9. A driver notices that her 1150 kg car slows down from 85 km/h to 65 km/h in about 6.0 s
on the level when it is in neutral. Approximately what power (Watts and hp) is needed to
keep the car travelling at constant speed of 75 km/h?
Given data
Mass of car is m=1150kg
Initial velocity is v0=85km/h=23.6m/s
Final velocity is v=65km/h=18.06m/s
The acceleration of the car is
a=tv−v0=6.018.06−23.6=−0.924m/s2
The force acting on the car is
F=ma=(1150kg)(−0.924m/s2)=−1062.7N
The amount of power is needed to keep the car travelling at constant speed of 75 km/h is
P=Fv=(−1062.7)(75(5/18))=−2.214∗104W
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