Question

While visiting a friend in San Francisco you decide to drive around the city. You turn a corner and are

driving up a steep hill. Suddenly, a small boy runs out on the street chasing a ball. You slam on the

brakes and skid to a stop leaving a 50 foot (600 inch) long skid mark on the street. The boy calmly

walks away, but a policeman watching from the sidewalk walks over and gives you a ticket for speeding.

You are still shaking from the experience when he points out that the speed limit on this street is

25 mph. After you recover your wits, you examine the situation more closely. You determine that

the street makes an angle of 20 degrees with the horizontal and that the coefficient of static friction

between your tires and the street is 0.80. You also find that the coefficient of kinetic friction between

your tires and the street is 0.60. Your car’s information book tells you that the mass of your car is

1570 kg. You weigh 130 lbs. Witnesses say that the boy had a weight of about 60 lbs and took 3.0

seconds to cross the 15 foot wide street.

(a) Is the police officer’s account that the entire scenario took less than two seconds an accurate

statement?

(b) Will you fight this speeding ticket in court by using physics to prove you did not break the law?

Answer 1