You are driving your car at 35 mph when another car crosses the road at an inter

Question

You are driving your car at 35 mph when another car crosses the road at an intersection in front of you. To avoid a collision, you apply the brakes and skid to a stop, leaving 24m skid marks on the road. The mass of

your car is 1390 kg. The coefficient of kinetic friction between the tires and the road is k = 0.70.

a) What is 35 mph in meters/sec?

b) What is the change in momentum of the car?

c) What is the force of kinetic friction between the road and the car?

d) How long does it take the car to stop?

Explanation / Answer

a) 1 mph = 0.447 m/s

35 mph = 15.65 m/s

b) change in momentum = 1390*15.65 = 21753.5 kg m/s

c) fk = muek*n = muek*mg = 0.7*1390*9.8 = 9535.4 N

d) from the equation

v^2-u^2 = 2as

a = -15.65^2/2*24 = -5.1 m/s^2

v = u+at

t = u/a = 3.07 sec

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