A car of mass 1.3X10^3 kg is traveling east at a speed of 21 m/s along a

Jerry

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horizontal roadway.? When its brakes are applied, the car stops in 6.9 s. What is the average horizontal force exerted on the car while it is braking?
 
u = 21m/s
v = 0m/s
t = 6.9s
a = ?

v = u + at
-> a = (v-u)/t = -21/6.9 = -3.043m/s

F = ma = 1300*(-3.043) = -3956.5N

So 4000N west (two significant figures)
 
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