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Warm Up
Day 85(5)
A 1000 kg car is stopped in 10 meters
from an initial velocity of 12 m/s.
What is the total force of friction on
the tires?
How much energy is absorbed in the
brakes while bringing the car to a
stop?
How many seconds does it take to
stop the car?
Solution
F=ma
0=v0^2-2ad
a=v0^2/2d=144/(20)=7.2m/s^2
F=7200 Newtons
Energy =1/2mv^2=500x144
E=72,000 Joules
d=d0+v0t1/2at^2
t=-vo+-sqrt(vo^2-4do(1/2a))/a;
…or v=v0+at
t=(v-v0)/a=-12/-7.2=1.67 sec

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