In 1964, after the 1250-kg jet-powered car Spirit of America lost its parachute
ID: 1422665 • Letter: I
Question
In 1964, after the 1250-kg jet-powered car Spirit of America lost its parachute and went out of control during a run at Bonneville Salt Flats, Utah, it left skid marks about 8.00 km long. (This earned a place in the Guinness Book of World Records for longest skid marks.) (a) If the car was moving initially at a speed of about 690 km/h, and was still going at about 350 km/h when it crashed into a brine pond, estimate the coefficient of kinetic friction k. (b) What was the kinetic energy of the car 24 s after the skid began? MJ
Explanation / Answer
from the given data
initial velcoity, u = 690 km/h = 690*5/18 = 191.7 m/s
final velocity, v = 350 km/h = 350*5/18 = 97.2 m/s
d = 8 km = 8000 m
Apply, Workdone by friction = change in kinetic enrgy
Fk*d*cos(180) = 0.5*m*(vf^2 - vi^2)
-N*mue_k*d = 0.5*m*(vf^2 - vi^2)
-m*g*mue_k*d = 0.5*m*(vf^2 -vi^2)
==> mue_k = -0.5*(vf^2 - vi^2)/(g*d)
= -0.5*(97.2^2 - 191.7^2)/(9.8*8000)
= 0.174
b) acceleration of the car, a = -g*mue_k
= -9.8*0.174
= -1.705 m/s^2
so, velcoity of the car at t = 24 s,
v = u + a*t
= 191.7 + (-1.705)*24
= 150.78 m/s
KE = 0.5*m*v^2
= 0.5*1250*150.78^2
= 14.2 MJ