Motion with Meaning: Exploring Human Dignity Through Kinematics

An object is going 30 m/s and slow to a stop in 5 seconds. What distance did this cover?

Every day, millions of people walk in cities to get to work, school, or home. But walking in urban areas can be risky—especially at intersections, crosswalks, or where cars travel at high speeds. According to the National Highway Traffic Safety Administration, thousands of pedestrians are injured or killed each year in traffic accidents, often because a driver could not stop in time.

How can physics—through the use of kinematic equations—help us understand and improve pedestrian safety?

Let's include reaction time

A car is approaching a crosswalk where the speed limit is 15 m/s . A pedestrian steps off the curb 40 meters ahead of the car.

• The driver’s reaction time is 0.75 seconds.

• The car’s braking acceleration is –6.0 m/s² once the brakes are applied.

Unfortunately, the driver was speeding, traveling at 20 m/s instead of the posted 15 m/s.

Will the car stop in time to avoid hitting the pedestrian? Show all work using kinematic equations. Compare what would happen if the driver had been obeying the speed limit.