Howard J. answered • 10/12/19
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Principal Mech Engr with 35+ years' on-the-job physics experience
A car is travelling at 50 km h-1 brakes as hard as it can and stops in a distance of 15 m. Suppose that the maximum braking force is no dependent on speed i.e. the coefficient of kinetic friction is constant. What is the shortest stopping distance when the car is travelling at 75 km h-1?
The work done by friction Wf = the kinetic energy prior to braking KE
Wf=fD1 where f is the opposing friction force and D1=15 m
KE=0.5Mv2
f(15)=0.5M[(40x103)m/hr(hr/602sec]2
15f=61.73M
f=4.11M
Since f=constant,
fD2=0.5M[(75x103)m/hr(hr/602sec]2=
D2=217M/f=217M/4.11M=53 m
Solution: 53 m
