2*a*s
a = (v^2-u^2)/(2*s)
= (0^2-11^2)/(2*0.7)
= -86.43 m/s^2
we know
v = u + a*t
t = (v-u)/a
= (0-11)/(-86.43)
= 0.127 s
b) workdone by friction = chnage in kinetic enrgy
-mue_k*m*g*d = 0.5*m*(v2^2-v1^2)
mue_k*g*d = 0.5*(v1^2-v2^2)
2*mue_k*g*d = v1^2 - v2^2
v1^2 = v2^2 + 2*mue_k*g*d
v1 = sqrt(v2^2 + 2*mue_k*g*d)
= sqrt(11^2 + 2*0.55*9.8*122)
= 37.9 m/s <<<<<<<<------------Answer
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