Question

Traveling at 40.2 m/s, a driver applies the brakes to his fast-moving car and skids out of control on a wet concrete horizontal road. The 2000 kg car is headed directly toward a student waiting to catch a bus to campus who is standing 58.0 m down the road. Luckily, Superman is flying overhead and surveys the situation. Knowing that the coefficient of kinetic friction between rubber and rough wet concrete is .800, he determines that friction alone will not stop the car in time. So, he flies down and exerts a constant force of F=13000 N on the car's hood at a downward angle of 30 degrees. Show that (a) Superman was correct in his determination that the force of friction (assumed constant) alone would not stop the car in time; (b) Superman's applied force saves the day; and (c) demonstrate how close the car came to the student before stopping.

Answer #1

(a) The retardation due to the friction will be

The distance in which the car will stop with this retardation will be

This distance is much greater than the distance of boy who is at 58 m from the car. Hence, the force of friction alone would not stop the car in time.

(b) When Superman applies the force then various force acting on the car will be

Then distance traveled by car will be

(c) Hence before stopping of the car, the car will be from the student

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