At 55 mph, a tractor-trailer requires how many times the braking distance of a typical automobile to stop?

At 55 mph, a tractor-trailer requires how many times the braking distance of a typical automobile to stop?

• A. One time
• B. Two times
• C. Three times
• D. Four times

Answer: (B)

The main idea is that stopping distance grows with the amount of energy the brakes must dissipate. A tractor-trailer has far more mass and momentum than a typical car at the same speed, so there’s more kinetic energy to remove when you brake. Even though the brakes on a truck are powerful, they have to handle that extra energy, and tire grip limits combined with weight transfer mean the deceleration isn’t as large as in a lighter car. All of this adds up to needing more distance to come to a stop. In practice, at 55 mph a tractor-trailer needs about twice the braking distance of a typical automobile, under normal dry-road conditions. Of course, road conditions, cargo load, brake condition, and slope can change the actual distance.

The main idea is that stopping distance grows with the amount of energy the brakes must dissipate. A tractor-trailer has far more mass and momentum than a typical car at the same speed, so there’s more kinetic energy to remove when you brake. Even though the brakes on a truck are powerful, they have to handle that extra energy, and tire grip limits combined with weight transfer mean the deceleration isn’t as large as in a lighter car. All of this adds up to needing more distance to come to a stop. In practice, at 55 mph a tractor-trailer needs about twice the braking distance of a typical automobile, under normal dry-road conditions. Of course, road conditions, cargo load, brake condition, and slope can change the actual distance.